·No text on this page.

·RADIATION,
GENES,
AND MAN

·No text on this page.

·RADIATION,
GENES,
AND MAN
BY
BRUCE WALLACE
Cornell Universily
AND
TH. DOBZHANSKY
Columbia University
A HOLT.DRYDEN BOOK
Henry Holt and Company, New York

·Copyright © 1959 by Henry Holt and Company Inc.
Library of Congress Catalog Card Number 59 14275
29198-0119
Printed in the United States of America

·Preface An event that may prove to have been a turning point of world history took place on December 2, 1942, on a squash court under the stadium of the University of Chicago On that day, unreported because of wartime secrecy, there was started the first man made self sustaining nuclear chain reaction, for the first time men obtained energy from a source not derived from the fires of the sun Science has thus forced man to enter a domain from which there is no tuming back and no escape Only two roads lie open One leads to material advance, universal brotherhood, and spiritual fulfillment The other leads to disaster and possibly to extinction The use of atomic energy opens before mankind vistas of unheard-of power, its misuse endangers the most precious possession man has or can have-his innermost nature Such misuse threatens genetic damage to generations yet unbom, it is a menace to our posterity, both close and remote. This book is about the threat of genetic damage to man--a topic which has been discussed frequently in books, in joumal articles, and in the daily press But the appearance of yet another book dealing with it hardly needs justification The subject will doubtless remain in vogue for a long time, since man cannot afford to be indifferent to his future As we improve our un derstanding of the situation created by widespread and everincreasing use of high energy radiation and atomic power, it will become necessary from time to time to restate and revise the conclusions concerning the dangers of genetic damage We have endeavored to make this book understandable to ·21. PRETACE the general reader, not to biologists alone But we resolved not to impose upon the reader our own opinions and conclusions without giving him at the same time some competence to judge their validity for himself A number of scientists, including very eminent ones have made pronouncements, some declaring that the genetic damage from radiation places mankınd on the brink of disaster, others contending that the damage, if any, will be slight or negligible We felt that merely adding declarations of our views to this not very edifying multiplicity of beliefs would be less than useful Probably the most important fact concern ing the issue is that the data evidence, and information on which, and only on which, conclusions should rest are as yet wholly inadequate Partisan statements usually conceal this in sufficiency of knowledge to avoid exposing their partisan nature We intend, on the contrary, to stress the need-Indeed, the urgency-of further research on the genetics of populations and on radiation genetics An evaluation of the magnitude of genetic radiation dam age to human populations can be at best very tentative at present To appraise such evaluations, the reader must be able to see the strengths and weaknesses of the arguments on which they rest This has necessitated the discussion of some topics that will perhaps be regarded as abstruse, particularly in Chap ters 6 through 8 If, despite our efforts to alleviate the difficul ties, the reader is terrified by these chapters, he can be advised merely to skip the difficult places We hope that the conclusions are stated understandably, but we prefer not to have them accepted on faith by those who are able to assess their validity We do not claim for these conclusions more than that they seemn reasonable to us on the basis of the information now available New discoveries, new information, may well change them Science is often most convincing when it is least dogmatic B W Th D July, 1959

·Contents
vü
1
Preface
1. Atomic Energy–Friend or Foe?
Prometheus, Old and New, 1
Do We Need Atomic Energy? 3
The Dangers of Atomic Energy, 7
Scientific and Ethical Aspects of Genetic
Radiation Hazards, 9
13
2. Heredity, Environment, Genes, and
Chromosomes
Man and Biology, 13
Individual Variation, 15
Nature and Nurture, 17
Heredity Is Not a Status but a Process, 19
Chromosomes and Nucleic Acids, 21
Meiosis 24
Genes 25
Mendel s Law of Gene Segregation, 27
Mendel s Law of Gene Recombination, 31
33
3. Spontaneous Mutation
Heredity and Mutation, 33
Mutation in Man, 35
Measurement of Mutation Rates in Drosophila, 37
L.

·X CONTENTS Mutation Rates per Gene, 40 Chromosome Mutations, 43 Mutation and the Welfare of the Organism, 45 Useful Mutants, 47 Adaptive Value of Human Genetic Variants, 49 Is Mutation Good or Bad? 54 57 4. Atoms and Radiations Life and Radiation, 57 Molecules and Atoms, 58 Anatomy of the Atom Radiations, 60 Effects of Radiation on Matter, 63 Measurement of Radiation, 66 Sources and Amounts of Natural Radiation, 67 Man made Radiations, 70 79 5. Induction of Mutation by Radiation Discovery of the Speeding up of Mutation by X rays, 79 Direct Proportionality between the Amount of Radiation and the Number of Mfutations, 81 Doubling Dose and Frequency of Mutation per Roentgen of Radiation, 83 Mutations in Different Organisms, 84 What Radiations Are Mutagenic? 86 The Directness of the Genetic Effects of Radiaton, 88 Somatic Mutation, 89 Chromosome Breakage by Radiation, 91 Radiation, Carcinogenesis, and Cancer Therapy, 94 Radiation Illness, 96 Genetic and Physıological Radiation Damage Compared, 99 101 6. Genes in Mendelian Populations What Is a Population? 101 The Gene Pool of a Population, 103 ·CONTENTS The Hardy Weinberg Theorem, 105 Random Mating, 107 Changing Gene Frequencies by Mutation and Selection, 108 Genetic Equilibrium, 110 Genetic Death, 113 Genetic Loads in Drosophila Populations, 116 Genetic Loads in Human Populations, 118 Incompletely Recessive Defects, 119 How Much Time Is Required to Reach Genetic Equilibrium? 122 127 7. Genetic Effects of Radiation on Populations Surveying the Problem, 127 Radiation induced Recessive Mutants at Equilibrium, 130 Radiation induced Recessive Mutants on the Way to Equilibrium, 133 Radiation induced Mutants That Harm Heterozygotes, 136 Recessive Mutants Produced by Temporary Radiation Exposures, 139 Semidominant Mutants Produced by Temporary Radiation Exposures, 145 Attempts to Detect Genetic Radiation Damage in Man, 148 151 8. Some Unsolved Problems Introductory Remarks, 151 Heterosis and Mutation 152 Genetic Structure of Natural Populations Classical Hypothesis, 156 Genetic Structure of Natural Populations Balance Hypothesis, 159 Role of Mutation under the Balance Hypothesis of Population Structure, 162

·CONTENTS
Does Mankind Face Extinction Because of
Genetic Radiation Damage? 166
Maintenance of Population Size, 167
Survival and Extinction of Irradiared
Drosophila Populations 170
Unlikelihood of Extinction of the
Human Species, 174
179
9. Concluding Survey
The Need for Research in Genetics 179
Radiation Damage and Value Judgmient, 183
Nuclear Bomb Tests, Fallout, and Genetic
Damage, 186
Should Man Control Man's Evolution? 188
Deficiencies in Our Present Knowledge, 192
Epilogue, 193
Glossary
197
Suggestions for Further Reading
201
Index
203

·RADIATION,
GENES,
AND MAN

·CONTENTS
Does Mankind Face Extinction Because of
Genetic Radiation Damage? 166
Maintenance of Population Size, 167
Survival and Extinction of Irradiated
Drosophila Populations 170
Unlikelihood of Extinction of the
Human Species, 174
179
9. Concluding Survey
The Need for Research in Genetics, 179
Radiation Damage and Value Judgment, 183
Nuclear Bomb Tests, Fallout, and Genetic
Damage, 186
Should Man Control Man's Evolution 188
Deficiencies in Our Present Knowledge, 192
Epilogue, 193
Glossary
197
Suggestions for Further Reading
201
Index
203

·1 Atomic Energy- Friend or Foe? Prometheus, Old and New According to Greek myth Prometheus stole fire from heaven, brought it to earth, and gave it to men to make them happy and powerful and proud For this act of defiance, he was doomed by Zeus to endless suffering Although nowadays people are dısın clined to invent myths of the kind the ancient Greeks created so well, the release of atomic energy was a feat of almost Promethean daring To all but a handful of physicists, the release of energy through the fission and fusion of atomic nuclei seems like magic If we had not seen photographs of atomic explosions and of the havoc wrought by them we might disbelieve the whole notion of atomic energy Even with all this evidence before us, the fact that the amount of energy released in these explosions can be calculated only with the aid of a formula which involves multiplying the speed of light by itself defies ordinary com prehension There is no way of telling just when and how fire was first tamed by man The real Prometheus is lost in the mists of ·% • ATOMIC ENERGT-FRIEND OR FOE? prehistory We know only that remains of charcoal from the hearths built by Peking Man are found in abundance, together with his bones, in the cave shelters not far from Peking, China. Peking Man is one of the links connecting biologically modem man with his apelike ancestors, his remains are perhaps as much as half a million years old The use of fire was doubtless one of the great achievements in the long and slow ascent of our remote forebears from an animal to a true human level Fire permitted our ancestors who were originally inhabitants of warm coun. tries, to invade lands that were otherwise too cold for them in winter, and eventually to spread to all conunents and most islands all over the world Fire rendered possible the cooking of food, and thus increased immensely the range of foodstuffs with which man could satiate his hunger Much more recently, some 4.000-5,000 years ago, fire was used to extract metals from ores and to work them into tools which gave man his dominion over his environment Especially from the time of the Industrial Revolution, fire has been the basis of many of man's labor. saving devices The history of man's mastery over fire is, however, a history of both good and evil Many a house, village, and city, with all its inhabitants, has been given to fire and sword Only few cen turies ago, thousands and thousands of people, whose crimes were chiefly those of conscience, were burned at the stake Today we have fire extinguishers in many buildings and main tain town fire brigades Folk wisdom warns that it is inadvisable "to play with fire Yet it occurs to nobody to suggest that we ought to give up the use of fire, we simply minimize fire hazards Man has learned to live with fire He will have to leam to live with atomic energy The story of atomic energy is in some ways like the story of fire, although the atomic Prometheus was our contemporary This new Prometheus was not a single person but a group of brilliant physicists and mathematicians, who toiled for many years, and in different countries, to uncover secrets of nature, unmindful of any possible practical valuc of their findings Many of them, in fact, belonged definitely to the class of im pracucal dreamers commonly known as eggheads, and they и сте ·DO WE NEED ATOMIC ENERGY? - 3 so regarded even by some of their more hardheaded, or real Istic, colleagues One of the authors of this book remembers a discussion, around 1940, with a very able but very realistic physicist, who gave the work of Einstein and Bohr as an exam ple of a kind of science that would never benefit humanity Little did he know that perhaps at that very moment, Franklin D Roosevelt was authorizing considerable expenditures to make possible scientific research recommended by Einstein and others A few years later, under the leadership of Enrico Fermi among others, this research led to the release of atomic energy in the first atomic pile The scene of this event was a squash court of the University of Chicago None of the scientists who worked to bring about the release of atomic energy was chained to a cliff, like the Titan Prometheus But perhaps the consciences of some were lacerated by the dreadful use awaiting their glorious discoveries For the first willful employment of atomic energy was in the atomic bombs which killed and maimed perhaps more people in Hiro. shima and Nagasaki than were ever burned at the stake by the Holy Inquisition Do We Need Atomic Energy? There is a popular belief, which may or may not be true, that monkeys can be trapped through the use of a small mouthed jug containing several rather large nuts The monkey, so the tale goes, finds the nuts in the jug and reaches his paw in to take one Grasping the nut the monkeys paw is too large to be withdrawn from the jug Escape could be accom plished simply by releasing the nut and walking away But the monkey refuses to let go, he is trapped Much as we may hope that letting go' offers one possible solution to the problems raised by the Atomic Age, it is highly unlikely that we will escape any more successfully than our simian friend The atom is too tempting, we will refuse to let go The reason the atom is here to stay in spite of its manyom ·2 • ATOMIC ENERGY-FRIEND OR FOE? prehistory We know only that remains of charcoal from the hearths built by Peking Man are found in abundance, together with his bones, in the cave shelters not far from Peking, China Peking Man is one of the links connecting biologically modern man with his apelike ancestors his remains are perhaps as much as half a million years old The use of fire was doubtless one of the great achievements in the long and slow ascent of our remote forebears from an animal to a true human level Fire permitted our ancestors, who were originally inhabitants of warm coun tries, to invade lands that were otherwise too cold for them in winter, and eventually to spread to all continents and most islands all over the world Fire rendered possible the cooking of food, and thus increased immensely the range of foodstuffs with which man could satiate his hunger Much more recently, some 4,000-5,000 years ago, fire was used to extract metals from ores and to work them into tools which gave man his dominion over his environment Especially from the time of the Industrial Revolution, fire has been the basis of many of mans labor saving devices The history of man's mastery over fire is however, a history of both good and evil Alany a house, village, and city, with all its inhabitants, has been given to fire and sword Only few cen turies ago, thousands and thousands of people, whose crimes were chiefly those of conscience, were burned at the stake Today we have fire extinguishers in many buildings and main tain town fire brigades Folk wisdom warns that it is inadvisable "to play with fire Yet it occurs to nobody to suggest that we ought to give up the use of fire, we simply minimize fire hazards Man has learned to live with fire He will have to learn to lise with atomic energy The story of atomic energy is in some ways like the story of fire, although the atomic Prometheus was our contemporary This new Prometheus was not a single person but a group of brilliant physicists and mathematicians, who toiled for many years, and in different countries, to uncover secrets of nature, unmindful of any possible practical value of their findings. Many of them, in fact, belonged definitely to the class of im practical dreamers commonly known as eggheads, and they were ·DO WE NEED ATOMIC ENERGY? able in a great many areas of the earth and that, even in the United States it is sufficient to supply only a fraction of the total energy we now consume This point can stand further amplification In an ordinary home, it is common for several lights to burn throughout an evening, sıx 100 watt bulbs burning for three hours is not unusual The energy required to keep these bulbs lit for this period of time is equivalent to nearly 5,000 000 foot pounds, that is, to the energy that would be embodied in a Cadillac hitting Thirty fourth Street if it were thrown from the top of the Empire State Building The Niagara Falls are 160 feet high, about 15 tons of water must pour over the falls in order to supply the energy required for lighting this one house This assumes that hydroelectric plants are 100 percent efficient, if their real efficiency is nearer 10 percent, 150 tons of water are required This does not seem to be a particularly great amount of water (at the usual rate of flow for the Niagara River, it would require about 0 0025 seconds for this much water to flow over the brink) untıl one starts counting the number of homes with electric lights Add to this the number of TV sets watched during the same evening Include various household appliances, power tools, and oil burners And finally, include industrial processes such as the extraction of aluminum, which demand large quantities of electricity Rather than add all of these individual demands, it is simpler to proceed from the other end The total production of electricity in the United States in 1955 was approximately 600 000,000,000 kilowatt hours We saw that 1 8 kilowatt hours (600 watts x 3 hours) was equivalent to 5,000,000 foot pounds The total consumption of electricity per year, then, is equivalent to 15 X 1018 (1,500 000 000,000 000,000) foot pounds With a mean height of 160 feet, a flow of 200,000 cubic feet per second, and a 10 percent efficiency of hydroelectric plants, it would require about 250 Niagara Falls to satisfy this demand In fact, if every drop of water that falls on the United States as rain were collected and were forced to run to the sea by way of a hydroelectric plant, it is doubtful whether our present demand for energy could be satisfied in this way What, then, are our sources of energy other than water ·ATOMIC ENERGY-HARD OR FOE? 700 600 500 400 electric energy produced (bill ons of lowatt hours) 300 200 100 1930 1935 1 1945 1940 1950 1955 FICURE 1 Annual production of electric energy in the United States between 1930 and 1955 Upper curve-total energy produced lower curve energy produced by hydroelectric scacions power? About 80 percent of our total electrical output comes from burning coal, gas and oil Figure 1 shows that the demand for electricity has climbed at an ever increasing rate for the past 25 years today we consume about six times as much as we did in 1930 During the thirties water power was a substantial con tributor to the total amount of electricity consumed At present it is only a minor contributor, moreover, the amount of elec tricity produced in this way has become nearly constant As the demand grows then water will become even less important in the over all picture By fulfilling an ever increasing demand for power by burning fossil fuels-coal, gas and oil-we are de pleting stores which are not being replaced These are non renewable natural resources and we have not been overly prudent in their expenditure Despite claims that there are gas fields as yet undiscovered and coal fields still un worked whatever quantity exists is fixed In time-and in an alarmingly short time if the demand grows as it has for the past quarter of a century-these fuels will be gone There are two energy sources that may replace fossil fuels oil and ·THE DANGERS OF ATOMIC ENERGY 7 as the supplies of the latter approach depletion One is radiation from the sun, the other is nuclear energy-that derived from the fission of heavy atoms and the fusion of light ones It may be noted that all energy which man has used so far is derived ultimately from the life giving warmth of the sun Plants capture solar energy by means of their green pigment, chlorophyll, and store it in energy rich chemical substances The existence of all other organisms-men, animals, and plants without chlorophyll—ıs entirely dependent upon their being able to utilize these substances as foods, and thereby to utilize at second hand, as it were, some of the solar energy captured by chlorophyll The energy obtained by burning wood is again that of the sun stored by a green plant, the fossil fuels represent accumulations of solar energy stored by plants of ages long gone by And finally, the energy of running water and of wind is derived from the warmth of the sun rays, which causes the evaporation of water from the surface of the sea and the circulation of air masses The tremendous historical significance of the release of atomic energy is that man has, for the first time, learned how to obtain energy from a source that is not merely transformed solar energy. It might seem that capturing solar energy directly would be preferable to utilizing atomic energy, with all the dangers inherent in the latter However, the practical utilization of solar energy at present is restricted to rather small solar batteries It is possible that these will eventually be developed into a substantial source of energy At the moment, though, atomic energy is far in the lead We know that enormous amounts of energy can be released by fission and fusion processes We know how to exploit the energy of fission In the case of fusion, we have yet to learn how to control its release, but this will probably be accomplished in the not too distant future Man will certainly not release his grip on this source of energy. The Dangers of Atomic Energy If playing with fire is dangerous, playing with atomic energy is far more so Animals, at least those living on land in ·• ATOMIC ENERGY-FRIEND OR FOE? years after tropical and temperate climates, often encounter rocks and soil heated to excessively high temperatures by the summer sun Living beings possess sense organs that detect such excessive heat in the environment and stimulate them to move away to cooler places Atomic radiations of considerable intensity, how ever, do not occur naturally on earth, they are man made Our bodies, and those of other organisms, have no sense organs that detect exposure to such radiations and warn us by giving a danger signal Yet these short wave radiations (see Chapter 4) produce, in a matter of days or weeks after exposure, burns just as dangerous as those produced by excessive heat Still more insidious is the power of these radiations to evoke malignant growths, or cancers These malignancies may not appear for exposure However dangerous radiation burns and radiation induced malignancies may be, these maladies affect only persons directly exposed to radiation Ailments caused by radiation in cxposed persons are referred to collectively as “physiological radiation damage' One may derive some comfort from the fact that physiological damage is at least confined to the generation irradiated More ominous is the genetic damage induced by radiation The sex cells of a person exposed to atomic radiations may be altered so as to produce death, disease, monstrosity, in validism, or simply delicate health in the descendants of irradı ated individuals This morbid alteration of the hereditary materials may menace any number of persons, because it may be transmitted generation after generation for a long time to come A great many persons believe that genetic radiation damage may endanger the future of mankind The seriousness with which physiological and genetic dan gers are regarded can be judged from the actions taken by many governmental and quası governmental agencies The United Nations Scientific Committee on the Effects of Atomic Radiation is attempting to gauge, from existing data, the dan gers of irradiation and to indicate the lines of research that may make more accurate judgments possible in the future The World Health Organization convened a committee of geneticists in Copenhagen during the summer of 1956 to consider similar problems ·SCIENTIFIC AND ETHICAL ASPECTS OF GENETIC RADLATION RAZARDS 9 In the United States we have had hearings conducted by the Special Subcommittee on Radiation of the Joint Congressional Committee on Atomic Energy the subject of these hearings at which many scientists testified was the nature of radioactive fallout and its effect on man The National Academy of Sciences has made a study of the biological effects of atomic radiation The governments and scientists of many other countries-Great Britain India and Japan for example-have delved into these same problems Scientific and Ethical Aspects of Genetic Radiation Hazards Perhaps no other scientific discovery has had the emotional impact that the release of atomic energy has had But the pride elicited by this brilliant attainment of the human spirit is mixed with fear The destructiveness of atomic explosions and the hazards of radiation have aroused the fear that if not our own generation then our descendants will find the Atomic Age an impossible one to live in And if man does survive there is the appalling possibility that human nature itself will be deformed and wrecked by misuse of this terrible new power The public even the so-called educated public gathers its information about atomic energy and the dangers of radiation chiefly from the daily press and from articles in popular maga zines Unfortunately newspaper and magazine reports have all too often dealt with bogus sensations-impending extinction of manhind through sterilization or through wholesale production of inhuman monsters On the other hand efforts more often glib than well grounded have been made to minimize the very real hazards of radiation How can the public arrive at a reasonable attitude toward both the need for and the dangers inherent in the use of atomic energy? One possibility of course is that the matter is alto gether beyond human reason Faced with a seemingly incom prehensible situation one can forsake reason entirely and rely solely on instinct or intuition We are not this pessimistic We ·The 10 • ATOMIC ENERGY-FREND OR FOE? believe that the situation calls for an understanding of the problems involved Under pressure of public opinion, a variety of official decisions regarding possible courses of action will be taken If these are to be intelligent decisions, if they are to stand in the face of opposition arising out of selfishness or ignorance, it is imperative that the public comprehend the issues at stake purpose of this book is to spell out, to the best of our ability, the genetic aspects of the problem of atomic-and otherradiations We do not believe, however, that all the answers to the problems created by atomic energy are to be found in genetic or any other scientific information Some of these problems involve ethical rather than factual considerations Con. sider, for example, a statement not unlike some that have repeatedly appeared in the press Fallout causes leukemia, bone cancer, and endangers future generations, therefore, bomb tests must be stopped Although the premise is certainly not irtelevant to the conclusion, it is not in itself sufficient to make the conclusion necessary It is possible that the testing of bombs should be stopped immediately even if fallout were harmless on the other hand, it may be that these tests must be continued regardless of their dangers even though these be a hundred or a thousandfold greater than we think they are There are people who hold that scienusts should not leave their laboratories and attempt to write a book of this kind But scientists and nonscientists must share equally, man for man, the responsibility for the ultimate decisions regarding the use of atomic energy It is wrong to restrict either responsibility or privilege in these matters We reject the view that only those with access to secret information are qualified to hold opinions about these issues, "secret information is used here both with reference to classified information and with reference to infor mation that is not generally known If decisions based on rational thinking are preferable to those based on intuition, then anyone who has relevant infor mation or an opinion based on such information ought to share this knowledge with others. Sharing information does not guar antee unanimity of final judgment, it does, however, offer each ·SCIENTIFIC AND ETHICAL ASPECTS OF GENETIC RADIATION HAZARDS . 11 person an opportunity to identify that point at which his views diverge from those of his neighbor A scientist for example who finds that his technical information is accepted but his recommendations on policy are not is entitled to know why not he then has an opportunity to re examine his own reason ing Every individual is entitled to information pertinent to a given decision If one finds it impossible to agree with conclu sions reached by others he is entitled-more than that obligated --to inquire whether pertinent information has been denied hım He is obligated to inquire and if necessary to demand that this information be placed at his disposal ·20 ATOMIC ENERGY FRIEND OR FOE? believe that the situation calls for an understanding of the problems involved Under pressure of public opinion, a variety of official decisions regarding possible courses of action will be taken If these are to be intelligent decisions, if they are to stand in the face of opposition arising out of selfishness or ignorince, it is imperative that the public comprehend the issues at stake The purpose of this book is to spell out, to the best of our ability, the genetic aspects of the problem of atomic-and otherradiations We do not believe, however, that all the answers to the problems created by atomic energy are to be found in geneuc or any other scientific information Some of these problems involve ethical rather than factual considerations Con sider, for example, a statement not unlike some that have repeatedly appeared in the press rallout causes leukemia bone cancer, and endangers future generations, therefore bomb tests must be stopped Although the premise is certainly not irrele vant to the conclusion, it is not in itself sufficient to make the conclusion necessary It is possible that the testing of bombs should be stopped immediately even if fallout were harmless on the other hand it may be that these tests must be continued regardless of their dangers, even though these be a hundred or a thousandfold greater than we think they are There are people who hold that scientists should not leave their laboratories and attempt to write a book of this kind But scientists and nonscientists must share equally, man for man, the responsibility for the ultimate decisions regarding the use of atomic energy It is wrong to restrict either responsibility or privilege in these matters We reject the view that only those with access to secret information are qualified to hold opinions about these issues, secret information is used here both with reference to classified information and with reference to infor mation that is not generally known If decisions based on rational think sng are preferable to those based on intuition, then anyone who has relevant infor mation or an opinion based on such information ought to share thus knowledge with others Sharing information does not guar antee unanimity of final judgment, it does, however, offer each ·2 Heredity, Environment, Genes, and Chromosomes Man and Biology An understanding of the genetic problems raised by irradı ation requires some knowledge of biology, particularly of genetics, which is the science of heredity, and also of physics, particularly of radiation physics In this and in the following two chapters we shall attempt to give a simple account of the fundamental and relevant facts of genetics and of radiation physics which will be needed as a background for the discussion to follow, Quite often this discussion will focus on experiments carried out with organisms other than man-Drosophila (the vinegar' or 'fruit" fly), mice, and even bacteria This may seem strange to some people, who will contend that problems per taining to human beings must be solved by studies on human beings Others may say that organisms such as monkeys or, at the very least, mice can throw some light on man, but that vine gar flies and plants and bacteria are too much unlike man to tell us anything worth while. 13 ·14 . HEREDITY ENVIRONMENT GENES AND CHROMOSOMES We do not for a moment believe that man is nothing but an animal In man biological evolution has transcended itself Man is not an overgrown fruit fly, or an overgrown mouse, or merely an especially clever monkey Man alone is the creator and possessor of culture, he alone can thinh in abstract symbols and use symbolic language Organisms other than man become adapted to their living conditions, to their environment, chiefly by changing their biological nature through the relatively slow process of evolution Man alone can adapt himself by changing his environment as well All organisms, including man transmit their biological heredity to their offspring by means of materials contained in their sex cells Man alone can transmit his thoughts his inventions, and his skills by means of instruction and learning, to other human beings who are neither his children nor even his contemporaries Clearly, we must avoid both types of error-that of treating man as though he were nothing but an animal and that of forgetting that human nature does have a biological basis It is this biological basis which may be en dangered by radiation exposure Biological sciences progress by building intellectual out posts far beyond the frontiers of general knowledge These bold advances are made as the result of new techniques and new ideas, they are based generally on the study of the most diverse organisms The outposts then establish connections and out of the union of distinct types of studies further spectacular ad vances are made Meanwhile many gaps are left in our knowl edge These are slowly filled by studies of more and more types of plants or animals Long before the gaps are filled, however, missing information can be predicted with remarkable accuracy from established points of cross reference Who for instance, has ever seen a hummingbird Lilied by lightning? But who doubts that lightning can kill a hummingbird? In the case of genetic studies, man has served primarily, though not exclu shely, as material with which to confirm and to which to apply the discoveries made with other organisms And so, despite the fact that we are concerned with the effects of radiation on man, our factual knowledge of the types of damage and the relation between damage and exposure will come largely from common ·15 LYDIVIDUAL VARIATION experimental organisms such as bacteria yeasts molds fruit flies and mice Individual Vanation It is a matter of general knowledge that no two persons are ever exactly alike Even so-called identical twins are never really identical although they may be more strikingly similar than persons who are not twins Identical twins result from the division of a single fertilized egg cell into two or more separate fetuses which eventually become separate infants and separate adults Such cwins inherit precisely the same genes from their parents They are identical in their heredity Other twins (called fraternal) result from different maternal eggs fertilized by dif ferent paternal spermatozoa They are really brothers or sisters born simultaneously they differ in their heredities and they are often quite dissimilar in appearance although no more so than brothers and sisters who are not twins What is the source of the differences between individuals? This is a very general biological problem for just as there are no two absolutely identical persons so there are probably no two identical individuals of any biological species Part of this variation we recognize as environmental and part we know is inherited It is common knowledge that the weight of an indı vidual is related to the amount and type of food he eats Under nourished persons lose weight and become emaciated It is how ever not rare to find that of a number of persons with essentially the same diet some are stout while others are thın (see Figure 2) In the case of skin color we know that some persons remain dark even when not exposed to sunlight On the other hand we know of other persons who do not tan upon exposure but burn or freckle instead Sometimes we are acquainted with the parents or grandparents of these same persons and we are familiar with the fact that similarities may run in families for Inany generations

·28
HEHEDITY ENVIRONMENT GENES AND CHRONOSOMES
HEREDITY
ENVIRONMENT
STARVATION
AVERAGE
COPIOUS
THIN
GENOTYPE
FAT
GENOTYPE
FICURE 2
Interact on of heredity (genotype) and envsronment
Persons who inhent from their parents genotypes wh ch prechos
pose them to being skinny or thin lose or gain weight depending
upon their diets So do persons who inhent a predisposition zo be
corpulent fat or obese However if the diets are very different
Some people with thin genotypes may actually weigh more than
persons with fat genotypes
Some famıly traits are passed on from one generation to the
next in a much clearer fashion than others Eye color hair color
blood types pattern baldness and certain abnormalities of the
fingers and toes are affected only slightly by the conditions
under which a persoss is raised by examining family albums
one can discern the occurrence of some of these traits through
a number of
generations Other traits such as height weight
intelligence and personality characteristics are profoundly in
fluenced by such factors as diet education and the entiron
mental tenor of the home It is frequently difficult to decide

·17 NATURE AND NURTURE whether heredity has played any role at all in the determination of these traits Nature and Nurture Labile traits-those which are easily influenced by the en vironment-often comprise the most important or the most interesting aspects of an individuals personality in his relation to other persons This fact brought about the nature versus nurture controversy, which was particularly lively several decades ago The debate centered on the relative importance of environment (nurture) and heredity (nature) in the develop ment of a human person As a matter of fact, the controversy still reappears every now and then, although to a large extent the debates have been rendered meaningless by modern genetics Every person is born with a certain genetic endowment inherited by him in the egg and the sperm from which he arose This endowment enables the individual to survive under a certain range of environmental conditions What, then determines the path which the development of a person will take? The genetic endowment, the individual's genotype, interacts with the particular environments—intrauterine, individual, family, group, class, regional, and national-in which the person lives his particular life These interactions determine the character istics by which we know the person, his phenotype In a limited sense we can think of the genotype as estab lishing certain potential capacities, which are then realized with greater or lesser success under various sets of environmental conditions Thus, certain congenital idiots by virtue of their genotypes, are incapable of learning even the simplest things, no amount of teaching will result in normal mental develop ment On the other hand, persons capable of leaming a great deal inay not have an opportunity to develop their potentialities Since the potentialities of any one individual form a complex pattern, it is doubtful whether any environment suffices to ·18. HEREDITY, ENVIRONMENT GENES AND CHROMOSOMES develop every facet of an individual to its utmost In certain instances the development of different facets is undoubtedly mutually exclusive At some stage in life a person of many potential talents must choose the few on which he will concen trate his efforts the others will suffer by his choice Some hereditary traits, however, seem to be unaffected by the environment The best examples of such traits may be found among the properties of the blood, or, more precisely, of the red blood cells These properties have much to do with the safety, or danger, of blood transfusions If two persons belong to the same blood type, the transfusion is safe if to differ ent blood types, the transfusion may or may not be dangerous Blood types are inherited according to simple Mendelian rules of which we will say more below The aspect of the matter which interests us here is that the blood type of a person be comes recognizable even before birth, persists throughout his entire life, regardless of his state of nutrition and health, and can be diagnosed even in his bones many years after death Such traits which are apparently independent of the environment are in reality traits which develop in all, or at least in all known, environments in which life is possible In any case, it is impossible to draw a sharp distinction between hereditary traits and environmentally conditioned traits The nature nurture controversy is thus reduced to the relatie importance of heredity and environment in the desel opment of this or that trant Relative importance, in turn, be comes a function of the distribution of environmental and hereditary factors in space and time Malaria may be absent in some areas because the malarial parasite or its carrier mosquito is absent, in other areas where malaria is prevalent, resistance or nonresistance may be conditioned by heredity, as it is in some areas of Africa In the northeastern United States tan skins are a function of the season, in the Southwest, tanging may be the normal state of affairs individuals who are unable to develop slın pigmentation in that region are unable to do so because of their particular genotype For personality character istics, one need not pass from one geographical region to 211 other in order to illustrate the alterations in the relative im ·19 KREDITY IS NOT A STATUS BUT A PROCESS portance of environment and heredity one need only go from home to home An understanding of the interactions between the environ ment and a person s genotype does much to abolish the resig nation which used to accompany the discovery that a given condition or disease was hereditary Frivolous examples of how ladies are able to defy their heredity can be derived from the role of beauty parlors in producing desirable women Such examples are not entirely irelevant for in a sense medical advances in the treatment of hereditary diseases are based on the same principle modification of the environment so that a given genotype will produce a normal healthy individual in stead of the expected infirm or crippled one Thus although no amount of training can at present improve the mental status of certain congenital idiots it is not beyond the realm of possi bility that man may some day discover how to correct the physı ological defect which is responsible for this hereditary trait Heredity Is Not a Status but a Process We have spoken above of the inheritance of traits-eye or skin color blood types and hereditary diseases—as though each trait were inherited separately and independently of all others This manner of speaking is convenient because it is tooted in everyday language one hears of a child having in herited his eyes from his mother the shape of his face from his father and his freckles from one of his grandparents But it can be seriously misleading if taken literally Heredity is not a gradual addition of unit traits or characters the sum of ilich makes up the person or the individual We do not really inherit eyes noses freckles or diseases what we do inherit are genes contained in the sex cells whose union starts the develop ment of a new person This matter is understood most easily if approached not from the end product the adult organism but from the or more ·20 HEREDITY ENVIRONMENT, GENES ANO CHROMOSOMES ganism's beginning, the newly feruilized egg The fertilized egg in which every human starts his existence, is a uny speck of jellylıke material barely visible to the naked eye But this very special jelly grows and develops into an embryo, a fetus, an infant, a youngster, an adolescent, an adult, an oldster, and, finally, a cadaver It grows and exists because it consumes food which it takes in from its environment-at first from the body of its mother, and then in the form of regular meals It trans forms its food into the constituents of a living body, into itself There is, however, nothing haphazard about this transforma tion, the development of a body proceeds along a definite path, from an upswing of vitality in childhood and youth to decline and final dissolution in old age What heredity really does is to define the path taken by the development of a particular individual in the particular sequence of environments encountered by this individual dur ing his lifetime The path is unique for each individual human being, but at the same time, it is related to that of his parents and ancestors It is unique because, as we shall see below every person inherits a constellation of genes wicht is different from that of every other person and because no two persons have exactly the same sequence of life experiences It is related be cause the genes which comprise the hereditary constellation of each individual are derived from ancestors close and remote back to the beginning of life on tlus earth When we speak of a person and try to describe this person to others, we can do so most casily by specifying separately the traits and characters which we observe This isolation of traits and characters is then, a necessity, or at least a convenience, imposed by the structure of the language we speak or write Let us not, however, be misled by this language into thinking that heredity is equivalent to fate Since the developmental path of every person is determined by his heredity and by his en vironment, it can in principle be modified by either If a given genotype yields an undesirable trait, such as a hereditary disease, in one environment, it may yield a different, healthy trait in a different environment The problem is wliat environment, or rather what sequence of environments, the genotype of each ·CHROMOSOMES AND NUCLEIC ACIDS • 21 particular person needs to produce the best result, the best phenotype Public health and educational authorities endeavor to devise environments in which most human genotypes-loosely referred to as normal -yield reasonably satisfactory persons, workers, and citizens Those genotypes which do not do so well in these environments result in people who are abnormal, deviant, or diseased Some of these unfortunates could, however, do better in environments better suited to them Fındıng out how to create such special edivironments is the task of medical science, which has already made great strides in this direction Medicine is not yet able to engineer good environments for certain human heredities, so these continue to produce incurable diseases Some of these conditions will certainly be alleviated in time Chromosomes and Nucleic Acids The formation of a new individual begins when a sperm unites with an egg Sperm and eggs are the sex cells, or gametes produced by the individual s father and mother respectively The genetic endowment-the genotype-of the person is con tained in the two uniting sex cells Fertilization, the union of an egg and a sperm brings about two important events First, there is a union of genetic material The egg and the sperm each contain, in man 23 chromosomessmall bodies which, in microscopic preparations stain with cer tain dyes A fertilized egg cell comes, then, to contain 23 pairs of chromosomes 46 in all, one member of each pair derived from the mother and the other from the father Second, a fertilized egg cell divides, and redivides again and from an initial glob of jellylıke material smaller in size than a period (), a human being is formed At every division of the egg cell and of the cells that are derived from it, every chromo some is duplicated, one of the resulting new chromosomes passes to each daughter cell Therefore, most cells of the body contain again, until, ·21 HEREDITY ENVIRONMENT GENES AND CHROMOSOMES 46 chromosomes Furthermore every cell contrins a chromosome descended from each of the 23 maternal and 23 paternal chromosomes that were combmed at fertilization We are chil dren of our parents in every one of the many billions of cells that compose our body The tu o parents mother and father contribute equally to the heredity of the child Yet the male and female sex cells are about as unlike as cells can be An egg cell has a mass thousands of times that of a sperm while the sperm has a tail by means of which it can swim about the egg is immobile ils cytoplasın being laden with strient materials for the new organism The chromosomes are the only constituents which are alike in the female and male sex cells It was this similarity of the egs and sperm chromosomes which led some nineteenth-century biolo gists to inſer that chromosomes are the physical carriers of heredity This inference has since been amply confirmed in many ways In recent years some insight has been gained into the chemical composition and structure of the chromosomes for the first time consequently we have an inhling of how they function as carriers of heredity Apparently the most important constituent of the chromosome is an extraordinary chemical substance known as DNA (desoxyribonucleic acid) The molecules of this substance are long stringlile fibers Because they are bundled into chromosomes these fibers can be apportioned in an orderly way at each cell division, in the absence of such packaging the cell nucleus would resemble a thread factory operated without spools DNA molecules are frequently compared to the tapes which control modern business machines because it is almost certain that the playsıcal structure of DNA fibers does indeed carry the code for all of the vial functions performed within living growing and dividing cells The incredible efficiency of the DNA coded tape can be sllustrated by saying that all of the chromosomes present initially in the ferulized eggs from wluch the present population of the world (some tho and a half billion people) developed would occupy a volume about equal to that of an ordinary aspirin tablet In addition DNA has a further property not shared by ·CHROMOSOMES AND NUCLEIC ACIDS . 23 any man made tape Its structure is such that by attracting to itself the relatively simple chemical substances of which it is composed and which are present within cells, it can make a complementary copy of itself Because of its chemical structure and the spatial relations between the constituent molecular parts it is capable of self duplication, correct in every detail Of what use is a complementary copy, a mitor image of the molecule? The answer to this question reveals another remarkable property of the DNA molecule As shown recently by ) D Watson and F H C Crick, the structure of DNA con sists of two chains which are complementary copies of one another Separate these chains let each build a mirror image, and you have two molecules of DNA where you originally had one And the two new molecules are precisely the same in structurel Self duplication is, of course, the basis of life We see it on a grand scale by watching the multiplication of living things But self reproduction must happen within organisms, on a molecular level, as well We said that the newly fertilized egg divides over and over again At each division both daughter cells get a full complement of 46 chromosomes, but the repeated division of these chromosomes does not mean that the chromosomes get smaller and smaller The process of division,' or mitosis, is actually one of duplication or replication With the possible exception of the first few divisions of the egg, development involves growth-the manufacture of proteins and enzymes, and the construction of new molecules of DNA Each of the 46 human chromosomes attracts to itself the material necessary to make an exact duplicate of itself, a duplı cate whose precise similarity to the original is guaranteed by the chemical structure of the DNA molecule Only then does the cell divide each of the 46 chromosomes going to one of the new cells while its newly constructed duplicate goes to the other Hence, cach cell in the body, with the exception of a few expendable kinds such as red blood cells, has its full quota of chromosomal material copied very precisely from the original models carried by the uniting egg and sperm ·26. HEREDITY. XVIRONMENT, GENIS ARD CHRONOSOVES Meiosis We might ask what happens when an adult individualman or womanmproduces sex cells of his own We have seen that, unlike the cells of the body, each sex cell contains 23 chromosomes rather than 23 chromosome parts Obviously, the number of chromosomes must somehow be reduced by half 8 W 2 21 5 18 149 17 6 FICURE. 3 Sex cells produced by a person may carry different combinations of his chromosomes A spermatozoon with 23 chro mosomes shown in black and an egg cell with 2$ chromosomes shown in white are represented schema:cally on the left. Union of these sex cells yrelds a zygote a person whose body cells con tain 16 chromosomes (shown in the middle) The sex cells of this person will again carry 23 chromosomes but, as shown on the nght different sex cells will have different numbers of black (B) and of white (1) chromosomes. This reduction occurs in two special divisions of the cells con tained within the reproductive organs, or gonads-lestes in the case of males, ovanes in the case of females These special disasions are called mesohe divisions or simply, meiosis During these tnp divisions the chromosomes build replicas of themselves only once The no members of each pair, each member with its nessly manufactured copy, approach one another sery closely to ·GENES 23 form 23 four parted compounds The cell then divides twice The first division separates the two "original' members, eachaccompanied by its cops-going to a separate cell This division is followed immediately by a second, which separates each chromosome from its copy Thus, the reduction from 23 pairs of double chromosomes to 23 single chromosomes, one of each of the original pairs, is accomplished Although this process involves some additional steps, it 15 sufficient for our present purpose to regard these cells, with their reduced number of chromosomes, as the final sex cells Thus we have seen how two sex cells, each carrying 23 chromosomes unite to give an individual with 23 pairs of chromosomes, and how the individual, with his 23 chromosome pairs, produces sex cells with 23 chromosomes each (Figure 3) Genes So far we have discussed the chromosomes as units, and in their maneuvers during cell division they behave as such In a physiological sense, however, a chromosome exerting its control over life processes is a body composed of a great many smaller units These are the genes There may be hundreds or thousands of genes in every chromosome Genes are localized regions of the chromosome-probably of the DNA molecule-which control specific physiological or biochemical processes Just how sharply neighboring genes in a chromosome are demarcated from one another, or how distinct they are in a physical sense, is somewhat uncertain In classical genetics, the genes in a chromosome were often compared to a series of beads on a string the string being some sort of skeleton of the chromosome Certainly, this bead on string model otersimplifies the actual situation Perhaps a better way of visualizing the relation between the two is to say that a chromosome is composed of genes just as a motion picture film is composed of different scenes Although the transition from a love scene to a chase to a gun fight may be gradual, still there are these different scenes A given region of a chromosome ·29 • MEREDITY, ENVIRONMENT GENES AND CHROMOSOMES is concemed primarily with a given physiological and biochemical process, and in this serise, at least, different regions can be referred to as units, as genes All the genes contained by the chromosomes act in concert to bring about the development of the organism A given gene seems to function by manufacturing a biochemical substance, such as an enzyme which facilitates certain chemical reactions Brochemical geneticists have found a possible, chiefly in simpler organisms such as bacteria, yeasts, and molds, to associate many genes with the enzymes and the chemical reactions they control If a certain gene functions properly, the plıysiological process which it controls proceeds normally If, however the gene 15 destroyed or altered in some way, the bacterial cell which lacks the gene, or carries only its altered form, may be unable to catry out a certain physiological function and may die In some instances bacteria with such a hereditary disease can be "cured' by adding to the nutrient medium on which they grow the chemical substance which the defective gene fails to supply but which normal bacteria are able to manufacture in their own bodies It is by means of this technique that we learn of the nature of chemical reactions vital to life itself Some hereditary diseases in man have genetic and brom chemical bases almost as simple as those of the bacteria men tioned above Thus one of the defective genes in man, when inherited both from the mother and from the father, produces a disease known as thalassemia or Mediterranean anemia In dividuals carrying this defective gene in double dose show symptoms of a severe anemia and usually die before the age of adolescence L Pauling and H A Icano discovered that the red blood cells (erythrocytes) of the anemics carry, instead of the normal red pigment, hemoglobin, a chemically altered form of the hemoglobin molecule Persons who are deterozygous for thalassemia--that is, who have inherited the defective gene from only one of their parents, enjoy normal health or are only mildly anemic The red blood cells of such persons contain both normal and abnormal hemoglobin It is as though the normal and the abnormal variants (alleles) of this gene facilitated chem ·MENDEL'S LAW OF GENE SEGREGATION • 27 ical reactions yielding the normal and the abnormal hemoglobin respectively The chemical substances produced by gente controlled re actions are, however, by no means always known Ordinarily, genes are recognized because they occur in two or more forms, which produce alterations of externally yışıble bodily traits In fruit flies, for instance, the normal functioning of many genes leads to the development of the usual dark red eye color But some of these genes from time to time suffer alterations (referred to in genetics as mutations) which produce variant (allelıc) forms of the genes Several of these altered genes result in eyes of a Very bright red color, instead of the normal dark red Similarly, the malfunctioning of a number of other genes makes the eye color brownish red When a fly carries both the altered genes yielding the bright red and the brownish red eye colors, the combined action of these genes results in an eye color of pale pink or white White eyes are also produced by the mutation of still other genes, which evidently control the production of both the bright red and the brownish red pigments A great many traits are known to be controlled by genes Even those controlled by the genes located in one chromosome can be of the most varied kinds In Drosophila, for instance, some well known genes (well known to geneticists that is, and of especial importance to their work) located in one particular chromosome control body color, bristle formation eye color, size of the fly, eye shape, texture of the eye surface, leg shape, and wing venation In man a list of traits controlled by genes located in the chromosome that also determines the sex of an individual includes color blindness, several skin abnormalities, and an abnormality of the retina of the eye Mendel's Law of Gene Segregation We are already familiar with the fact that human sex cells, eggs and sperm, contain 23 different chromosomes Fertilization, ·28 STEREDITY ENVIRONMENT GENES, AND CHROMOSOMES the union of the two sex cells, produces an individual with 46 chromosomes-23 pairs The two corresponding (or homologous) chromosomes of each pair possess the same spots at which genes are located, these spots are known as gene loci Hence, cach indi vidual carries thousands of pairs of genes done up very neatly into pairs of chromosomes It does not follow, however, that each individual carries tivo identical genes at each gene locus The two genes may be the same, or they may be different A person with blue eyes, for example, almost certainly carries two similar genes at the respon sible locus these genes are those whose action leads to the production of the blue eye color A person with brown eyes, on the other hand, may carry two genes which are alike, each lerd ing to the production of brown pigment, or he may carry one gene for brown eyes and one for blue Since an individual carry ing one gene of each type is brown eyed, we refer to the gene for brown eyes as the dominant gene dominance is a frequent but not a necessary property of one of the forms (alleles) of genes at a given locus The gene for the blue eye color is a recessive gene In the shorthand of genetics we say that there are two types of homozygous individuals in the case of genes for brown and blue eyes, these are the individuals who carry two representi tives of the same gene-BB or 66, where B is the gene for brown eyes and b is the gene for blue There are also heter ozygous-B&-individuals wio carry one gene of cach sort As far as the genes for eye color are concerned, homozygous indi viduals produce only one type of sex cell since both members of the chromosome pair carry the same gene Heterozygous indt viduals, on the other hand produce two types of sex cells in equal numbers since at merosis half of the sex cells will get a chromosome with gene B while the other half will get a chromo some with gene b At fertilizauon sperm and eggs unite at random, that 23, there is no tendency for sperm of one kind to scek out preferen tally eggs of that or any other kind Consequently, in the progeny of tho heterozygous brown eyed individuals (B6), the chances of a child being brown cycd are three times as great as ·MENDEL'S LAW OF GENE SEGREGATION . 29 the chances of its being blue eyed (Figure 4) If a heterozygous individual, Bb, marries a blue-eyed partner, bb, and they have a number of children, half of these will be heterozygous (Bb) and half homozygous (bb) This is a statistical expectation in small families, there may be only brown eyed (Bb) children, or only blueeyed (bb) children, or some of both types but not necessanly in equal numbers oo 36 ତି ® 1 5 JO FIGURE 4 Mendelian inheritance in three genera tions of the blue (light) and the brown (dark) eye colors The blue is a recessive and brown is a domi nant character Some brown-eyed persons are heterozy gous as shown by the fact that some of their children may have blue eyes The principle that we have been considering is that discovered by Mendel in 1865 but not appreciated by the scientific community until rediscovered independently by three different investigators in 1900 Mendel discovered the principles, or laws, of heredity (now known as Mendel s laws) not by observing inheritance in man but by crossing varieties of the garden pea. However, the basis of heredity is, most remarkably, the same throughout the ---- The la... Nfandal fondun nal..

·50% 50% A 25 % 25% 50% AA 25 % 25 % 50% e Aa aa for eggs Tor #993) A A 100% (or #peron SO % AA 50% Aa for sperm) IAa) 100% Alal (Aa) 100% FIGURE 5 This is the first of a series of "checkerboard diagramu such diagrams are useful for illustrating the frequencies with which certain genetic endowments will occur The upper part of the diagram allustrates the frequencies with which ofspring of various genotypes are formed following the mating of two individuals heterozygous for the genes A and . Each parent produces sex cells of which hall contain the gene A and the ather half. Thes fact is represented by the division of the top and left sides of the square into equal segments The ekss and sperta unite at random, this as allustrated by the subdivision of the large square into four smaller ones of equal size The offspring of this mating, then use \AA Yo Yaa These proportions can be deraonstrated in the progeny of a single pair of fires by the kernels on a single car of corn, or by the matings of other organisms, in the case of human beings and other less producene organisms the same frequencies can be demonstrated by the summation of the progenies of many couples The two smaller diagrams in the lower part of this figure illustrate the relatise proportions of olispring following the mating of (I) a homo zygore with a heterozygote and (2) two dissimilar homozygotel ·31 MENDEL S LAW OF GENE RECOMBINATION to peas apply also to fruit flies to man and even in modified form to bacteria and other microorganisms To recapitulate Mendelian inheritance in man and in other higher organisms is predicated on the following phe nomena (1) The possession of pairs of genes by every individual (2) The occurrence of meiosis during the formation of the sex cells At meiosis the chromosomes (and the genes) segregate so that each sex cell carries one member of each pair (3) For any one gene an individual may have either two identical alleles (in which case we say he is homozygous for that gene) or the two gene members may differ somewhat (the individual is heterozygous) (4) As far as their gene content for a given locus is concerned all sex cells produced by an induvidual may be identical or half may be of one sort while the other half are of a different sort (5) Sperm and eggs unite at random during fertilization (see Figure 5) Virtually all of the higher forms of life are equipped with pairs of chromosomes rather than sets of single chromosomes as some lower organisms are We saw above that a person canty ing the genes B and b is brown eyed the gene for brown eyes (B) is said to be dominant to the recessive one for blue eyes (6) Many recessive genes are harmful to their carriers when homo. zygous persons carrying both the recessive gene and its normal dominant counterpart are spared this harm There are many known recessive genes whose action in the homozygous condi tion leads to death or to the most pathetic deformities Fortu nately most of these are so rare that two persons carrying genes for the same abnormality are by the operation of chance alone unlikely to marry Occasionally however they do and it is for the guidance of such couples that heredity clinics are being established at many of the larger hospitals and medical schools Mendels Law of Gene Recombination The fact that genes segregate at meiosis has a number of consequences the most important of which se the one of ·32 HEREDITY TVIRONMENT GANES AND CHROMOSOMES genetic endowments capable of being transmitted by an inde vidual heterozygous for several genes So great is this variety that perhaps no two persons (except identical twins) have ever had the saine genotype We have already seen that during the formation of sex cells in man the 23 pairs of chromosomes characteristic of every body cell are reduced to 23 chromosomes One member of each of the orginal 23 pairs was originally contbured by the individuals mother while the other member ssas contributed by his father So among the gametes being formed, there is one type contain ing entirely paternal chromosomes There are 23 different types of gametes in which all of the 23 chromosomes but one are of paternal origin Similarly there are 253 different combinations of 21 patemal and 2 maternal chromosomes Without lisung all the combinations of paternal and maternal chromosomes which are possible we can simply say that over 8 million different com binations of paternal and maternal chromosomes are possible sk these chromosomes are treated as indivisible units Actually, even this is an oversimplification There are exchanges of genes of paternal and maternal origin within individual chromosomes during merosis If these are talen into account in computing the number of possible gene combinations any individual can produce one gets a tremendous figurema figure so large that no tso of the billions or trillions of sperm produced by any man during his lifetime are likely to contain exactly the same combination of genes It is almost certain that no two individuals (barring as stated above identical twins) have ever in the course of mankinds existence had the same genotype It is from this wealth of material and through diffes ences in the number of offspring produced by persons with diferenz gene combinations that natural selection molds the genetic composiuon of human populations This process lies 25 the bottom of evolution evolution w an inescapable con comitant of life itself ·3 Spontaneous Mutation Heredity and Mutation Heredity is a conservative force It tends to make offspring resemble their parents If heredity were perfect, all living indi viduals would be exactly alikemand exactly like the first form in which life existed The fact that they are not alike indicates that the force of heredity is sometimes partially overcome by another force wluch induces change in the hereditary materials win the genes This lateer force is mutation in the preceding chapter we saw that genes reproduce themselves, that is, they cause the synthesis of copies of themselves from nongenic ma terials present in the cell, and ultimately from the food the organism consumes But occasionally the process of copying goes wrong and the new gene formed is not quite like the original Such imperfect copying is known as gene mutation, and it is the source of varsation, of individuality, and of evolution It is because of mutation that some people have blue and others brown eyes, some have zype blood and others A type or Brype, some enjoy good health and others suffer from Malassemia Of course, some of these mutations may have occurred many genera tions ago, some may have occurred only once, others may have occurred repeatedly ·34. SPONTATEOUS MUTATION Genetics is a science whose very existence depends upon observable differences between individuals If the individual members of each species were identical, if every man were pre cisely the same as every other man, we would still have the sciences of physiology of developmental morphology, of boo chemistry, and of taxonomy (Perhaps scientists in these fields would even be extremely happy, individual differences and idiosyncrasies are nothing but a source of trouble as far as they are concerned) We would still have doctors of medicine dentists, pharmacologists and optometrists but we would have neither genetics nor geneticists In order to trace patterns of notre we must have recognizable differences to trace trent in genetics a gene is a $ nund Txed pens in a double sens, order, simple cherical structures called iteru that there exist only four kinds of nucleoudes commonly found in chromosomes How can four kinds of nucleondes make up the countless different genies which exist in the living world? The answer seems to be that the sequence in which these four linds are arranged within the gene serves as a code much as do the holes in the tape or cards controlling modern business ma chines The code embodied in the arrangement of nucleotides within a gene determines just what the gene will do There are many different ways in which a thousand objects of four dif ferent kinds can be arranged to supply each gene with its unique cocle and, hence, with its unique function Gene mutation is, then, most easily imagined as resulting from a mistake in the coded sequence of nucleondes within the gene An error, perhaps that occurs during chromosome duple cation consisting of the loss of a fcu nucleotides or of a change in their position relative to one another Or an error caused by thic collision of a chemically lughly active molecule, such as a peroxide or a free radical with the chromosome At any rato, whatever the cause a gene that has been performing one funt tion occasionally ceases to function, functions at a different rate or functions in an entirely different manner The mutated gene, during the course of subsequent gene and chromosome duplications reproduces itself in its altered ·MUTATION IN MAN + 35 form There is no known process by which DNA can correct a mistake of this sort, once it has occurred, except by chance back mutation, an event as rare as or rarer than the original error Blue eyes or brown, black hair or blonde or red, blood types A, B, or O-all of these well known differences between persons have their origin in these errors in gene reproduction and are expressed as obvious differences because of the effects these changes have on the physiological functioning of genes Mutation in Man Mutational changes in any one gene are (with infrequent exceptions which we need not consider here) rare events This is a different way of saying that, ordinarily, the genes reproduce themselves accurately Mutations were first discovered by the Dutch botanist Hugo de Vries soon after 1900, in a plant called the evening primrose (Oenothera) Beginning about 1910 T H Morgan and his associates observed and described many mutants in several species of Drosophila Since about 1945 the phe nomenon of mutation has been studied extensively in bacteria and in other microorganisms, in which, with the aid of proper methods many billions of individuals may be rapidly screened for the occurrence of mutations Although man is not a favorable subject for studies on mutation, it is certain that mutations do occur in man and ingenious methods have been devised for their investigation One of these is referred to as the "direct method Consider, for example a fairly common type of dwarfism in man in which the head and the trunk are of normal size but the limbs are abnor mally short If persons afflicted with this condition-chondrodystrophia-survive early childhood, they enjoy reasonably normal health as adults but are obviously handicapped in many kinds of work This type of dwarfism is inherited as a dominant trait That is children who are chondrodystrophic dwarfs are usually born in families in which at least one of the parents is also such a dwarf We may say that the dwarfism is due to a dominant gene D, while the normal body build is caused by the ·39. SPONTANEOUS MIUTATION corresponding recessive, d. The dwarfs are heterozygotes, Dd, and normal people homozygotes, dd. It does happen occasionally, however, that both parents of a chondrodystrophic duyarf are of normal stature. The normal recessive gene d evidently mutates to the dominant gene for dwarfism, D. To determine the rate of this mutation, many thousands of birth records must be surveyed. One scores the number of dwarf and nommal children and then checks the parents of every child in the survey, removing from his data those children whose parents (one or both) were dwarfs. The remaining children born of normal parents, would consequently be expected to be normal too. However, in one such study made by a Danish geneticist, it was found that about one child in 12,000 born to normal parents was aßicted with this type of dwarfism. Since the normal parents did not carry the dominant gene, these children must represent new mutants. In each instance, the mucation could have occurred in either the gene supplied by the mother or in that supplied by the father; since 12,000 children represent 24,000 genes (at this one gene locus, remember), the mutation rate is 1:24,000, or approximately 0.000,04. In other words, about four per hundred thousand sex cells produced by normal people carry a newly arisen mutant gene for chondrodystrophic dwarfism. The "direct method" of determining the frequency of mutation is applicable only 10 dominant traits, since the emergence of a recessive trait (such as the blue eye color) in the offspring of parents manifesting only the dominant trait (brown eyes) simply means that the pares carry the recessive gene in hetero zygous condition. The frequency of mutation toward recessíve genes which produce recessive hereditary diseases or mallorma. tions can someumes be estimated with the aid of the "indirect method." Let us take as an example hemophilia, or bleeders' disease. which is produced by a recessive gene located in the chromosome that is also concerned with the determination of sex. The blood of hemophiliacs does not clot, and so even small cuts and abrasions may result in fatal hemorrhages in these persons. In fack. only a fracuion of all infants who are born with hemophilia reacht ·MEASUREMENT OF MUTATION EATES IN DROSOPHILA . 37 maturity, the remainder die in childhood Nevertheless, new hemophiliacs are born in every generation, almost always to parents with no blood disease How is this possible? One might think that a gene which kills most of its carriers would soon eliminate itself from the population We are forced to make one of the following two assumptions First we may suppose that the supply of hemophilia genes in human populations is renewed in every generation by the occurrence of new mutations If, then, the number of hemophilia genes removed by early death is approximately balanced by the number of newly arisen mutants an equilibrium is established at which the chance of a child being born with hemophilia remains roughly constant generation after generation On the other hand, it can be imagined that the present frequency of hemophilia genes repre sents only a residue left over from higher frequencies existing in the past. This second assumption is demonstrably absurd It would imply, for example, that Hannibal s army and the Roman legions that opposed him were composed mostly of hemophiliacs History has certainly failed to record any such condition We are left, then, with the assumption that for hemophilia and similar hereditary diseases, there exist, in human popula tions approximately stable equilibria between the elimination of harmful genes by death or sterility and their origin by muta tion knowing the proportion of hemophiliacs who survive to adulthood and produce children of their own, one may calculate the frequency of new mutations that must occur to maintain the equilibrium Such calculations have been made by J BS Haldane and other geneticists for a number of disease producing genes in man The results are fairly consistent in giving muta tion rates ranging from 1 10 100,000 Mutation rates determined by the direct method' also fall within this range to Measurement of Mutation Rates in Drosophila Man is an intractable, expensive, and unreliable animal to work with when it comes to studies of mutation rates The ·48 • SPONTANEOUS MUTATION amount of time, labor, and money needed for census-taking examination of old records, and the tracing of families for personal interviews is often prohibiane Consider that to obtain reasonably reliable information about the rate of mutation of a gene that mutates only once in 300,000 sex cells, we must examine birth records for at least 250,000 chuldren. It is no wonder, then, that the best data concerning mutation rates are obtained from organisms suitable for experimental work in laboratories The availability of a prolific laboratory animal, large numbers of which can be raised » a short time, does not in itself guarantee that reliable data can be obtained Such data come more often from the use of efficient techniques, such as those developed in the mid twenties by H J Muller For per fecting these techniques and for vigorously pursuing the lines of investigation they opened-especially in the field of radiation genetics Muller was awarded a Nobel Prize in 1946 Not only because an understanding of these techniques will be essential later in this book but also because of their intrinsic interest, we shail consider them here in some detail We have seen that the mutation rates for individual human genes are generally very small To obtain information about these mutation rates, one is faced with the necessity of studying extremely large numbers of individuals If one wishes to study differences in the rate of mutation of different genes, or the possible effect of some physical or chemical agent on the muta tion rate of a particular gene, the numbers of individuals required for the study become prohibuively large-even for a small animal such as the fruit fly Indeed, in refined studies of this type, the investigator can no longer be satisfied with rough escimates of mutation rate it becomes necessary to arrass sufficient data to guarantee that the observed rate of mutation is correct within very narrow limits Muller's method of keeping the experimental errors 10 minimum and, at the same time, keeping the amount of nork within feasible limits was extremely clever (One must remember that thirty or forty years ago the amount of money available for genetics and other "useless" research was even more limited than ·MEASUREMENT OF NUTATION RATES IK DROSOPHILA - 99 it is today, only a few of the larger universities could afford such luxuries) Muller saw the necessity of studying mutation rates, not in single genes, but in groups of genes carried in the same chromosome A chromosome may contain hundreds or thousands of genes If there were some aspect of development that was affected in a similar way by many of these genes, and if all these genes could be held together and analyzed as a single unit, the over all mutation rate-the frequency with which at least one of these many genes mutated-would be relatively great And if the over all rate were considerable, proportionately fewer observa tions would suffice to obtain an accurate estimate of this fre quency Muller reasoned further that the most common gene changes and the ones most likely to occur in many different genes, were lethal changes Lethal mutations are those which alter development (in this case, of the fruit fly) so drastically that the mutant individuals die in human terms, lethal genes produce hereditary diseases or malformations grave enough to cause the death of affected individuals before the advent of adolescence and sexual maturity The use of lethal mutations for studies of mutation rates has a further advantage It eliminates what is sometimes spoken of as the personal equation of the investigator Suppose, for example, that we are looking for mutants which alter the color of the Drosophila fly s eyes Some of these mutants are so con spicuous that everyone will notice them for example, a com monly used mutant has white instead of the normal dark red eyes But there are also mutations which change the eye color 50 slightly that a careful examination is needed to detect them And the fact of the matter is that some observers can perceive slight changes in color more easily an others This makes com parisons of the data obtained by different observers unreliable, and even the same observer may overlook more mutants when he 15 cired than when his attention is at its heenest The experi ments with lethal mutations, on the other hand are contrived so that one has to examine and score not individual flies but cultures of flies for the presence or absence of an entire, clear-cut class visibly distinct from other classes in the same culture, a class of flies that should be represented by at least several dozen ·40 SPONTANEOUS MUTATION individuals Even a novice can make accurate observations of this sort For the first time, then, different investigators could perform studies of mutations and arrive at comparable results Scientific literature dealing with problems in this field became a meaningful body of knowledge, from which one could proceed into unexplored fields without repeating old experiments merely in order to evaluate one's own ability to perform the type of work The frequencies of mutations observed in experiments using Muller's techniques are, of course, much larger than those mentioned above for human material As stated above, the reason for this is that one determines the probability that at least one of a great many genes will mutate One particular chromosome in the common fruit fly, for example, mutates from a lethal free condruon to one in which it carries a lethal gene at rates varying from 1 1,000 to 3 1,000, depending on the strain of fles studied Another chromosome of the same fly, one that is about twice the size of the first, acquires a lethal mutation at a rate of about 5 1,000, that is, about 5 of every 1,000 sex cells produced by a fy contain a newly arisen lethal mutation in this chromosome It was by using this technique of measuring the frequency of lethal mutations in entire chromosomes of Drosophila that Muller was able to prove that mutations OCCUT more frequently in the progenes of flies irradsated with x tuy's than an dies not subjected to such creatments This topic be discussed in more detail in Chapter 5 For the time being, we are concerned with spontaneous" mutation, that 15 with mutation which occurs in Hies or other organisms not subjected to any Inown mutation inducing treatment 'Spontaneous, mutation must surely have a cause, and calling it 'spontaneous" means simply that we do not know what that cause is 3231 Mutation Rates per Gene How can the mutation rates determined per chromosome 13 Drosophila be compared with the rates per gene estimated, as ·MUTATION RATES PER GENE. 41 discussed above, for some genes in man? This is a difficult and highly technical problem, but we must acquire at least a very general idea of its solution What is needed here is an estimate of the numbers of genes carried per chromosome in a favorable experimental animal such as Drosophila We have stated that there are hundreds or even thousands of genes in each chromosome In genetically well studied organ isms, such as Drosophila, dozens or even hundreds of genes have been identified by the changes they produce in the char acteristics of the organism when they undergo mutation These are, however, only a fraction of the total number of genes pos sessed by the organism This total number can be estimated by studying the genes whose alteration leads to similar end results We have seen that white eyes can arise in a number of genetically different ways in Drosophila There are even more mutations that result in the death of individuals carrying only the mutant gene and not its normal allele If we collect a num ber of these lethal" mutations, each of which is known to have had an origin separate from that of all others (that is, each has been obtained through mutation from a different, lethal free individual), we can ask how often the same (allelic) gene muta tion has been obtained two or more times by chance Obviously, the greater the number of different genes that give rise to lethal mutations, the smaller the chance that any two will be the same In fact, the frequency with which two such mutations prove to be identical is equal to 1 divided by the number of places where the mutation can occur This type of test indicates that there are at least 500 different gene loci in one chromo some in Drosophila that can yield lethal mutations For a variety of technical reasons, this is known to be a minimal estimate, more probable estimates lead to the conclusion that there are some 1,000 genes in that chromosome or about 5,000 genes in the entire chromosome complement of this fly As stated above, the rate of origin of new lethal mutations in a certain chromosome of the vinegar fly, Drosophila mela nogaster, is about 5 per 1,000 chromosomes per generation, or 0005 Since this chromosome contains at least 500 genes capa ble of producing lethal mutants, the rate of mutation per gene ·41 . SPONTANEOUS MUTATION is 0005 divided by 500 or 0000 01 (I per 100 000 genes per generation) It is remarkable that this estimate agrees with the value obtained for human genes (see page 37) If as is probable the Drosophila chromosome actually has more than 500 genes Drosophila genes would have to be considered somewliat less mutable than human genes These figures may also be compared with those obtained from various microorganisms such as the common colon bacillus the pink bread mold and bacteriophage (a virus that ss parasitic on bacteria) Using organisms such as these one can study enot mous numbers of individuals on a culture plate four inches in TAYLEI Examples of mutation rates per gene per generation in venous organusms (10 or 10-8 means that approximately one individ wal per thousand or one individual per million acqiures a newly arisen mutation in a given gene in every generation for further explanation see text) Alutation rates Organism Trom To Bacterial virus (bacteriophage) 10- Colon bacillus Indian corn 10 Drosophula (fruit fly) Man IO 10- 10-1 10- diameter more indsvidual bacteria can be grown than diere are human beings on the face of the earth By using special techniques involving agents such as antibiotics that kill normal bacteria 11 is posssble to determine the frequency with which mutant resistant individuals arise The range of mutanon sates determined in this way extends from 0 00003 or higher values down to values of 0 000 000 000 1 (from 1 per 100,000 to 1 per 10 billion cell generations) Some mutation rates found in a variety GE słown in Table 1 The table suggests that there is a general tendency for mutation rates to increase as one moves from ysruses and bacteria toward the ingher organisms Howeves some mutation rates in lower organisms are as high as those organisms arc ·CHROMOSOME MUTATIONS 13 Izsted for Drosophila or man Of course these are the mutation rates per generation the length of a generation in man is about 25 years in Drosophila about the same number of days while in bacteria and bacteriophages it is estimated in minutes If we were to estimate the mutability per unit of time the genes in bacteria would prove to be most mutable and those in man least mutable On the other hand it is possible that the data for man are biased in the direction of giving higher estimates than they should If one were to use the direct method outlined above in the search for mutations occurring in man with a frequency of 1 to 10 billion sex cells per generation (as in bacteria) and if one were to limit the study to children born in the United States (at a rate let us say of 150 million per generation) one would have to accumulate data for 350 generations or roughly 10 000 years in order to observe about 10 such mutations Small wonder that mutation rates of this sort have not been found in man) But with bacteria a study involving mutation rates as low as this can be made conveniently within a period of 48 72 hours In addition to the limitations imposed on an investigator of human populations by the number of individuals he can handle there is a further limitation imposed on huis estimates of mutation rates Without casting aspersions let us say simply that people will be people and that there is a certain amount of infidelity in this imperfect world The frequency of uncon trolled and uncontrollable conceptions establishes a base line beyond which studies of mutation rates cannot go except by the severest limitation of the types of material studied-a limi tation that restricts even further the number of persons available for study Chromosome Mutations Thus far we have restricted our discussion to mutations of which most non-nhall.. dr. sc.L. -lee- ·44 SPONTANEOUS MUTATION Another kind of mutation involves breakage and reunion of chromosomes which as we know are the gene carriers Chro mosome mutations are important genetic consequences of radi ation exposure (see Chapter 5) A chromosome like a piece of string may be broken into two or more pieces Such accidents happen to chromosomes only rarely under ordinary circumstances but the frequency of chromosome brealages is increased by exposure to atomic radiations and to certain chemical treatments A cell in which one or more chromosomes have been fractured is in a precarious situ ation To carry on its normal functions indeed to remain alle a cell generally needs all the genes it normally contains Few if any genes are dispensable The cell needs its full complement of chromosoma) material Now as shown in Chapter 2 during cell division the chromosomes divide and one complete set of chromosomes moves into each of the daughter cells This move ment is however not a property of the whole chromosome il depends upon one spot in the chromosome which contains a specialized body called the centromere Consequently wlien a chromosome is broken one of the pieces--the one without the centromere-lacks the ability to migrate to its appointed place when the cell divides. As a result, one of the newly formed cells does not get its full quota of gener and generally dies However unlike a string a chromosome possesses socky raw ends wherever a break occurs if two of these ends meet they will rejoin and the break will be healed Consequently when a single breah occurs there are trvo poss? ble outcomes Esther the one piece is lost at the next cell divi sion as mentioned above or prior to the division the brolen ends meet rejoin and restore the chromosome to its normal condition Other possibilities arise if two or more breals occur in a cell simultaneously One or more of the chromosome fras ments may be lost and the cell may die or the breaks may and the original chromosomes may be restored Two chroma some fragments which lack centromeres may join or to from ments both including centromeres may join cells in which this happens are not viable The piece of one chromosome laching 3 centromere may join the piece of the other chromosome with a centromere and the other pieces may also join this forming heal ·MUTATION AND THE WELFARE OF THE ORGANISM • 15 two new chromosomes containing one and only one centromere each Such new chromosomes, which contain blocks of genes pre viously carried in different chromosomal bodies, may function in a perfectly normal way in ordinary cell divisions Genes are suffciently independent in their action to produce a normally viable and healthy organism even if reshuffled between different chromosomes As a matter of fact, some of the cells in an adult body do not divide at all, and fragmentation of their chromo sorties need not destroy their ability to live and to carry on their functions We shall see later that this fact throws light on such important problems as destruction of cancer cells by x rays or radium treatments, and radiation sickness produced by the exposure of the body to excessive amounts of high energy radiation Mutation and the Welfare of the Organism We have stated above that gene mutation can be visualized most readily as a mistale in the chemical pattern of the gene which arises during the process of gene reproduction A mutant gene is essentially an incomplete, imperfect, defective copy of the parent gene Now, the genes which an organism carries are there not by a lucky accident but because of the slow, steady, unrelenting process of fitting or adjustment to the environment in the course of evolution Genes in healthy, vigorous human beings, or for that matter in healthy and vigorous mice or flies or com plants, are really masterpieces of biological evolution You would hardly expect a masterpiece, such as a painting by El Greco or Rembrandt, to be improved if some dauber cook it upon himself to change it Mutation spoils a gene much more often than it improves it for exactly the same reason And yet, one should not jump to the conclusion that mutation can never improve the gene After all, the paintings of El Greco and Rem brandt were improved—ın fact, they were created-by the brush strokes made by their authors But let us set aside the analogies and look at the facts Among the meants in Drosophula the largest and most easily ·SPONTANEOUS MUTATION detected group are the recessive lethals which kill their carriers when homozygous Moreover, just as there are countless diseases liereditary and otherwise, which kill men, so there are many ways a lethal can kill a fly About 5 to 10 percent of the lethals kil) embryos before they have left their egg cases about 80 per cent kill during the larval stages the remaining 10 to 15 percent Lill pupae or young adults while they are emerging from their pupal cases In addition to lethals, there are mutations that simply lower the chances of a fly attaining adulthood Some of these muta tions are associated with externally visible effects (almost all of the mutations used by geneticists in their studies are of this $ort), opers are not Very fesy of these semilethal or subvita} mutants have distinct intermediate effects that is, a gene muta tion affecting development generally kills all or almost all fies carrying it, or it has a much more subtle effect which results in the death of a relatively small percentage of its carriers Just why some individuals live and some die when they carry these semilethals is hard to tell Perhaps variations in the severity of the environment are responsible just as in man some persons afflicted with a hereditary disease may die while young, others at a later age, and still others may live and even enjoy fair health The mutants of Drosophila which are pictured in textbooks of genetics or of biology are mostly defective in some way or another The white eyed mutants have lost the eye pigment normal flies have and with it hase lost their visual acuity Wing mutants usually make the wings less serviceable for flying or reduce them to useless vestiges A mutant that makes the fly yellow instead of the normal brown might seem to have caused no appreciable harm, and yet yellow males are less successful is mating with females than are males of normal color However, these textbook mutants are apt to conveys false impression They are a selected loi-selected because they produce sharp, easily visible casily scored changes The more radical the change, the less likely it is to be harmless, not to speak of sts bessig useful Yet mutants do occur which make the fly a tiny bit larger or smaller, develop a little faster or more slowly, have slightly more or fewer bristles on certain parts of that ·USEFUL MUTANTS . 17 the body, deposit on the average slightly more or fewer eggs Some of these unspectacular mutants sometimes called poly genic ones, may be neutral or even favorable The evidence for this comes from studies on populations of flies living in their natural habitat The individuals in such wild' populations differ from one another in just such polygenic traits, and the only way in which this normal variation could have arisen is by mutation Similarly, with normal people, some are taller and others shorter, some have darker and others lighter hair or eyes some exhibit differences in facial features which are diffi cult to describe in words but are easily percepuble to us as the recognition marks of our friends Useful Mutants Gene structure is evidently capable of multitudinous modi fications and some of these chiefly those which produce minor alterations in the characteristics of the organism may be innoc uous or even useful and may become incorporated into the array of genotypes we describe as normal for the species Moreover, and this is important, it is often the environment which deter mines whether a given mutant is harmful or useful Diagram matically clear examples of this can be cited especially in microorganisms We have already mentioned mutants in bacteria which are resistant to antibiotic substances that kill normal bacteria. Are these resistant mutants good or bad for the bac teria? It stands to reason that they are good in an environment containing the antibiotic, for in such an environment the non mutants are unviable However, nature did not make a mistake when it evolved bacteria that are sensitive to these drugs In drug free environments the sensitive varieties often enjoy varı ous advantages which the resistant varieties do not have The latter may be more fastidious in requiring various nutrient substances not required for growth by the former It is known, for example, that some of the streptomycin resistant mutants do not ·SPONTANEOUS MUTATION grow at all except on nutrient media containing streptomycin What was a poison to "normal" bacteria has become a necessity to the resistant strain Breeders of agricultural plants and animals are always on the lookout for mutational changes of possible economic use fulness Since the mutation process is speeded up by high energy radiations (see Chapter 4), the possibility arises that among masses of worthless mutants there may be found a few useful ones Ale Gustafsson, a Swedish plant geneticist, is the pioneer in this type of worh, which more recently has been taken up by some other breeders in various countries Table II loss sombe TABLE 11 Some useful mutants obtained in the progeny of plants exposed to high energy radiations (after Brock modiked) Plant Description of mutants Barley Dense spike, large grains broad leases, tall and short, stilf straw, high yield Improved malting quality Higher protein content Wheat Resistance to black stem rust Increased yield, suf straw, increased grain weight increased tillering, improved haking quality Oats Increased yield Surf straw, increased grain weight Rust resistance, improved quality Peas Increased yield, more branches, more pods, later maturity Soybeans Early ripening Increased yield, shortened antemodes, increased branching, more pods, increased seed weight, earliness rax Increased yield of long fiber Rust resistance Jute Increased fiber yield and quality Peanuts Increased yield and disease resistance Oil turnip Earlsness, faster seedling growth Increased yield ·ADAPTIVE VALUE OF HOMĂN GENETIC VARIANTS 49 examples of radiation induced mutants in various agricultural plants that may prove serviceable in breeding work The existence of these useful mutants certainly does not contradict the statement made above that a mutant gene is usually a more or less defective copy of the gene from which it arose In the first place the mutants listed in Table II are a tiny residue selected from a much greater mass of mutants most of which were discarded because they were useless or harmful Second consider the fact that any agricultural plant exists in what is a new environment,new compared with the environ ments in which the plant lived before man began cultivating it Finding useful mutants among agricultural plants is no more surprising than finding bacteria resistant to antibiotics among billions of bacterial cells exposed to selection by antibiotics that kill off all the nonmutant individuals Adaptive Value of Human Genetic Variants In the case of man we are familiar with the common gene mutants that for all practical purposes seem harmless The eye color genes mentioned several times serve again as an example So too do the genes which are responsible for the different blood types of different individuals thanks to the existence of blood banks most of us have become familiar with the blood types AB A B and and with Rh positive and Rh negative Skin pigmentation hair form and color shapes of ears and noses degree of hairness and many other common differences be tween persons are known to have a relatively simple genetic basis Most of these variations make no great difference to a person s health But one should not jump to the conclusion that these gene-controlled variations are absolutely neutral and under no circumstance make any difference to their possessors You may have learned from your blood bank whether your blood is thesus positive (Rh+) or thesus negative (Rh-) This is important information because it has been found that a serious-often ·(a) FIGURE 6 Examples of mutant genes in man, which produce hereils tary diseases defects, or malformations (a) progressive muscular dys crophy, (b) achemopodia (absence of hands and fees), () achondroplası, or chondrodystrophic, dwarfism, (dl) xeroderma pigmentosura (abnormal freckling and light sensitnity of the skin). (c) ichthyosis congenita (leathery skin with deep, bleeding cracks à usually icthal condion) 1 ·(c) (0 (1) syndactviy (fusion or absence of fingers) (g) brachydactyly one Junted fingers) (h) Harelip (failure of the two halves of the upper jaw to fuse properly) for purposes of illustration, we have selected mutants which produce drastic and easily visible changes in their cartiers, 10 should be kept in mind that many moutants cause only slight variations, of the sort in which "normal' persons often differ ·$2 SPONTANEOUS MUTATION fatal-condiuon of newbom infants is caused by the occasional interaction of a baby s blood with that of its mother The abnormal condition known technically as erythroblas tosis fetalis, is evoked ordinarily by an unfavorable combination of paternal, maternal, and offspring genotypes The gene in question has two forms Rh and rh, RhRh and Rhrh individuals have Rh positive blood (The Rh, incidentally, is an abbrevi ation for rhesus monkey and is used because blood from Rh positive persons will react with blood serum obtained from a rabbit that has been previously injected with blood from such a monkey) Individuals homozygous for the th gene (rhrh) have Rh negative blood A man and wife who are Rh positive and Rh negative, respectively, may have children who are all Rh positive, or half may be positive and half negative During preg nancy there may be a leakage of blood from the developing embryo into the mothers blood stream, and vice versa If the child is Rh positive his blood carries into his mothers blood a protein which to her body is strange, her body manufactures antibodies to fight this protein just as it would to fight any infection Apparently the amount of leakage is not great, at any rate, the first Rh positive child-and even the second and third-may be bom in a perfectly normal condition There comes a time during a later pregnancy, however, when the developing child is Rh positive and the mother has built up enough antibodies to cause trouble Because of placental leak age, some of the mothers blood finds its way into the embryo Here the maternal antibodies find the strange protein they are designed to fight Unfortunately, this strange protein happens to be in the red blood cells of the unborn child These cells are largely destroyed, and after birth when the baby must rely on the red cells to transport oxygen to different parts of his body, his blood does not function properly At present the lives of some such babies can be saved by means of total transfusionsreplacement of all the blood in the infant s body The origin or erythroblastosis fetalis is an illustration of the general principle that common and apparently innocuous gene differences may become quite important under certain circumstances Evidence is grou ing that not only the Rh blood ·ADAPTIVE VALUE OF HUMAN GENETIC VARIANTS • 53 types but also the better known A B O system of blood group genes may differentially predispose their carriers to certain ail ments Thus persons with the O blood type are slightly, but significantly, more frequent among sufferers from duodenal ulcers than they are in the general population TALE 111 Some mutations in man Mulation Tate Disease Description (X 10-) Healt/" Achondroplasia Short limbed dwarfs high mortal 10-70 010 or chondro ity during early childhood af dystrophia fected women bear children by Caesarean section only Epilola Tumor like growths on many or 8-12 Very gans such as brain skin liver low heart lungs and kidney rela tively few individuals survive to 30 years of age Retinoblastoma Malignant tumor of the eye which Below necessitates removal of the eye normaal ball immediately (early child hood) one fifth of those affected must have both eyes removed Dystrophia Muscular disorder resulting from 8 0 30 myotonica disorders in the nervous system Marphans Long slender fingers and toes dis 5 050 syndrome placed lenses in the eyes fre quent heart disorders Anindia Absence or defect of 111s resulting 5 Below in seriously impaire vision normal Hemophilia Inability of the blood to cloc 25-50 0 25 20 Not all mutations in man have as subtle an effect as do the Rh th genes in bringing about erythroblastosis fetalis In Table III we have listed a number of the better known human heredi tary defects along with a description of their effects, their esti mated mutation rates and an indication of the health of the affected individual as measured by his ability to leave surviving offspring Most of the conditions listed are semilethal or worse, ·52 SPONTANEOUS MUTATION fatal-condition of newborn infants is caused by the occasional interaction of a baby s blood with that of its mother The abnormal condition known technically as erythroblastosis fetalis, is evoked ordinarily by an unfavorable combination of paternal, maternal, and offspring genotypes The gene in question has two forms Rh and th, RhRh and Rhrh individuals have Rh positive blood (The Rh, incidentally is an abbrevi ation for rhesus monkey and is used because blood from Rh positive persons will react with blood serum obtained from a rabbit that has been previously injected with blood from such a monkey) Individuals homozygous for the th gene (rhrh) have Rh negative blood A man and wife who are Rh positive and Rh negative, respectively, may have children who are all Rh positive, or half may be positive and half negative During preg nancy there may be a leakage of blood from the developing embryo into the mother s blood stream, and vice versa If the child is Rh positive his blood carries into his mothers blood a protein which to her body is strange her body manufactures antibodies to fight this protein just as it would to fight any infection Apparently the amount of leakage is not great at any rate, the first Rh positive child-and even the second and third-may be born in a perfectly normal condition There comes a time during a later pregnancy, horever when the developing child is Rh positive and the mother has built up enough antibodies to cause trouble Because of placental leak age, some of the mothers blood finds its way into the embryo Here the matemal anubodies find the strange protein they are designed to fight Unfortunately, this strange protein happens to be in the red blood cells of the unborn child These cells are largely destroyed, and after birth when the baby must rely on the red cells to transport oxygen to different parts of his body his blood does not function properly At present the lives of some such babies can be saved by means of total transfusionsreplacement of all the blood in the infant s body The origin or erythroblastosis fetalis is an illustration of the general principle that common and apparently innocuous gene differences may become quite important under certain circumstances Evidence is growing that not only the Rh blood ·3. COCO C2 ? . 53 price of having to care for some defective. Such a darger, if it exists at all, is remote. In the first place, we could not, a: the present level of our knowledge, bring the mutation process to a halt even if we wished to do so. The issue before us or is that of mutation becoming more frequent second, the count of genetic variability—the raw material of evolutionary changeavailable in human and other sexually reproducing populations is amply sufficient to sustain a great deal of further evolution; we shall see later that in any one generacion the contribution of mutation to this store of variability is infinitesimal compared with that which has already accumulated within a species. This last point deserves some further comment S.S. Chetverikov, one of the pioneer geneticists, said that a sexually reproducing species is like a sponge. absorbing mutants as they occur. Human individuality-the fact tha: every person has a unique, unprecedented, and nonrecurrent genotype-is a consequence of the perpetuation of the generic variability that has arisen by mutation. However, generic variants that, in combination with other genes carried by the organism, lead to vigor and good health are perpetuated in populations rather more easily than arc detrimental genes. Not that the latter are a rent-we shall see in Chapter 6 that populations do cany "genetic loads." Nevertheless, the variety of genes present in sexually reproducing populations does represent a store of genetic raw material from which adaptive changes can be constructed in the process of evolution. Lack of genetic variability for further evolution of the human species is something we need not worry about.

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·4
Atoms and Radiations
Life and Radiation
The sun and its life giving rays have inspired wonder and awe
in men of every age sun worship was a part of many religions
Here are some lines from the Hymn to the Sun composed by
Ikhnaton, a pharaoh of Egypt, more than three thousand years
ago
Thou art he who createst the man-child in woman,
Who makest seed in man,
It ho grueth life to the son in the body of his mother,
TV ho soothest him that he may not weep,
A nurse even in the womb,
Tyho giveth breath to animate every one that he maketh
Ihnaton was right life endures on earth because living
matter is adapted
to capture and to store within itself some of
the solar energy that reaches the earth Life has existed on earth
for perhaps two billion years, it has learned to cany on, literally
and metaphorically, under the sun" As was pointed out in
Chapter 1, unul man started to use the energy of atomic fission,
every bit of energy he consumed came ulumately from solar
radiation
SYT

·$8 ATOMS AND RADIATIONS But there exist other kinds of radiations in nature which are weal or absent in the sun's rays when the latter reach the earth's surface Some of these radiations are destructive to biological organization Outstanding among these destructure agents are the radiations variously called ionizing, penetraling, short wave, or high energy radiations, which are produced by radium and other radioactive substances Man has learned to create them artificially, some of them are generated in the proc esses of atomic fission and fusion It would be out of place in this book to discuss the physical nature of radiations in detail Such discussions can be found in a large number of books written on different levels of physical and mathematical sophistication For our purposes we must, however, be familiar with some facts concerning different kinds of radiations, their measurement, and the amounts man encoun ters in the natural environment, and in the environments cre ated by our modern industrial civilization Molecules and Atoms The world about us is composed of countless types of substances, some solid, others liquid, sull others gaseous Numerous words in our vocabulary are names of these different substances -water, alcohol, kerosene, salt, sugar, soda, aspırın, etc. We recognize these substances by their consistency, color, smell taste, their effects on ourselves or things or processes in our environment. These properties and effects depend largely on the molecular structure of the substances concerned If we tale a lump of sugar and divide it into smaller and smaller pieces he still have sugar left after each division until se artise at very small units called molecules If a molecule were to be magnified about 100 million times, it could be conveniently represented by a picture on a page If a lump of sugar seiglung one pound were to be magnified 100 million umes it would be almost as large as the earth Molecules can be broken into still smaller parts For ex ·MOLECULES AND ATOMS. 59 ample, if we heat sugar we get two quite different substances, namely a black residue of carbon and-although we may not see it as it boils off-some water The water given off when sugar is heated is exactly the same as that coming from the faucet above the sink, and the carbon is exactly the same as that found in a piece of charcoal or in soot Water can, in turn, be broken down into two further substances If an electric current is passed through water, two gases-hydrogen and oxygen-are produced Again, the oxygen and hydrogen we get from water are precisely the same as those obtained from an or other substances What we are left with when we break apart different sorts of molecules is a relatively small number of elements There exist many thousands, perhaps millions, of different kinds of molecules, but they are made up of only about 92 naturally oc curring elements, and even some of these are quite rare in nature How does an element differ from a molecular compound such as sugar or water? As we have seen, sugar and water can be decomposed into simpler substances quite easily But even if an elementary substance, such as carbon or oxygen or iron, is divided again and again, it still remains carbon oxygen, or iron The ultimate particle of an element, the atom, defies subdivision by ordinary maltreatment A molecule of sugar is com posed of 6 atoms of the element carbon 6 atoms of the element oxygen, and 12 atoms of the element hydrogen The properties of sugar are determined by the number and relative position of these atoms within the molecule Molecules can be broken into atoms by heat, by electric currents, or by reactions with other substances Atoms cannot be broken by such manipulations The word atom means indivisible in Greek The picture of the world composed of a fairly small num ber of indivisible and immutable atoms has the virtue of great simplicity and clarity Moreover, this picture is adequate for most purposes of practical life and even of rather advanced technology During the second half of the nineteenth century and the early years of the current one, most physicists and chem ists thought that atoms were, indeed the ultimate reality of the material world That this conception was erroneous became dear through the discovery, by Pierre and Marie Curie, of an ·60. ATOMS AND HADIATIONS extraordinary element called radium This amazing substance was found to emit, continuously and without any outside source of energy, powerful and penetrating radiations, and to transform itself, slowly but inexorably, into a quite different ele ment-namely, lead Atoms thus proved to have a complex internal structure, and atoms of one element were shown to be capable of turning into those of another Anatomy of the Atom: Radiations The atoms of the 92 elements known before 1940, and a few more discovered since then, can be regarded as consisting of three basic particles-electrons, protons, and neutrons These particles are extremcly small_-10 000 times smaller in diameter than the atom itself Moreover, the interior of the atom resem bles, of all things, the solar system in the center there is the atomic nucleus compounded of procons and neutrons, the nucleus may be likened to the sun in the solar system In the space around the nucleus, and spinning around it like the planets around the sun, are a number of electrons The number of orbital electrons is characteristic for atoms of each element. The simplest atom is that of the element hydrogen A hy drogen atom has a diameter of about 1 000 000 005 of an inch The center of the atom is occupied by the nucleus the nucleus contains but a single proton, which is an electrically charged particle positive in sign Rotating around the nucleus is a single electron The electron is also electrically charged, but its charge is negative, thus balancing the positive charge of the nucleus. The mass of the electron is, however, very tiny-about 1/1 810 of that of the proton Most of the weight of the atom is thus concentrated in the nucleus Atoms of other clements are more complex Their nuclei contain varying numbers of protons and neutrons depending upon the element, and many electrons gyrate around the nuclei in fixed orbits For each element the number of electrons balances the number of protons in the nu cleus Thus, an atom of one of the heaviest elements, uranium, ·ANATOMY OF THE AIOM RADUATIONS 01 has a nucleus compounded of 146 neutrons and 92 protons 92 electrons spin around the nucleus in several concentric shells Atoms of other elements range in complexity and in weight between those of the lightest element hydrogen and up to and beyond uranium at the upper end If the properties of electrons are to be studied easily they must be removed from the atoms containing them One of the early tools used for this purpose by physicists was the Geissler tube a glass tube from which most of the air has been evacuated Inside at each end of the tube is a metallic plate connected to the poles of a high voltage source of electricity by a wire that pierces the glass When the current is turned on the cube gives off a glow which is caused by the passage of a stream of elec trons through the rarefied air The electrons can be demon strated to leave the plate connected to the negative pole and to jump across the tube to the plate connected to the positive pole By coating the positive plate with a phosphorescent substance (zinc sulfide) one is able to discern the impact of the electrons as pinpoints of light In 1895 a German physicist W K. Roentgen discovered that hitherto unknown radiations were coming out of Geissler tubes The properties of these radiations seemed mysterious enough for them to be called x rays, x designating the un known nature of the rays X rays are invisible to the human eye but they can be detected by photographic plates strangest of all * rays easily penetrate screens of black paper and other materials completely opaque to ordinary rays of light The penetrating powers of x rays made them important in ways which were not obvious to their discoverer and to many others They became indispensable tools for the diagnosis of many diseases and for the treatment of some of them Well equipped hospitals and even doctors and dentists offices now usually include x ray machines Unfortunately together with properties useful to man these rays proved to have when im properly used some sinister ones as well They can cause severe damage to the body and as we have known since 1927 to the units of heredity the genes X rays are not streams of electrons escaping from the Geiss ·EZ ATOMS AND RADIATTOR ler tube, electrons cannot pass easily through glass or heary paper X rays are a part of the radiation spectruna, of which visible light is one part and radio waves sull another The prop crties of radiation depend upon the length of its wales As shown in Figure 7, radio waves vary s length from several kilometers to a few meters, microwaves (radar waves) from meters to millimeters, infrared (heat) wates from about a milli meter to 8/10,000 (8 x 10-) of a millimeter visible light Occupies the narrow band from & X 10-4 to about 10-s of a millimeter utraviolet from 10-8 to 10-, while x rays extend mutage rodation rays Coming ront ultro intored voet heal ays microwavet rodar dugu 3:05 3610 3x100 3 NOT Il matary TIRE? Diagrammatic representation of the spectrum of electromagnetic radiations Proceeding from the left cach division represents a lenfold increase in wavelength the dis tance from Cst to crest of the wave Visible light occu pies only a small portion of the spectrum of radiations smalier in fact than that shown from some 10- (soft x rays) to 10- (hard x rays) and to 10-* (known also as gamma rays) Sall shorter wavelengths (whose characteristic of cosmic rays) are knonn Another basic fact of radiation physics u that radiacion behases as if it comes in discrete packages, these are called quanta Each quantum of radiation has in it a certain amount of energy, the shorter the was elength of radiation the greater the energy contained in iis quanta Thus, a quantum of 1sible light has relatsely little energy, quanta of ultraviolet light are more powerful, and those of x and gamma rays are more ener geuc still This is why x rays and gamma rays are Inout as high energy radiations ·FECTS OF RADIATION ON MITTER 63 Efects of Radiation on Matter an atom its When visible light falls on a material body the light quanta are either absorbed in or reflected from the body The energy carried in the quanta of visible light is however not sufficient to affect the structure or behavior of most atoms Quanta of light resemble peas shot from a toy gun against a wall the peas bounce off and do not go through the wall Only some few substances are phosphorescent This means that when they are exposed to light electrons are displaced in their atomic orbits and the substance gives off a glow (that is emits visible light) for a short time after the exposure stops The visible radiation is emitted as electrons return to their normal position Other substances such as silver bromide which is used in photographic film are light sensitive their molecules undergo chemical changes because of the energy delivered to them by light X rays gamma rays and other high-energy radiations have quanta so powerful that they may perhaps be likened to bullets shot from a nfe When such a quantum hits energy is imparted to one of the electrons circling around the atomic nucleus An energized electron may be ejected from an excited atom and fly at an enormous speed into surround ing space where it hits other atoms Now an atom that has lost one of its electrons has lost one unit of negative electric charge It will therefore be a positively charged atom or a positive ion Such ions are chemically much more active than electrically neutral atoms they will react easily with other atoms or atomic groups which are able to share an electron with them The electron ejected from an atom darts through the sur rounding space where it may collide with other electrons in neighboring atoms These other electrons may in turn be knocked from their orbits A hit by a powerful x ray quantum thus produces a whole series of agitated electrons and ionized atoms The disturbance caused by the initial hit gradually subsides as the energy of the absorbed quantum is distributed among more and more electrons Tinally no one electron has enough energy left to displace other electrons and the residue ·ATOMS AND BADIATIONS of the energy is dissipated as heat By then, however, a number of electrons may have been displaced from their atoms the atoms that have lost electrons are left behind as positive ions the displaced electrons or the aroms that have picked them up are negatzve ions Each displaced electron results in a pair of ions, one positive and one negative This is why high energy radrations are also called ionizing radiations Absorption of quanta of high-energy radiation by atoms is not the only cause of ionization High speed electrons, called beta Tays, exist within x ray tubes in fact x rays are produced when these electrons strike the metal in the positive electrode Disintegrating atoms also eject electrons from their orbit at a high speed beta rays are in fact, a characteristic emanation of dis tegrating radium atoms These rays have a wide range velocities and, hence, of characteristic energies Although more energetic beta rays are known, most of those produced by atomic disintegration are capable of passing through only thin layers of alumnum or about 15 feet of air They slow down rapidly as they collide with other electrons their initial energies are rapidly dissipated by ejecting these other electrons from their orbits Again, the result of irradiation by beta rays is the forma tion of many pairs of ions Since they are simply rapidly moving electrons beta rays are electrically charged Another type of charged particle is the alpha particle, a rather ponderous particle bearing two positive electric charges it is in fact, the nucleus of a helium atom Again velocities of alpha particles vary according to their source, a value of about one twentieth the speed of light is not unusual For all thesr weight, alpha particles are not efficient penetrators a sheet of paper suffices to stop most such particles However, in living tissue the extent of their penetration is not negligible, especially if their source is inside the body Through out the distance they do penetrate, their effect is tremendous, as they roar past and through atoms, they knock electrons out of their orbits at a tremendous rate Not only electrons but esen whole atoms may be dislodged by the blows of these relatively massive particles Neutrons have effects of their own Neutrons, st may be recalled, together with protons make up atomic nucles when ·ES AFFECTS OF RADIATION ON MATTER atomic nucler dısıntegrate neutrons are quite frequently ejected These may travel at a fast or slow speed Since they have no electric charge they can rush or drift right through the elec tronıc orbits of other atoms Eventually they are captured by the nuclei of these other atoms For reasons that are not com pletely known certain numerical combinations of protons and neutrons are unstable atoms having such unstable combinations undergo spontaneous nuclear disintegration This disintegration or fission may result in the ejection of additional neutrons If as in the case of uranium 235 the aver age number of ejected neutrons is larger than 1 there are more free neutrons after the disintegration of each atom than before It thus becomes possible to initiate and sustain a chain reaction This reaction may be slow and controlled (as in atomic piles) or almost instantaneous and of great violence (as in atomic or fission bombs) Electrons are also frequently expelled from unstable atoms as high speed beta particles Furthermore the jos tling of electrons in their orbits because of nuclear adjustments frequently results in the ejection of a quantum of gamma radı ation Gamma rays are as we have seen above very powerful x rays Thus the disintegration of radium atoms results in the release of all three kinds of radiation-alpha beta and gamma rays These rays are all 10nızıng and the gamma rays have in addition tremendous penetrating powers The disintegration of atomic nuclei poses a novel type of biological problem since the atoms involved change from one chemical element into another Thus upon capturing a neu tron an atom of common sodium disintegrates into an atom of iron plus an atom of helium The isotope (variety) of iron produced in this process 15 unstable and quickly changes into neon It is obvious that such changes if they occur in a living cell may result in grave disturbances Suppose that a physio logical reacuon depends upon the presence of sodium atoms if some of these atoms suddenly change to helium and neon the normal course of the reaction will be disturbed The existence of a radioactive isotope of phosphorus poses a special problem for hereditary materials The desoxyribonucleic acids (DNA) which are the most important constituents of the chromosomes contain a great deal of phosphorus If a radioactile variety ·in the ens cell 66 ATOMS AND RADLATIORS phosphorus becomes incorporated into chromosomal DNA, the affected molecule of DNA is doomed Radioactive phosphorus exists for a few days only, and then changes into sulphur, an element entirely unsuitable for the chemical constitution of DNA The normal flow of life processes depends upon the pres ence, in the proper place and at the proper time, of certain chemical molecules This is especially true of the processes of heredity A change in a single gene molecule in the sperm or may make the difference between a normal child and a pialul invalıd High energy, or ionizing, radiations disrupt the physical structure of atoms and initiate novel chemical reac tions The addition of a hydrogen atom here, its loss there, the tonization of a formerly uncharged atom, the production of hydrogen peroxide, the physical disrupuon of long, chainiske molecules-these and a dozen other effects or immediate after effects of radiation may raise havoc With the cells basic organ ization In many instances, of course, the damage is done to an expendable subscance and is repared by the cells built in de fense mechanisms in many other instances, however, damage 15 done to a unique substance not easily replaced or repared Here the effects of radiation are serious There is no more unique, no more important, no more complex substance, nor one less capable of being repaired, than DNA-the chemical that con dicions the hereditary processes and directs the manufacture of enzymes and proteins in every cell of the body Measurement of Radiation The effects of radiaton upon a living cell or a liung body depend upon the amounts of energy the living substance ab sorbs from this irradiation It is obuously important to measure the quantiues of radiation applied and to relate them to the biological effects produced Thus, exposure of the body to high energy radiations may result in radiation sickness and in death (see Chapter 5), we must know, then, how much radiation of 2 giren kind wl} be fatal to a mouse, or to a man, or to some ·SOURCES AND AMOUNTS OF NATURAL RADIATION 67 other organism We must also know how great are the amounts of radiation to which people are exposed from fallout products of testing atomic weapons, from x rays used in medical practice, and from other sources The most useful unit for measuring exposure to x or gamma rays is the roentgen, usually abbreviated as 'r" It is so named to honor the discoverer of x rays One roentgen corresponds to the amount of x or gamma radiation which produces about 2 X 10' ion pairs per cubic centimeter of air This meas urement is made fairly easily with the aid of an apparatus (dosimeter) which measures the leakage of electricity from a charged object The amount of leakage depends on the number of ionized molecules, the greater the amount of x rays, the greater the number of ions, and the greater the loss of electric charge through leakage The number of ions produced by a given amount of radia tion is, however, different in different types of substances One roentgen of x rays yields for example, about 18 X 1012 ion pairs per gram of living tissue The measurement of types of radiation other than x and gamma rays is more involved The basic unit of the radiation absorbed is the 'rad The absorption of I rad delivers 100 ergs of energy per gram of matter The relative biological effectiveness of i rad of alpha rays is, however, approx imately equivalent under certain conditions, to that of 10 rads of gamma rays In practice, the biological effects of these radia tions are measured by equating them with effects produced by a given dose of x rays as measured in roentgens Thus we have a unit called 'rem (= roentgen equivalent for man) For measurement of very small amounts of radiation there are millırads (mrad one thousandth of a rad) and mullirems (mrem, one thousandth of a rem) Sources and Amounts of Natural Radiation All life is continuously exposed to high energy radiations Such 'natural," or background, radiations are present every where, though their intensity is generally very low There is ·68 • XTOMS AND LADUATIONS every reason to think that such radiations occurred on earth no less abundantly in the past, in fact, we do not know whether the first form of life arose because or in spite of radiation. It would probably be impossible, or at any rate very difficult to create an environment completely free from such radiations Some of the sources of natural radiation are extemal others are internal, within living bodies By far the most omnipresent among the former are cosmic rays These, as their name sug gests reach the earth from outer space At the earth's surface cosmic rays consist chiefly of extremely high energy quanta cor responding to wavelengths considerably shorter than those of Mirad per TABLE IV Exposure to cosmic rays in mrad per 30 years in differen! countries (after the report of the U N Saentific Committee on the Efects of Atomic Hadiation) Place of Place of Mrad per observation 30 years observation 30 years Great Britain 840 Japan 1020 United States 870 Northern Argentina 600 Austria 810 Southern Argentina 810 France 720 the gamma rays of radium So great are their penetrating poners that the amounts of this type of radiation received by all organs of the human body are uniform and practically the same out doors and inside dwellings These rays have been detected in mines a quarter of a mile or more below the earth's surface The intensity of cosmic rays varies, however, with the elevation of the locality above sea level it increases about threefold as one ascends from sea level to 10 000 feet. It also varies with gra graphic latitude, being generally lower in the tropics than near the magnetic poles of the earth Since we are interested in the genetic damage which radiations may produce in human popula tions the data in Table IV are gien in mrad So years exposure This interval of ume has been chosen because it corresponds coughly to the middle of the reproductive life span of modem man (the average age of parents at the time hall per ·SODIC KXD XXOURTS OP RATOBAL BADITIOX their total number of children have been bom) The radiation exposure during 30 years is then about the average exposure per human generation Other external sources of background radiation are radioactive elements which though in very small concentrations are widely distributed over the earth s surface The amount of radia tion received by people from these sources is however highly variable because of the uneven distribution of radioactive sub Mirad per TABLE Background radiation levels (including cosmic radiation in murad per 30 years in some cities of the United States (measurements made during August 1957) Mrad per City 30 years City 30 years Harrisburg Pa 2 640 Denver Colo 4 410 Pittsburgh Pa. 2,880 Colorado Springs Colo 5040 Cleveland, O 2730 Grand Junction Colo 4 340 Toledo o 2,280 Albuquerque N A 3480 Ch cago IN 2 640 Amarillo Tex. 3 240 Madison Was 2,590 Oklahoma City Okla 2,520 Minneapolis Minn 2 760 Tulsa Okla 2760 Sioux Falls SD 2 850 Little Rock Ark. 3 180 Cheyenne Wyo 4 260 Memphis Tenn 2850 stances in different rocks and different soils Thus the radio active elements radium thorium and a radioactive isotope of potassium (k 39) occur in greater concentration in granite than in basalt and in the latter in greater concentration than in sedimentary rocks External radiation rates are lower in build ings constructed of wood than in those made of brick and an the latter lower than in those of concrete or granse Because of these variations the total radiation doses recented from sources external to the body are rather different in different places Table V contains estimates made by the investigators at thic Health and Safety Laboratory of the U S Atomic I nergy Com mission in New York (L R Solon and others 1958) Ilıcxc estimates include the radiation received from cosmic rays as well as other external sources ·ATOMS AND RIDIATIONS - 30 years) Background radiation levels appreciably higher than those summarized in Table V occur however in some parts of the World This is particularly true of a region in the state of Kerala India and in the state of Espirito Santo Brazil where many people inhabit regions of so-called monazıtıc sands which contain the radioactive mineral thortum The population exposed to these high radiation levels in herala is estimated to number about 100 000 persons The average radiation level in Espírito Santo is around 500 mrad per year (15 000 per while in Kerala estimates between 131 and as high as 2 814 mrad per year (about 4 000 to 84 000 mrad per 30 years) have been obtained Minute quantities of radioactive elements occur normally as constituents of human bodies and of course of all other living creatures A certain number of atomic desintegrations occurs therefore within our bodies and certain amounts of luigh energy radiations are produced by these internal sources. Thus the body of a man weighing about 150 pounds contains approximately 17 milligrams of radioactive potassium (2-40) and it has been estimated that human gonads (sex glands) receive some 500 mrad of gamma radiation from this source in 30 years The presence of even smaller amounts of a radioactic isotope of carbon (C-14) exposes human gonads to about 16 mrad per year (about 50 per 30 years) The Report of the UN Scientific Committee on the effects of Atomic Radiation est mates the average total dose of radiation received by human gonads from both external and internal radiation sources 23 100 mrem per year or 3 000 mren or 3 rem per 30 years Man made Radiations Three rem per generation may be regarded as close to the irreducible minimum of background radiation to which man kind was is and will continue to be exposed In addition to this rock bottom minimal exposure mankind is and probably ·MAN MADE RADIATIONS • 71 will continue to be exposed to some radiations from man made sources Among the latter, the radioactive fallout from the test ing of nuclear weapons has in recent years been incessantly dis cussed in learned articles, popular books, newspapers, and magazines, and from political tribunes It may, therefore, surprise many readers to learn that the amount of exposure from fallout products is decidedly smaller than that from less pub licized man made sources--namely, from sources used for medical and industrial programs The story of the fallout is, very briefly, as follows The explosion of atomic bombs and superbombs generates numerous unstable and radioactive isotopes of many chemical elements Among these, strontium 90 and cesium 137 are considered most important, both because these radioactive atoms are produced in large amounts and because they decompose rather slowly, continuing to give off dangerous high energy radiations for a long time Strontium 90 has a half life of 28 years, cesium 137 a half life of 27 years The half life of a radioactive element 18 the length of time required for one half of the substance to decompose and to lose its radioactivity But after two half lives -that is, after 56 years, one quarter of the original radioactivity of the strontium 90 fallout will be preserved, after 84 years one eighth, etc Last but not least, strontium 90 and cesium 137 are important because they are absorbed and retained in the bodies of plants animals and men Atomic explosions hurl radioactive materials high up into the atmosphere, even into the stratosphere These materials are carried aloft and then, for a certain time, transported about the earth by winds Eventually they fall once more to the surface of the earth-hence, the term fallout A part of fallout is local fallout, these are materials which settle in the neighborhood of the test site within a few days after the explosion But lighter particles in the upper reaches of the atmosphere and in the stratosphere may remain aloft for several years may be carried around the whole earth, and may settle only slowly-but not slowly enough to have lost their radioactivity Since most of the atomic test sites have thus far been located in the temperate and subtropical belts of the Northern Hemisphere (see Figure 8), ·wa FIGURE & A schematic representation of world wide fallout parterns 24 one of the more common by products of atomic and thermonudear aplouons latitudes of the Northern Hemisphere, the lowest averages (1 os 2 microCUTICS erable vanation *hich is not represented in this figure and which results froco amount of fallout in a given area

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to
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these are revealed by the quantity of urnntrum 90 in the soil Strontium
The highest average (about 10 microcuries per square mile) occur in elics
per suare melt) occur near the equator and ac die (wo potes There is con:
local climatic conditions, rainfall ti cipecially important in determining

·7. ATOMS AND RADIATIONS fallout in this region has been greater than in the Arcuc, along the equator, or in the Southern Hemisphere Nevertheless, fall out is a world wide problem Some strontium 90 and cesium 137 may enter the human body through actual inhalation of air containing fallout par ticles By far the most important source, however, is vegetables that have absorbed fallout products from the soil, or milk and milk products from cows that have been pastured on contami nated vegetation A great deal of research in several countries has attempted to estimate the amounts of strontium 90 and cesium 137 which people accumulate, and the amounts of zonizing radiations which will reach human sex cells from these radioactive atoms Indeed, strontium 90 has made its untelcome appearance in milk, in cereals, and in vegetables, and it can be detected in the bones of persons of various ages, the concentra tion being especially high in the bones of growing children The genetically significant dose of radiation from this source is, how ever, not easy to determine On the assumpuon that atomic weapons tests will not resume after the end of 1958, the average dose to human sex cells per 30 years will apparently be about 10 mrem, that is, about one third of one percent of the amount received from natural sources (see Table V) If, however, weapons tests are continued at the present rate for the next century, the exposure will be increased, in the generations to follow, some four to ten times The radiation exposure of human sex cells through the testing of atomic weapons will, thus, amount to only a fraction of the presumably irreducible exposure to radiation from natural sources As we shall attempt to show in the chapters that follow, this does not mean that the additional radiation is negligible Moreover, if a war were unleashed in which nuclear weapons were used, the radiation exposure would be much greater than that resulting from either the testing of weapons or natural (background) sources By far the most important source of radiation to which human populacions are al present exposed is neither background radiation nor atomic bombs bus the x ray machines in hospitals and in doctors' ofices X rays have a very important place in ·MAN MADE RADUMONS . 75 modern medical practice They are virtually indispensable in diagnosing many ailments, from toothache to tuberculosis and cancer They are essential preliminaries for many surgical opera tions Equally important is the therapeutic use of radiation Treatments with radiation are applied in cases of malignant tumors and for many other lesser ailments Radioactive sub stances particularly wodine 131, phosphorus 32, and gold 198, are given internally to some patients, usually in small quantities, and chiefly for diagnostic purposes The unquestionably beneficial uses of radiation in medical practice are unfortunately, coupled with some attendant danger of genetic damage Genetic damage arises when high energy radiations impinge on reproductive organs This occurs to some extent no matter what part of the body is irradiated inten tionally If a beam of sunlight enters a room part of the light is reflected and scattered throughout the room Sımılar scatter ing occurs also with x rays and other radiations When, for example, a beam of x rays is directed at the chest of a patient to examine his lungs a fraction of the rays also reaches the sex cells This occurs even with dental x rays When x rays are directed at the pelvic region as in obstetrical examinations the gonads of the mother as well as those of the unbom child may be directly in the intense field of the primary beam More and more persons are employed by industrial plants that use x or gamma radiations to inspect their products One Teads with increasing frequency of this or that company which has obtained a cobalt 60 source of radiation for some aspect of industrial research Quality control through automatic ma chinery activated by radiation sources is becoming common place, all TV viewers are aware by now that the manufacturer of one brand of cigarettes claims greater uniformity through this type of control Finally, both in industrial and research laboratories the use of radioactive atoms has become extremely common Radioactive atoms do not differ from ordinary atoms in their chemical behavior Consequently, in the study of industrial or physiological chemical processes, radioactivity offers a means of tagging certain atoms and following them through a variety of reactions The technique used is basically the same ·78. ATOMS AND RADUATIONS whether one wishes to determinat deposits paras millum bor the genetic of rac wuul on human populations, it makes no difference what problem is being investigated, the increased use of radioactne isotopes means that more members of the population are being exposed to radiation How much radiation exposure is incurred by people be cause of the medical occupauonal, and technological use of radioactive substances? This is very difficult to estimate with any precision The sources of uncertainty are many First of all, 110 evident that people in technologically advanced countries are exposed to radiations more frequently than people in primite or underdeveloped countries Bur even in countries with nell organized research facilities it is no easy matter to find out the genetically important exposure dose-that is, the dose whisch reaches the sex cells. Tahe, for example, che sadrauons used to treat patients with malignant growths Even if massive doses of radiation are applied, the generic effect may be sliglie, indeed, most such patients are beyond the childbearing age, and for many of them even the chances of survival are small The Report of the U N Scienufic Committee on the Effects of Atomic Radiation has collated and attempted to interpret a mass of data on man made radiation exposures in Austria, Denmark, England, France, Japan, Sweden, and the United States The figures arrived at vary from 20 to 150 mrem of exposure per year from diagnostic radiations, from 1 10 30 mrem from therapeutic exposure less than 1 mrem per fear from radioactive substances laken inernally, and less than 2 mrem per year from occupational exposure These are alerages per person, certain individuals of course recente considerably more than others Multiplying these figures by 30 to obtain genetically effectne doses per human generation, we obtain torals of 720 mrem to 5,500 mrem In other words, people in technologically advanced countries arc, on the average, exposed to a total amount of man made radiation about twice 35 steal as the average natural bac.ground (see Table 1 Thus trebling of the nacural radiacion exposure is certainly ·MN-MADE RADUATIONS. 17 disquieuing since, as we shall see belowy, it is bound to produce some genetic damage Most certainly, we should not be fright ened to the point of denying people the benefits which legitimate medical uses of radiation are capable of giving These benefits are too obvious and too important to be renounced But it is also certain that precautions should be taken to minimize radia tion exposure, particularly of the sex cells, as far as possible This can be done by using screens made of materials opaque to x rays to shield the reproductive organs of patients as well as physicians and medical technicians Recognition of the genetic and other dangers of radiation should make us avoid unneces Sary exposure It is, for example, very doubtful whether x ray machines should be allowed to be used for shoe-fitting One need not rely solely on genetic arguments to justify this statement, the growing bones in children s feet, because of the rapid divi 310n of bone cells, are especially sensitive to x ray exposure, just as are other tissues including cancerous growths-in which cell divisions are common Solely on genetic grounds one can argue, however, that temporary male sterility should never be induced by irradiation, too many alternative and convenient techniques are available to justify the use of an agent which may doom a future individual to a life of misery and unhappiness ·76 - XYOMS AND RADIATIONS genetic effect whether one wishes to determine what compound in gasoline deposits carbon on valves in automobile engines or what part of parasitic virus enters and kills a bacterium For the of radiacion on human populations, it makes no difference what problem is being investigated, the increased use of radioarte 150topes means that more members of the population are being exposed to radiation How much radiation exposure is incurred by people be cause of the medical, occupational, and technological use of radioactive substances? This is very difficult to estimate with any precision The sources of uncertainty are many First of all it is evident that people in technologically advanced countries are exposed to radiations more frequently than people in primitne or underdeveloped countries But even in countries with well organized research facilities it is no easy matter to find out the genetically important exposure dose-that is, the dose which reaches the sex cells Take, for example the radiations used to treat patients with malignant growths Even if massive doses of radiation are applied, the genetic effect may be slight indeed most such patients are beyond the childbearing age, and for many of them even the chances of survival are small The Report of the U N Scientific Committee on the Effects of Atomic Radiation has collated and attempted to interpret a mass of data on man made radiation exposures in Austria Denmark, England, France, Japan Sweden, and the United States The figures arrived at vary from 20 to 150 mrein of exposure per year from diagnostic radiations, from 1 to 30 mrem from therapeutic exposure, less than 1 mrem per scar from radioactive substances taken internally, and less than 2 mrem per year from occupational exposure These are averages per person, certain individuals of course receive considerably more than others Multiplying these figures by 30 to obtain genetically effective doses per human generation we obtain totals of 720 mrem to 5,500 mrem In other words people in technologically advanced countries are, on the average, exposed to a total amount of man made radiation about twice as great as the average natural background (see Table 7 This trebling of the natural radiation exposure is certainly ·5 Induction of Mutation by Radiation Discovery of the Speeding up of Mutation by Xrays Genes are the carriers of genetic information This fact makes them the controlling influence in determining the path which the development of an individual will follow in a given suc cession of environments The genes present in a fertilized egs cell can be passed faithfully to every one of the 247 cells (a fifteen place figure) composing the body of an adult man as well as from generation to generation This transmission is possible because of the unique chemical properties of the DNA molecule which ensure a faithful replication of the gene pattern and also because of the wonderfully efficient organization of living matter This organization includes the consolidauon of groups of genes into chromosomes and the elaborate mechanisms of cell division It is known that penetrating or high energy radiations are destructive to biological organization Radiation striking a molecule is likely to change it from its original to a biologically abnormal condition of necessity then radiation must threaten 79 ·OD INDUCTION OF MUTATION BY RADIATION the normal functioning of the genes and of the living body which is dependent on their proper action The mutagenic (mutation inducing) effect of x rays was dis covered by Muller in 1927 As described in Chapter 3 Muller realized the difficulty inherent in studies on mutation rates of single genes He solved the difficulty by treating chromosomes as units and by studying a type of mutation-recessive lethalswhich occurs in many genes within a chromosome For example if there were 300 genes for each of which mutations from normal to lethal occurred at a rate of 0000 01 the mutation rate observed for the chromosome as a whole would be 300 times 0 000 01 or 0 003 (3 lethal chromosomes per 1 000) Mullers technique made the quantitative study of mutation rates feasible To see what the Mullerian approach accomplishes suppose that in order to prove that the rate of mutation is increased by X radiation we must obtain about 10 lethals in a control series and about 30 lethals in a corresponding experimental (x rayed) series If we limit ourselves to a study of a single gene for which the spontaneous mutation rate is 0000 01 and for which the mutation rate under the influence of irradiation is trebled or 0000 03 we shall have to make some 2 000 000 observations to be reasonably sure that the radiation is responsible for the in creased mutation rate If however we use Muller s technique our task is to demonstrate the reality of the difference bett een the mutation frequencies 0 003 and 0009 this can be accom plished by making only some 6 600-6 700 observations Since we have studied simultaneously the mutation rates of 300 gene locı our labor has been reduced 800 times An experiment that would have required a heroic effort can now be performed by one person in a matter of weeks In Muller s classical experiments 1 177 chromosomes of Drosophila flies treated with a massive dose of x rays yielded 143 lethal mutants 741 chromosomes treated with a dose only one fourth as large yielded 59 mutants and in the control exper iment in which flies were not exposed to x rays 6016 chromo somes yielded 5 lethals The differences between the outcomes of the three series of experiments are far too great to be explained by chance alone It follows that lethal mutants are ·RADIATOR AND MUTATIONS . 11 more frequent in the progeny of flies treated with x rays than they are in the control experiment X rays are mutagenic. Direct Proportionality between the Amount of Radiation and the Number of Mutations Since x ray treatments speed up the mutation process, the problem that immediately presents itself is the quantitative relation between the amount of x rays applied and the frequency of mutations induced This problem was taken up by C P Oliver and by N W Timofeeff Ressovsky as well as by Muller himself Figure 9 shows some of the results obtained in X chromosomes of Drosophila melanogaster, the common vine gar fly Along the vertical axis in the diagram in this figure are plotted the percentages of all treated chromosomes found to be carrying recessive lethals Along the horizontal axis are plotted the various amounts of x radiation to which the chromosomes were exposed The measurement of x ray dosage, it will be remembered, is made in units designated as I (for Roentgen the discoverer of x rays), 1 r is that amount of irradiation neces sary to produce 1 8 X 1012 aon pairs per gram of tissue Ejecting an electron from one atom it may be recalled, results in the formation of two ions or charged particles one ion consists of the atom which has lost its electron the other of an atom which has picked up the extra electron Although 1012 (1 followed by 12 zeroes) is an enormous number, there remain 100 billion Cuines as many nonionized atoms in tissue following an exposure to lr The solid line of the graph in Figure 9 is drawn through the points represenung experimental findings, the dotted line 15 a straight line obtained by correcting the observed frequencies for instances in which one expects two or more lethals to be induced in the same chromosome and, hence, to be erroneously scored as a single lethal There are three important features of this diagram (1) The corrected curve is a straight line, every increase in the amount of x rays to which the chromosomes are exposed results in a corresponding increase in the mutation rate ·INDUCTION OF MUTATION BY RADIATOR (2) The line starts slightly above the horizontal axis the soutal height represents the spontaneous mutation rate Orcutting even in the absence of radiation (3) There is no known threshold below which irradiation is ineffective in producing mutations Dercent tethol Chomet 1Door 4000r scoor 2000 3000T amount of rad otson FICURE 9 The frequency with which sex linked lethals are induced in mature sperm of Drosophila melanogaster (the common vinegar Aly) as a result of exposure to various amounts of x radiatson Sex linked lethals are lethal mutations of genes which are located on the X chromosome the chromosome which is im portant in the determination of the sex of an indi vidual The solid line corresponds to actual observations the dotred line which represents the d rect propor tionality between the frequency of andured lethals and the dosage is obtained by correrting the data for chromosomes carrying or more lechals The value of 25 r is the lowest dosage studied in fruse flics Recent studies with bacteria have been made with doses as low as 8 r, these low exposurcs also result in the producuon of gente The results of the experiments summarized in Figure 9 are Very important They show that no matter how small the amount of radiation to which an organism is exposed, a certain calculable mutations ·RADIATION AND MUTATIONS number of mutations will be induced by it As far as the genetic effects of radiation are concerned there is no such thing as a permissible or safe amount below which mutations are not in duced This could by no means have been predicted or assumed on theoretical grounds As a matter of fact many physiological effects of radiation (of which more will be said below) do not behave in this way Large amounts of radiation produce dan gerous radiation burns in the exposed individuals while small amounts have no immediately detectable harmful effects In general many physiological reactions in organisms have minimal thresholds--that is quantities of stimuli below a certain minimal amount are ineffective while quantities above that minimum produce effects at rapidly increasing rates Among physiological reactions there are many instances of all or none effects as in nerve stimulation Stimuli below a certain threshold cause no discharge along the nerve fiber while stimuli slightly above or very much greater than the threshold produce equal discharges The direct proportionality between the amount of radia tion and the frequency of mutations induced has permitted a number of interesting calculations to be made it was not sllogi cal for example to suppose that spontaneous mutations-those which occur in nonirradiated individuals—may be accounted for by cosmic and other natural radiations Physicists can measure the intensity of this omnipresent or background radiation Knowing how many mutations 1 r is likely to produce (see below) it is possible to show that considerably less than 1 per cent of the spontaneous mutations in Drosophila can be accounted for by background radiation In other words the mutations induced by radiation are superadded onto the amount which would be produced without radiation and which for a lack of knowledge we call spontaneous Doubling Dose and Frequency of Mutation per Roentgen of Radiation One of the ways in which the magnitude of the genetic damage produced by radiation may be expressed is based on ·LNDUCTION OF MOTATION BY RADIATION the amount of radiation necessary to double the spontaneous rate of mutation Turning to the data in Ligure 9, we can see from the graph that an exposure of Drosophila to 1,000 r results in approximately 3 percent lethal mutations in one particular chromosome The spontaneous mutation rate for that chromo some was given earlier as 0 001 or 0 003, depending on the strain of fies used for analysis We also said that certain inferences led to the conclusion that there are about 300 gene loci at which lethal mutations can occur in this particular chromosome What is the mutation rate that would result at one locus as a result of exposure to I r? To calculate this, we divide 003 by 1,000 to get the mutation rate per chromosome per r, and we divide this answer (0 000,03) by 300 in order to reduce the calculated muta tion rate to that for a single locus The answer is 0000 000,1, there is one chance in 10 million that exposure of a single gene to I r will result in its change to a state having a lethal effect The spontaneous mutation rate per gene locus can be calculated by dividing 0 001 or 0 003 by 300, the answer in this case lies between 0 000,003 and 0 000 010 The average mutation rate per locus is somewhat less than I per 100,000, perhaps 3 million is a better estimate Allowing for this uncertainty, it would appear that an exposure of flies to between 30 r and 100 r would double the spontaneous mutation rate This is a calculation based on averages for many genes, recent information obtained for various genes in bacteria indicates that the doubling dosethe amount of radiation required to give an induced mutation rate equal to the spontaneous rate-varies from about 10 rto over 100 r for different gene loci Thus the information obtained from lethals in Drosophila is borne out by these more sensite studies per Mutations in Different Organisms A flood of confirmatory reports followed Muller's discovery that x rays increase the mutation rate in Drosoplula These reports, which dealt with a wide variety of organisms both plant and animal, showed without exception that in every form ·WUTATIONS DUTERAT OSCATUS. US of life in which mustauons could be studied radiation was effec tie in increasing the mutation rate As the present time mas of all biological species of particular interest to the geneticist happens to be the only one for which there exists no completely consinang evidence that mutations are induced by radiation It kould be foolish indeed to regard this as an indication that man may be immune to the mutagenic effects of radiation Experz ments involving deliberately contrned and adequately con trolled crosses are obviously out of the question in man The geneticist is thus deprued of the sery techniques needed to demonstrate the induction of mutations A great deal of effort has been expended in studying radia tron induced mutations in mice Even though some people may argue that whatever we learn about mutations in mice will be scarcely more applicable to man than what we find in Drosophila 1 is nevertheless desirable to check our conclusions through studies of a form of life somesvhat closer to man than are fltes Mice like men are higher animals-mammals Not only do they live longer than flies do but-and this may be more im portant-the processes of development and maturation of the sex cells in the ovaries and testes are similar to those in man Techniques analogous to those devised for Drosophila by Afuller and others are not yet available for mice W L Russell and his collaborators in this country and T C Carter and his colleagues in England have had to use a more laborious method A strain of laboratory mice is prepared which is homozygous for recessive mutant genes these genes affect various easily visible characteristics of the animals chiefly the coat color so that it is possible to tell by examining a mouse whether it is or is not homozygous for a gis en gene Suppose then that a female mouse from this mulaply recessive strasn 15 crossed with a male from an ordinary normal wild type strain A normal mouse carties dominant variants of a}} the recessive genes in the tester strain Barring mutation the entire progeny (Fs genera tron) of this cross will consist of animals heterozygous for the recessive genes recersed from the tester strain to which their mother belonged and for the normal dominant genes received from the father This progeny will almost always be like the tester ·86. INDUCTION OF MUTATION BY RADIATION normal, wild type parent Only rarely does an individual exhibit the traits of one or another of the 7 recessive genes of the tester strain Such individuals arise when a sex cell of the normal strain (in this case a spermatozoon), happens to carry a mutant allele of one of the 7 genes of the tester strain The experiments are divided into two parts In one, females of the tester strain are crossed with normal males In the other, the reciprocal cross, mutant males are crossed with normal females The mutations detected in the first set of experiments must have arisen in the male parent, in the second, in the female parent Moreover, some of the normal parents are treated with a known amount of radiation, while others serve as non irradiated controls Some of Russell's results are as follows Mutations recorded Normal parent Male Male Female Female Female Dose of erradiation None (control) 600r None (control) 400r 2581 Ofspring examined 37,868 48,007 40,918 7,859 26,468 2 53 or 54 0 7 2 Among the 48,000 offspring of irradiated fathers there were 53 or 54 mutations (one was not quite certain), while only 2 mutants were found among almost 38,000 mice in the progeny of nonirradiated fathers A total of 9 mutants appeared in the progeny of irradiated mothers, and none in the control experi ments Irradiation is clearly mutagenic in mice, as 3t is in dies and in other organisms We shall have occasion to refer to these experiments with mice later in this book What Radiations Are Mutagenic? In Chapter 4 we saw that the physical properties of radia tions depend on their wavelengths Geneticists have studied the mutagenic powers of radiations of both short and long walc ·WHAT ELATIONS ARE MOTAGENIC?. lengths They have also studied the mutagenic powers of radia tions delivered in different ways-gradually, over periods of days or weeks, rapidly, in a matter of seconds or minutes, continu ously, or alternated with periods of "rest" Of necessity, these experiments were done mostly with fruit flies, corn, and some lower organisms It was quickly revealed that, with x rays and other high energy radiations, it is the total dose of the radiation-the total number of roentgens (see Chapter 4) to which a gene or a chromosome is exposed-which determines the frequency of the mutations induced As shown in Chapter 4, "soft x rays have relatively long wavelengths and consist of relatively small energy quanta, hard x rays and gamma rays have short wavelengths and high energy quanta The hardness or softness of the radia tion has no effect whatever on the mutations induced, provided only that the radiation is able to penetrate to the sex cells The number of mutations produced is determined by the number of ionizations caused by the radiation exposure, and it is the number of 10nizations that is measured in roentgens Even ultraviolet radiations are mutagenic The relation between mutation and exposure to ultraviolet rays is, however, a complex one and need not concern us in this book Ultraviolet rays have a much longer wavelength than the softest x rays, and their penetrating powers and energy quanta are low Visible light and infrared rays are not mutagenic The term high energy radiation does not apply to ultraviolet and to longer wave lengths The rate at which radiation is delivered has under most circumstances, no effect on mutation frequency With powerful radiation sources such as powerful x ray machines, radioactive cobalt, or atomic bomb explosions, massive amounts of radia tion can be delivered in a matter of seconds on the other hand, by placing flies at a distance from a radiation source, the treat ment may be prolonged for days or weeks The number of muta tions induced by a given number of roentgens is the same regard less of whether the exposure is short or long The same number of roentgens may also be delivered in a continuous exposure, or in several bursts anterspersed with periods of "rest The manner ·INDUCTION OF MUTATTOS BY BADIATION of delivery is, in general, unimportant as far as the number of mutations induced is concerned It is easy to see what this means to man Genetic radiation damage is an insidious thing, Sery small radiation exposures add up, and if continued for years may amount to as much as or more than an accidental brief exposure to intense radiation Since genetic damage is cumulatie, no radiation increment however small and insignificant by itself, can be neglected. What matters is the total exposure up to the close of the reproductive or childbearing age It should be noted that recent data obtained by Russell and his collaborators do suggest that, in mice, "chronic irradiation (that is, delivered gradually over a long time) may be less effec tive in producing mutations than 'acute" irradiation (adminis tered rapidly) It will be remembered that Russell and his group experimenced with mutations affecting 7 different genes in mice (see page 86) They obtained only 2 mutants among 26,469 mice whose mothers had received chronic irradiation, whereas they observed 7 mutants among only 7,859 mice whose mothers liad been subjected to acute irradiation This matter obviously needs further study The Directness of the Genetic Effects of Radiation Studies of the effects of radiation on mutation rates harc demonstrated that the action of radiation is direct That 25, if one is to study mutations saduced in the sex cells, one must irradiate sex cells (or the gonads-osaries or testes-in which these cells are produced) One cannot obtain mutations in these cells by irradiating the head of an animal, or its thorax, or 16 legs Radiauon does not result in the production permeate the body and, upon arnval in the gonads, product mutations The radiation must strike at or very near the gene which mutates The fact that the mutation inducing action of high-energy of pousons What ·SOMATIC MOTANON .69 radiations is direct suggested a 'target" theory, which was in vogue in genetics unul recently According to this theory, the quantum of x radiation acts like a small bullet, when this bullet happens to hit a gene it shoots a hole in it and thus results in mutation This view proved to be an otersimplification, the action of radiation is really quite different Irradiation results in the productson of highly reactive chemical substances in the cells and chromosomes However, these substances persist in the irradiated tissue for a very short time perhaps only a fraction of a second If one of these substances happens to be produced withm or very close to a chromosome, it can interact with the genetic material and induce a mutation These rwo theories of the mutagenic acuion of high energy radiations lead to very different views concerning the possibility of discovering an antidote for radiation damage If the radiation spinging on the gene acts like a stream of bullets then there w little hope of finding treatments protective against the muta trons in exposed cells The theory of mucation inducing chem ical intermediaries between the radiations and the genes does offer some hope an antidote, an antı mutagen, may possibly be discovered Such an antidote will perhaps be a chemical substance more likely to react with the muration inducing sub stances than are the genes themselves Somatic Mutation In order to induce mutations that can be observed in the progeny of an irradiated individual, one must irradiate his sex cells This does not mean that irradiation of body cells is ineffec llle in producing mutations in them However, the mutations induced in the body cells of a parent remain confined to these cells they are not transferred to the sex cells and consequently are not transmitted to the offspring If the gonads of the irra diated individual are protected from radiation whatever mula tions may be induced in his body cells are confined to the indı vidual exposed and will not affect his progeny ·90. INDUCTION OF MUTATION BY RADIATION It may be noted that this fact offers excellent evidence against some nineteenth-century theories of heredity and of sex cell formation (recently revived in Russia by Lysenko) These theories held that small particles from every organ and every tissue of the body migrated to the gonads, where they became organized into mature sperm or eggs In this way each inda vidual wis supposed to have obtained the genetic information needed for proper development, head particles multiplied and formed a head, eye particles located the eye properly, and so forth If this were the case, irradiation of one portion of an animal s body should result in defective particles from that part and, hence the corresponding part should be damaged in the new embryo Needless to say, no such effect has ever been demonstrated In order to observe mutation in body cells rather than sex cells experiments must be arranged differently, as they were in a study performed originally by J T Patterson Patterson reasoned that chromosomes are reproduced faithfully at each cell division and that each daughter cell is furnished with a complete set. He was also aware that the normal red eye of Drosophila is changed to white (pigmentless) by the mutation of a certain gene Drosophila larvae have no eyes however as larvae develop cells appear which are destined to form the eyes of the adult fly These cells form eye primordia, which are transformed into adult eyes during the pupal stage Patterson s experiment consisted of exposing normal larvaç of various ages to x rays and examining the predominantly red eyes of the emerging adults for white patches The results stere clear-cut White patches did appear The number of white patches increased with dosage Early exposure resulted in large patches (many cells carrying the same mutation) late exposure to radiation resulted in small patches (very few cell divisions after the mutation had been induced) Mutations then were induced at the time of exposure Genes carried by any cell of the body consequently, are subject to mulation when exposed to irradiation It seems highly possible that this fact has a bearing on the production of at least some forms of cancer in man Evidenice ·CESSOMOSOME BREAKAGE BY RADIATION . 91 particularly suggestive of this has recently been summarized by E B Lewis for a form of cancer of the blood known as leukemia Leukemia manifests itself in an uncontrolled multiplication of the white blood cells, it is usually fatal The frequency of leukemia is higher among persons exposed to radiation than among the rest of the population This is known to be the case among the Japanese who were exposed to radiations from atomic bombs, and it is found also among children of mothers exposed to obstetrical x ray examination during pregnancy The dou bling dose for leukemia appears to be about 30 r-an amount of radiation suspiciously similar to that calculated as the doubling dose for mutations In the face of this evidence, the implication that the origin of leukemia is mutation in a cell producing white blood corpuscles cannot be summarily dismissed, although A M Brues and others have recently questioned the valıdıty of this hypothesis Chromosome Breakage by Radiation We saw in Chapter 3 that the term mutation is applied to at least two rather different phenomena-qualitative changes in individual genes (gene mutations) and chromosome breakage and reconstruction (chromosome aberrations) Both kinds of mutation occur spontaneously, and, as was demonstrated in the classical work of Muller in 1927, both are increased in fre quency by exposure to high energy radiations However, the relation between the amount of irradiation administered and the number of mutations observed is quite different for genic and chromosomal mutations Although chromosomes undergo breakage spontaneously, in most organisms such accidental breakage is very rare, at least in sex cells and in the cells producing sex cells Suppose, then, that a chromosome breaks spontaneously in 1 of every 100 000 cells As stated in Chapter 3, a chromosome break may "heal', if it does not, the cell in which it has occurred is usually unviable A chromosome aberration can arise only if at least two ·BZ • INDUCTION OF MUTANION BY RADIATION Chromosomes in a cell are broken, or if one chromosome is broken in at least tivo places Only then can the brolen ends reunite in new combinations With a frequency of spontaneous chromosome breakage of I per 100,000 cells, two breaks will appear in the same cell by chance in 1/100 000 X 1/100,000, or in i per 10 billion cells The number of people now living on earth is estimated to be about 2,700,000,000, hence, the number of the sex cells in wluch they had their origin is 5,400,000,000 If the frequency of spontaneous chromosome brcalage in man is as low as we have supposed, a chromosome rearrangement of spontaneous origin enters the entire human species only once every 2 generations Exposure to high energy radiations greatly increases the frequency with which chromosomes undergo breakage More over, the frequency of single breaks increases in direct propor tion to the amount of radiation applied, just as gene mutation does This has been demonstrated by irradiating, for example, flower buds and then examining under a microscope the daud ing cells that gre rise to the pollen grains (pollen mother cells) Some lours or days after irradiation, many of these cells show one or more fragmented chromosomes The number of chromo some breaks required to produce the observed fragments can be calculated and compared with the amount of irradiation in the treatment The proportion of cells with two chromosome breaks may be calculated by multiplying by itself the probability that one break will be induced as has been done in the example above Suppose, then, that certain radiation exposures increase the rate of breakage by 10, 100, or 1000 times, these exposures sill increase the proportion of cells containing two breaks cach by 100, 10,000, and I 000,000 times respectuels The result is that the frequency of chromosome mutations in the progeny of Irradiated individuals will increase very much faster than ini direct proportion to the amount of radiation This has been amply demonstrated by studies on the progeny of irradiated Drosophila, as well as with experimental plants We may now consider the effects of chromosome brcalage in sex cells and in the early germ cells that give rise to the ·CHROMOSOME BREAKAGE BY RADIATION . 93 latter For convenience, we may divide the germi cells into two groups-those which have undergone the meiotic divisions reducing the number of chromosomes from 46 to 23, and those which have not Radiation effects in premerotic cells are com parable to those in any other system of dividing cells, a majority of the cells in which chromosome breaks occur will get into difficulties at or after the next division and will die The death of these cells is not particularly important, since it does not lead to any clinical symptoms The cells in the gonads are inde pendent of one another to the extent that each cell goes through the series of divisions and transformations leading to a func tional gamete, regardless of the fate of its neighboring cells A heavy radiation exposure is required to destroy most of the testicular or ovarial cells the loss of a few early germ cells as the result of small radiation exposures is not serious in itself-that 18, this loss does not threaten one with sterility Treatment with x rays has been used to induce temporary sterility in men as a method of birth control This practice is clearly dangerous and should be stopped Although most of the cells in which chromosome breaks have occurred die before forming mature sperm this is not true of gene mutations Muta tions induced in the young germ cells do appear later in mature sperm In the human male, mature sperm are manufactured continuously-at least several trillion of them in all Some groups of cells in the testes preserve the ability to divide, while the cells they produce by division proceed through the transformations-including the meiotic division-into mature sperm Mutations induced in those cells which act as the source of sperm are passed on faithfully if we may use this word, to all descendant sperm from the moment the mutations are induced Fortunately for man, radiation exposures are rare events, and so only a small fraction of irradiated mature germ cells are actually involved in reproduction Most single x ray exposures, and the greater proportion of one s cumulative exposure, affect mature sperm cells that never achieve fertilization Conversely, sperm cells that do fertilize eggs are exposed to radiation chiefly during their early germ cell stages This is fortunate because radiation is generally much more effective in producing muta ·94 • INDUCTION OF MUTATION BY RADLATION tional changes in mature sperm than in the early germ cells The DNA in a mature sperm cell is carried in an extremely condensed package, the sperm's "head 'In this condensed form, the chromosomal material seems to be about twice as Sensitive to the induction of gene mutations as it is in cells of earlier stages (In part this may be a spurious sensitivity, gene mutations that are deleterious to early germ cells are partially eliminated before sperm formation ) More serious than the excessive sensitivity of mature sperm cells to the induction of mutations is their sensitivity to raduz tion induced chromosome breaks Because of the condensed form of the chromosomes in the sperm head, these breaks are more likely to rejoin in erroneous patterns fatal to the cell at the following division The 'following division' for a mature sperm is the first division of the individual formed at fertiliza tion Sperm cells with these fatal chromosome breaks act as "dominant' lethals, egg cells fertilized by such sperm fail to develop Now, in discussing the fate of a fertilized egg we are no longer talking of cell death in the same sense in which we refer to cells abraded from the skin or plucked out with a hair We are talking of a new individual-a rather minuscule and unimposing individual at the time, but a new individual none theless Radiation, Carcinogenesis, and Cancer Therapy No group of human diseases is more feared or has attracted 1 the tumors or cancers The reason for this is that the fundlich causes which bring about these most insidious illnesses have proved extraordinarily difficult to understand Although much progress has been made in cancer studies in recent years, the fact remains that the mechanisms of carcinogenesis are sull largely unknown The reason we must discuss the problem of ·RADIATION CARCINOGENESIS AND CANCER THERAPY . 95 cancer at least briefly in this book is that it is related to radia tion in two ways First, it is well known that exposure to radia tion brings with it the risk of malignant disease Secondly, the same radiation is also used to destroy or at least to depress the growth of some malignant tumors One of the current though by no means proved, hypotheses regarding the origin of cancer is that the transformation of an ordinary, healthy cell into a cancer cell arises through one or several consecutive somatic mutations These mutations remove the cell from normal growth-controlling mechanisms and allow it to grow and divide much faster than it otherwise would A dangerous cancer is one in which the division of certain cells has gone out of control These cells multiply, develop into masses of unorganized tissue, deplete the nutritional supply of the body, and destroy the proper functioning of vital organs by mechanical displacement or by invasion and disruption The cells which are performing their proper functions reproduce at a much slower pare and are threatened with extinction in com petition with the cancer cells Evidence has been referred to above which shows that the quantitative relation between ex posure to high energy radiation and the incidence of one kind of cancer-namely, leukemia-is the same as that for radiation induced gene mutations This does not really prove that leu kemia arises by mutation, and still less that all forms of cancer arise in this way But it does show that the mutational hypothesis of carcinogenesis cannot be easily ignored It may well be that the occurrence of a mutation in a body cell is necessary, though not in itself sufficient, condition for the development of a cancerous growth Let us now recall that the occurrence in a cell of an unhealed single chromosome break, or of two or more breaks followed by an improper reunion of the resultant fragments, leads to the death of this cell Let us recall also that the frequency of chromosome breakage is augmented by exposure to radiations Furthermore, at certain times dividing cells are exceptionally sensitive to radiation damage The reason for this sensitivity is unknown, perhaps during cell division some pro tective substances are removed from the chain of DNA molecule ·98 INDUCTION OF MUTATION BY RADLATION cules For the purpose of cancer therapy massive doses of x ox gamma rays are administered to the part of the body contain ing the malignant growth The rest of the body is protected by a lead shield which is opaque to the raduation used The rapidly dividing cancer cells suffer the greatest amount of radiation damage Their chromosomes are sensitive to irradiauon and their breakage and rejoining into complexes with multiple centromeres or rith no centromeres leads to their death Radia uion is then almost a specific treatment for cancer the damage done to normal cells in the body is much less severe and because of the slow rate of replacement of most tissue cells this damage manifests itself slowly over a long period of time The body can cope with this damage much more easily than with the cancer Thus it happens that the same agency-radiation can be both the cause of cancer and a powerful and in its control Radiation Illness We have distinguished two kinds of biological radiation effects and radiation damage One Lind consiscs of physiological or somatic effectsmchanges produced in the individuals or per sons exposed to radiation The other kind are genetic effects which appear only in the descendants of exposed individuals Tlus book is concerned primarily with the genetic effects hos ever "e have seen that some somatic effects such as leulemia and perhaps other malignancies may have geneuc aspects if they are caused by changes (mutations) of the generic material (genes and chromosomes) in somatic cells Such genetic aspects can be found also in other somauc effects patescularly in radiation 11 ness produced by an excessive exposure of the shole body to radiation Short term and long term consequences of whole body ex posure can also be distinguished Follow up studies of the resulus of explosions of atomic weapons and of some accidents to workers in laboratories and other atomic energy installations have shown that about 50 percent of adult human beings ex ·RADUATION ELESS . 97 posed to 400 or 500 r of radiation die of an acute illness within a few weeks. This radiation illness, or radiation syndrome, begins with nausea, vomiting, and characteristic changes in the white blood cells, whose number in the blood rapidly falls below normal Later on there appear a high fever, reddening and hemorrhages in the skin, loss of hair, misshapen fingernails, ulceration of the mouth, throat, and intestines, and finally death Those in whom the symptoms are less severe may recover after a prolonged convalescence With smaller exposures, about 100, only some 15 percent of the persons exposed shon symptoms of a mild radiation illness, these persons recover fairly rapidly With still lower doses only transient changes in the winte blood cells can be diagnosed. The radiation syndrome can be understood if one remem bers that body tissues consisting of rapidly dividing cells suffer more damage from chromosome breakage than those in which cells are dividing only occasionally Every diagnostic symptom of radiation illness involves a cell system in which the cells are constantly dividing New blood cells are constantly being formed hair grows because cells divide in the hair follicles, fingernails are the product of continuous cell division The hning of the intestine and of the mouth and throat, and the upper layer of the skin, are continuously replaced by cell division Like cancer cells, these dading cells are acutely sensitive to radiation After 500 r of radiation enough cells are apparently destroyed to make repair impossible, and death 15 consequence An adult Drosoplula can withstand radiation one hun dred times more severe than that which is lethal to man Flies so treated become sterle but they develop nothing like the human radiation syndrome Why is the Ny so resistant? The answer 13 that in most adult insects there is very Bottle cell division The cells in the adult body are fixed in number But there is an ex ception--the reproductive organs There divisions do occur, and we find that germ cells are destroyed by much lower radiation exposures than the body itself can withstand The result is steril ity of the exposed fly In agreement with the same principle, fly eggs have none of the extraordinary resistance of the adult ·98 . INDUCTION OF MUTATION BY RADIATION insect These eggs are easily killed by small doses of radiation if they are irradiated very early during embryonic development while cell division is rapid It is likewise expected that children and infants and espe cially embryos during early stages of pregnancy, are more vul nerable to radiation injury than adult persons are Cell division is a requisite for growth The dose of radiation lethal to children and infants is not known, but it probably lies below 400 r Irradiation of pregnant mothers may cause miscarriages or still births as well as the birth of deformed children exhibiting more or less grave physical abnormalities All this may well be related to the more numerous cell divisions occurring in fetal and infant bodies Persons who recover from or escape radiation illness after exposure to intense radiation, and persons who are chronically exposed to small doses of radiation, may develop grave physical disturbances years after exposure It should be noted that chronic exposures may add up to total amounts considerably greater than 400-500 r-a highly dangerous amount of radiation if it is delivered in a single, brief exposure Note the difference between genetic damage and physiological damage the former is additive while the latter is not Nevertheless a study of radiologists-that is, people dealing professionally with x rays and other radiations in hospitals and laboratories-has indicated that they are more likely to die of leukemia than other members of the medical profession who are not radiologists It has also been suggested that people chronically exposed to small doses of radiation age more rapidly and die younger than do other persons This, at any rate, seems to be the case with chronically Irradiated mice The hypothesis that the physiological damage in these situations may be due to an accumulation of somatic gene mutations is certainly a reasonable one The effects of irradiating localized tissues of the body should also be mentioned Radium, strontium, and certain other elements, some of which are radioactive, are preferentially absorbed and lodged in the bones The resulting irradiation of the living cells of the bone tissue and the bone marrow may result in malignant tumors of the bones bone necrosis and ·GENETIC AND PHYSIOLOGIC RADIATION DAMAGE COMPARED • 99 anemias-sometimes as many as 10 and even 20 years after the ingestion of the radioactive materials This is what took place in the unfortunate case of a group of workers employed to paint watch dials with a luminous paint containing radium and mesothorium Because they were in the habit of moistening their paint brushes on their tongues, these people accumulated enough radioactive materials in their bones to cause severe invalidism and death Here again, the supposition that delayed onset of these diseases can be accounted for by the occurrence of mutations in the irradiated cells cannot be dismissed Genetic and Physiologic Radiation Damage Compared By way of summary, we may restate again the principal difference between the genetic and the physiological effects of radiation This is especially necessary since, as we have seen above, it is possible that at least some physiological damage can be accounted for by damage to the genetic apparatus of body cells—that is, somatic gene mutation and chromosome breakage Nevertheless, the damage to the body of an individual on the one hand and to his sex cells and hence to his descendants on the other, are profoundly distinct, and this distinction must be clearly understood With physiological damage, such as radiation burns and the radiation syndrome, there exists both a minimal amount (threshold) and a crtical intensity of radiation, if either the amount or the intensity is below the critical level, the radia tion has no obvious effect The cellular and bodily processes involved in producing radiation burns are dynamic chemical processes affecting relatively large quantities of different mole cules in a conunuous cycle of formation and destruction Healing processes can counterbalance the effects of radiation in this type of system provided the radiation is not too large at any one time With genetic damage, on the contrary, experimental results ·100 INDUCTION OF MUTATION BY RADIATION clearly show that there is no healing process following the induc tion of mutations The bulk of the experimental evidence indicates that the total amount of radiation the total number of ion pairs is the only important factor neither fractionation of the exposure nor delivery of the radiation at a low rate modifies the genetic consequences Each bit of damage induced at one time is added to that which has been induced earlier There is no half damage which can be undone by virtue of a respite from exposure These facts seem reasonable on the basis of what we have learned concerning the role of DNA in heredity and the special properties of DNA that insure its accurate self-duplication At the time the data of radiation geneticists were being obtained however the results were sur prising to those whose training was primarily in physiology even today many medical doctors largely because of the emphasis placed on physiology in their training find it difficult to visualize a biological phenomenon in which the ability to heal or to revert to normal is lacking ·6 Genes in Mendelian Populations What Is a Population? Up to this point we have considered heredity as it manifests itself in individuals and in families We have discussed the laws, discovered by Mendel, which govern the segregation and recom bination of genes in the progeny of heterozygous parents the interactions of heredity and environment, the origin of muta trons their frequency, their effects on the welfare of their carriers, and their induction by high-energy radiation But people live in groups, in communities from the members of which individuals usually select their mates and membership in which parents bequeath to their children Such communities -tribes, castes, social classes, nations, races-bound together by a network of marriages and of common descent, are Mendelian populations We must now consider the phenomena of heredity In such populations Genetics takes on an added dimension when : encompasses populations as units We speak of "Mendelian populations" because the word population has a variety of meanings both in everyday language S.M.S. ovlivnom LULLIGE, LIBRARY JAIPUR 101 ·302 GENES IN MENDELIAN POPULATIONS ond in biology Thus, one may speak of a 'population" of trees composing a forest, or of a bird 'population which inhabits this forest The population of the world is estimated to be close to 2700 000 000 people, that of the United States consists of some 170,000,000, that of Cleveland, Ohio, is somewhat less than 1,000,000, while that of McKean, Pennsylvania us about 300 These are the numbers of persons who live in a given terri tory at a given time A geneticist is interested in organisms that reproduce sexually and in populations as reproductie cum munities-n short, in Mendekan populations Mendelian populations have a temporal continuity, they exist through time The starlings in the United States, for ex ample, comprise a population that has existed since 1890, the year these birds were introduced from Europe In mon and many other species, individuals representing several generations mchildren, parents, grandparents, and great grandparents-may be living at the same time, and the successive generations need not be discrete In insect populations children may mate with their parents On the other hand, many organisms have but one generation per year, and the preceding generation is dead before the succeeding one reaches adulthood In man, population structure reaches its greatest com plexity Mankind-the human species, Homo sapiens is the most inclusive Mendelian population, one which inhabies nearly the whole globe lis generic unity is attested by the existence of English, German, Japanese and other war brides the convergence of immigrants from many countnes on Israel, the world wide dispersal of Russian refugees, etc But eies before modem means of transportation made these extensive migrations possible, there were people who wandered around as soldiers, traders, slaves, or simple agrants and shen people travel they are likely to leave some of thiess genes along the vay However, this does not mean that people intermory in * random manner with respect to the place where they were bom A girl from Dallas is much more likely to marry a Texan shan a Californian, a German, or a Chinese Mankind is, therefore, split up into geographic Mendelian populations whose bound aries are usually not sharply defined and which need are con ·THE GENE POOL OF A POPULATION . 109 cide er en approximately with poliucal units such as countries or states Nor are marriages contracted at random even between inhabitants of the same state city or village Differences of language religion economic stalus educational background and profession erect numerous although usually not sharply delineated communities of Mendelian populations Mankind is thus divided and subdivided into smaller and smaller popula tion units the smaller the unit the closer the approach to random mating between its members The Gene Pool of a Population Suppose that we examine the population of some Middle town which is reasonably uniform in social economic and religious respects and find that about 49 percent of the persons in this community have blue eyes and 51 percent have brown eyes Assume that the difference between blue and brown is due to a single gene and that blue is recessive (this is not strictly Correct since shades of eye color are determined by several genes and a person with greenish blue eyes may actually carry the gene for brown) Will the proportion of blue and brown eyed persons remain the same generation after generation? What proportion of brown-eyed parents will have some blue eyed children And should for some reason all blue eyed persons leave Middletown before reaching adolescence how many blueeyed persons will there be produced in succeeding generations? These are examples of elementary problems with which popu lation genetics has to deal With problems of this sort it is convenient to focus our attention not on individuals but on their gametes and the genes which these gametes contain Assume that every individual member of a Mendelian population contributes to thic gene pool (pool used in this sense is analogous to its use in motor pool of pooled resources not to a pool of water ) of this popula tion equal numbers of functioning sex cells and assume that these sex cells combine at random in fertilization These ·104 GENES I MENDELIAN POPULATIONS assumptions are not as unrealistic as they may seem to be lle know that some persons have more children than others hos ever, we are assuming here merely that brown eyed and blue eyed people have on the average, the same number of children Nor is it too farfetched to assume that people with eyes of a given color will not select their mates preferentially according to the eye color of the latter Suppose now that in the gene pool of the Afiddletown population 70 percent, or the fraction 070 of the total number of sex cells carry the gene a for blue eyes, and the remainderthat is, 30 percent or O 30 of the cocal, carry the gene A for brown eyes (0 70 happens to be the square root of 0 49 wlich is the frequency of blue eyed persons in our Middletown popu lauion, why we have chosen just these figures will appear shortly) Since the sex cells of this population combine at random, the sperm carrying the gene A will have 7 chances out of 10 of ferulizing an egg cell with the gene a, and 3 chances out of 10 of uniong with an egg cell carrying A Similarly the sperm with a will have a 7 in 10 and a 3 in 10 chance of uniting with a and with A eggs respectively The results of the union of these sex cells wilt, accordingly, be as follows Lgs cells Genc frequency 0 30 Sperm Gene Frequency 030 a 070 080 070 Children Gene Frequerey 007 da 021 Aa 090 023 070 0 ad In other words, 009 or 9 percent of the population will have brown eyes and will be homozygous for the gene A {AA} 042, or 42 percent will have brown eyes and will be hetero zygous for A and a (A) and 0 49, or 49 percent will base blue eyes and will be homozygous for 2 (aa) Since the homozygous (AA) and heterozygous (Ag) browns have eyes of about the same color, the frequency of brown eyed and blue-eyed persons wilt be 051 and 049 respectively The fundamental problem concemning us is ubat the nex?

·10$ THE BLEDY WEINBERG THEOREM. generation will be like Will the proportion of blue eyed and brown-eyed persons change, or will it remain the same? To solve this problem, remember that the homozygotes AA and aa will contnbute to the gene pool of the next generation only A or a gametes respectively, while the heterozygotes, Aa, will produce, according to Mendel s law of segregation, gametes A and a in equal numbers As shown above, the three genotypes AA, AA, and sa occur in the proportions 0 09, 0 42, and 0 49 Therefore, the proportion of A and a in the gene pool will be as follows A=009+021 = 0 30 4 3021 +049 = 070 The proportion of the genes A and a in the gene pool re mains the same as in the previous generation The incidence of blue- and brown-eyed persons in the populatson will remain constant generation after generation Neither the dominant nor the recessive gene will increase or decrease in frequency Do not make the error whuch beginning students of genetics are prone to make The dominant gene does not necessarily tend to dis place the recessive nor are dominant genes or dominant traits necessartly more frequent than recessive ones The Hardy-Weinberg Theorem The conclusion we have reached is valid regardless of the actual frequencies of the genes in the population Consider a population in which the gene A has a frequency p and the gene a the frequency q, such that p +9 = 100 Proceeding as before we have Egg cells Sperm Gene Frequency Cene Frequency A 9 9 A 9 Children Gene Frequency pa da 09 da 29 ag q?

·106 GENES IN MENDALIAN POPULATIONS The composition of the population will, therefore, be pt of AA homozygotes, 2pg of Aa heterozygotes, and qof aa homozy gotes. The frequency of the genes A and a in the gene pool of the next generation will be: A=pa + pq=p(p to 9) = = pq + q* = 96 +9)=9 P A Pa AA AA PA g? pg AL FIGURE 10 This checkerboard diagrain shows the relationship one expects to find between the frequency of 2 genes in a population and the frequency of inde viduals of the 3 possible genotypes in principle this diagram is identical to that in Figure 5 Gene frequen cies in a population need not be limited, as is that ol individuals, to the three values-100 50, and 0 percent but can assume any value, and, therefore, the duvid ing lines within the square have been displaced off center by an arbitrary distance This demonstration of the constancy of the gene frequency in the gene pool of a population was first given independently 1 in 1908 by G. H Hardy and W. Weinberg It is the basic theorem of population genetics (see Figure 10), ·RANDOM MATING 107 Random Mating The assumption we have made above is that marriages and the union of gametes occur at random The validity of this assumption may now be examined "Random mating' obviously does not mean promiscuity, it simply means, as already ex plained above, that in the choice of mates for marriage there is neither preference for nor aversion to the union of persons similar or dissimilar with respect to a given trait or gene Not all gentlemen prefer either blondes or brunettes Since so few people know what their blood type is, it is even safer to say that the chances of mates being simlar or dissimilar in blood type are determined simply by the incidence of these blood types in a given Mendelian population In addition to the ABO and the Rh blood group systems, there is another set of blood types called M, MN, and N, which is independent of the foregoing ones The interest of these latter types is that the heterozygotes in this case are easily distinguish able Group M persons are homozygous for a gene which we may call A1, these are AfM1 persons Group N persons are ho mozygous for a gene which we may call M* And finally, group MN persons are the heterozygotes, MIM* Neither gene is domi nant to the other A study of 6,129 white persons of the United States popu lation showed that 29 36 percent had M blood, 49 58 percent MIN, and 21 26 percent had N blood What is the composition of the gene pool of this population? Remember that persons of blood type a produce gametes with the gene AI, N persons, gametes with the gene A1®, and MN persons produce sex cells with M and with my in equal numbers Therefore, the fre quencies of the two genes in the gene pool are M = 0 2916 +0 2479 – 05395 M = 0 2479 + 0 2126 = 4605 = 4 Noss, if marriages in this population are at random with respect to these blood types, then, according to the Hardy Weinberg ·equation, the proportion of the three blood groups in the popu lation sliould be M = p = 0 542 = 0 2916 MN = 2pg = 2 X 0 54 X 046 = 14969 N=q2 = 046 = 02116 The predicted incidence of the three blood types coincides almost exactly with the observed incidence There is obviously random mating in the population of the United States with respect to these blood types Gene frequencies are not constant from race to race in fact, races can be understood and defined best as Mendelian populations differing in the frequencies of various genes Al though within each race matings may occur at random with respect to certain traits matings between races are less frequent chan would be expected chance Indeed, if random mating prevailed between races these races would fuse in a single Aſen delian population Although individuals in such a population would show a great deal of genetic variability, the races would no longer exist as distinct entries Now, in a group of American Indians the incidence of blood types was found to be N = 6000 percent, MN = 35 12 percent, and N=488 percent The fre quencies of the genes AI and M* in the gene pool can be calcu lated, as in the example above, to be M = 0776 and AI' = 0224 Ji mating were at random, the incidence of the three blood types should be M = 60 15 percent, MN = 34 81 percent N=504 percent The agreement between the observed and the calculated values is again extremely close But note that the gene pool of American whites has a composition different from that of merican Indians Changing Gene Frequencies by Mutation and Selection The Hardy Weinberg theorem shows that gene frequcn cies in a Mendeltan population tend to remain constant from generation to generation This is, however, correct only if (1) genes do not change by mutation, (2) the carriers of differen!

·. CEAXGING GENE TREQUENCIES BY MOTATION AND SELECTION 10$ genes are adaptively equivalent and leave, on the average, the same number of surviving offspring, (3) there is no immigration to or emigration from the population that would bring in or remote one gene in preference to the other, and (4) the popu Lauon is large enough so that accidental ſuctuations of gene frequencies may be ignored No population in reality satisfies these conditions completely రేపే FIGURE 11 An analogy to illustrate the idea of genetic equilibrium the more water flows from the faucet into the vessel the higher the level of water in the vessel becomes and consequently the faster it escapes from the hole in the bottom Equilibrium is established when the rates at which water enters and leaves the container are equal In living populations harmful mutations are constantly produced (water from the faucet) the more frequently murauons arisc per generation the greater the frequency with which they are found in the population (the level of water in the vessel) and the grcater the frequency of genetic death (water escaping through the hole in the vessel) We know that genes do undergo mutation, albeit usually with a low frequency such as I new mutant gene per 100,000 sex cells per generation Nevertheless, even such a low mutation pressure will eventually be enough to influence the gene fre quency in the population Suppose that the gene A is occasion ally converted into a by mutation The mulation of A to a 15 to the frequency of the gene a in a population as the flow of water from a faucet is to the amount of water in a tub in itself the mutation does not determine the final gene frequency, but ·110 GENES IN DENPELJAN POPULATIONS without mutation the frequency of the mutant allele would surely be zero The final frequency attained by a gene is one representing a dynamic equilibrium, this frequency is constant (or very nearly so) not because life is static and the frequency is frozen at one level but because rates of input and outſlow have become equal Using the tub analogy once more If the scopper is not placed properly water will drain out of the tub It will drain out faster as the water gets deeper and deeper Eventually the water in the tub may be deep enough to drain out as rapidly as the faucet is running the amount of water in the tub remains constant once this equilibrium is established (see Figure 1) The water in the tub is not however, the same from one minute to the next it is in dynamic equilibrium There are two forces which oppose mutation in bringing about an equilibrium of gene frequencies These are (1) back mutation of a to A and (2) natural selection of the two natural selection is of more importance and of greater interest to us Genetic Equilibrium The genetic equilibrium with respect to tuo variant (allelic) genes in a population is reached when one of these genes enters the population at the same rate by which it leaves It enters by mutation and leaves by back mutation and by selection Let us consider some examples of selection The simplest illustration 15 probably that of a fatal hereditary discase caused by a recessite lethal gene Assume that the normal gene A mutates to the lethal a once in every million gametes and that all as ind: viduals die before reproducing Equilibrium is established whert one individual in every million bom is homozygous on elim nation of the gene a by the death of an individuals then balances the origin of new a genes by mutation We saw above that the Frequency of aa individuals equals q?, therefore, q, the equi. librium frequency of the gene a, equals 0 001 the square most of the mutation rate (see Figure 12) ·GENETIC EQUILIBRIUM . 111 To better visualize this situation, imagine 170 persons in the population of the United States, or one person in a million, dying in each generation (25 to 30 years) from a recessive lethal mutation of this sort-an average of about 6 deaths per year in the whole country This may seem to be a very rare disease It is however, important to realize that although the number of polto A O Povu FICURE 12 Another checkerboard one which is intended to represent the equilibrium that is established between mutation and selection in the case of a recessive lethal gene (a) The rate at which a arises from gene A equals u usually a small fraction The shaded portion of the diagram represents aa individ nals who die because of the lethal nature of this gene Each death of an da individual removes two mutant genes from the population one from the initial supply of these genes among sperm and the other from the initial supply among eggs Consequently equilib rium is reached when the shaded area equals u Each side of the shaded square corresponds to the fre. quency 9 of the gene a in the population (see Figure 10) Hence g2 equals u ot q the gene frequency equals vÈ Note that the quantity 92 15 much smaller than a ·112 GENES IN MENDELIAN POPULATIONS homozygotes dying because of thus disease is small the number of people who are carriers of the recessile lethal gene in hetero zygous condition is relatively large According to the Hardy Weinberg theorem, the frequency of the heterozygous carriers is 2pq, or 2 X 0001 X 0999, or about 0 002-roughly I per 500 persons in a populavon as large as that of the United States, the number of such persons who enjoy normal health them selves but who nevertheless carry the lethal gene is about 340 000 This is about the same number of persons as the number living in Syracuse New York The great majority of recessive genes for lethal and semi lethal hereditary diseases ate, thus carried not in the homozy gotes who show the effects of these genes but rather in the heterozygotes who do not This fact rauses a tremendous diffi culty for any attempt to eradicate a recessive hereditary disease by sterilizing or otherwise preventing the reproduction of per sons who are affected with it In the example discussed above only 170 persons show the disease in a population as large as that of the United States but there are 340 000 heterozygous carriers Since we have not discovered a method of distinguish ing these heterozygotes from normal noncarriers there is little that can be done to prevent the birth of a new crop of affected homozygotes in every generation As we have seen in foregoing chapters not all tions are completely lethal Recessive semilethai and subvital mutants cause the death of fewer than 100 percent of their homozygous carriers in other words, some of the homozygotes manage to survive and even to beget children of their own In general, if a fraction s of all individuals homozygous for a gyen gene die before reproducing (or if the number of children born to such individuals is lower than the average number per couple in the population by a fraction s), then an equilibrium is estaba lished when sqa equals the muration rate We can translate sqa as the fraction lost per homozygote multiplied by the frequency of these homozygotes in the population if eve denote the muta tion rate by u, we have a stable equilibrium when $9% = 4 gene muta or, qu/s or, q = Vu7s ·CENETIC DEKK 113 Is equals l--that is of all homozygotes due or for other reasons lease no children-we have the equilibrium q = discussed abore To illustrate the more general relationship consider a gene B which mutates to b once in a million sex cells per gen eration The effect of the gene b is to reduce the average number of children born to bb parents (esther to bb mothers or to bb fathers what happens when both parents are bb is relatively unimportant because of the rarity of such matings) to 99 percent of the average of the population as a whole The value of s equals then 001 The frequency of bb individuals at equi hbrium will be, according to the formula above q? 0000 001/001 or 1 per 10 000 persons in the population within the United States there would be some 17 000 homozygous bb persons bom in each generation or some 600 per year The frequency of the harmful gene b in the gene pool at equilibrium Vats - VOOOOT 00L that is 1 percent of all sex cells will carry this gene The heterozygous carriers of this gene will amount to almost 2 percent of persons in the popula 2 X 001 X 0 99 00198 or a total of 3 400 000 heterozygous carriers in the population of the Unued States Gathered into one place these carriers would form a city large enough to be listed among the ten largest in the world! tron 29 Genetic Death It must be made clear just what is meant by the elimination of mutant genes from a population The symbol s used above stands for coefficient of selection This is a very smportant farsable and its meaning demands a fuller explanation Two rather striking phraſes have been used by geneticists in describe ing the action of selection on human populations genetic death' and genetic murder will out Genetic death encom passes, however a great deal more than death as we understand 1t from accounts of automobile accidents and hatchet murders First of all, complete or partial sterility is equivalent to ·114 GENES IN MENDALIAN POPULATIOMS complete or partial lethality as far as selection is concerned To a geneticist, any person who dies childless is counted as genetically "dead'. that is, whatever genes he carries are, by virtue of his actions or lack of actions, removed from the popu lation just as surely as if he had been stillborn It is obvious, however, that the emotional impacts of death and of childlessness are in no way comparable Second, if the mutant gene u lethal or semilethal, the actual death caused by the action of this gene can occur at any time from conception on The loss of an individual within the first few weeks after conception is often imperceptible and, hence, causes no emotional trauro.. Death at any later ume, however, is recognizable as such and is accompanied by at least some degree of sadness and sorrow We must, nonetheless, guard against ngdly equating the seri ousness with which we regard the deaths of individuals and the degree of mourning or sense of loss accompanying these deaths If there must be deaths, by all means let them occur early in development, so as to be as painless to the individual and to others as possible But, if deaths are unnecessary, let us not develop the callous attitude that undetected deaths are of no importance How easily this attitude can spread to encom pass even living persons! Finally, there is a perhaps unexpected property of selecuon that must be clearly understood We know that many miutant genes are lethal, and that some of these cause incurable heredi tary diseases which result in death only after much illness or agony On the other hand, a subvital gene opposed by a relatrely mild selection, such as we have discussed above (s=001), may act simply by lowering the vitality of its carriers to the extent that they leave 99 surviving children where an equal number of nonmutant individuals would have left 100 offspring At first sight, the lethal gene would seem to cause many more genetic deaths than the suby ital gene But this is not so at all Provided that the mutation rates, u, which yield the lethal and subvital genes are equal, these genes will cause the same number of genetic deaths in the population at equilibrium. This fact becomes apparent when we consider the implica tions of the equation ga : =u/s, used above to describe the equi ·GENETIC DEAR • 113 librum frequency of mutant homozygotes in the population It will be recalled that the selection coefficient s, is equal to the amount by which the chance of surviving or the average number of offspring is reduced in homozygous individuals The net effect of the mutant gene on the populatson, is u/s timess the frequency of affected persons times the average adverse effect per person This product equals 4, the mutation rate The con clusion is clear The effect of a recessive mutant gene on a population as a function of its mutation rate, and is independent of the effect as measured by the selection coefficient of the mutant on homozygous individuals This does not imply how €147, that the emotional impact of these various types of mutauna u at all similar To watch a child suffer from hemoplulia is quite different from realizing-if one could realize such things ---that a childs life expectancy is destined to be one or two years less than that of his playmates We have considered the genetic equilibrium for mutants with recessive harmful effects The situation with respect to genetic deaths us analogous for dominant deleterious mutants but the effect on the population is twice as large A dominant lethal which produces a fatal disease (such as some inherited forans of muscular dystrophy) or a subvital which produces only a slight handicap for survival or reproduction (such as brachy dacıyly short fingers) causes twice as many genetic deaths as there are mutations If the reader is still baffled by the equiv alence of the sery bad and the only mildly disadvantageous mutations perhaps the following argument will help Any harm ful mutant whether its effects are drastic or barely perceptible will sooner or later have to be removed from the population in which it occurs Indeed the genetic equilibrium is a situacion obtaining when the number of mutants produced is balanced by the number of mutant genes removed from the populauon A harmful gene that comes into a population eventually has to get out The facts listed above have an obvious bearing on the problern of gencuc damage caused by radiation in human and other populations As will be discussed in more detail in the next chapter, this genetic damage cannot be measured adequately by ·116. CENES IN MENDELIAN POPULATIONS taking into account only gave hereditary diseases Mutant genes with less deleterious effects result in genetic damage just as surely as the more drastic mutants Genetic Loads in Drosophila Populations We have seen thac mutants with recessive deleterious effects accumulate in populations up to certain theoretically calculable equilibrium frequencies For example, a recessive lethal that arises by mutation once in every million gametes will have an equilibrium frequency of one per thousand in the gene pool Since, at least in higher organisms, many genes are capable of producing delecerious mutants, the gene pool of a population will contain great numbers of genes for recessive hereditary dis eases, malformations, defects and weaknesses of all kinds in every generation some deleterious dominant and semidominant mutations will also be produced Since dominant genes harm heterozygous individuals, they do not accumulate appreciably in a populacion, they are, nevertheless, a source of all health and misery Deleterious mutant genes of all kinds constitute the genetic load of a population The magnitude and the composition of genetic loads in human populations cannot at present be estimated with any precision, although in the following section we shall consider some data relevant to this question Better data are asarlable for populations of several species of fruit flies, Drosophila The methods used to obtain these data cannot be discussed here in detail Very briefly, they consist of taking a sample of a Drosophila population in its natural habitat and making with these "wild fies a series of crosses in the laboratory These crosses are so contrived that eventually a series of cultures is obtained each of which contains both a class of Pues carrying in homo zygous condition a certain chromosome present singly in one of the wild progenitors and another class of fies, recognizable by its external appearance, wluch carries the same "asild chro mosome and also another chromosome of laboratory origin ·GENETIC LOADS E DROSOPHILLS POPULATIONS 117 Suppose now that a wild chromosome contains a recessive lethal The fly which in its natural habitat carned this lethal was seemingly normal because the lethal was concealed in the heterozygous state But in the laboratory culture in which a class of fies homozygous for this chromosome 15 produced the presence of the lethal will be evident because the class will be absent. If the captured fly carried not a lethal but a semlethal a subvital or a recessive gene producing some visible abnor mality in the fly's body each of these genetic variants will be detected in the test culture in which homozygous individuals have been formed The method is very sensitive if a sufficiently large number of wild chromosomes is examined one can make a census of the deleterious genetic vartants that make up the genetic load of the fly population The application of this method has disclosed tfiat natural populations carry a large number of deleterious mutant genes concealed in heterozygous condition The frequency of chese mutations will amaze many persons Let us examine the data for just one species Drosophila pseudobscura This species has 5 pairs of chromosomes of which 3 were examined Between 2 and 33 percent of each of these chromosomes proved to be lethal or semilethal to their carriers when homozygous Among the chromosomes that were free of lethals or serilethals be 11 een 41 and 95 percent were subvital in double dose-in other words they incapacitated their carriers percepubly but not to the extent of being lethal or semilethal Between 4 and 14 per cent of the chromosomes caused complete sterility of homozygous females and between 11 and 18 percent made homozy gous males completely sterile A considerable proportion the chromosomes made the homozygotes grow and develop more slowly than normal fies some of them as much as doubled the lime required by the fly to develop from an egg to the adult Stage Sall other chromosomes contained recessive mutants that changed the color of the fly s eyes of body the shape of the wings etc lle can predict that a fly wlisch ss free of deleterious genes of all sorts-lethals viability modifiers sterility factors and the rest-will be found only rarely if ever ·118 . GENES IN MENDELIAN POPULATIONS Genetic Loads in Human Populations Since experimental methods such as those used with Drosophila are inapplicable to man, we are reduced to inference by analogy and to evidence of an indirect nature Assume, as before, that a gene mutates to a recessive lethal state in one per million sex cells, and that its equilibrium fre quency in the gene pool is consequently 0 001 Should man have as many as 10,000 gene loci capable of giving rise to such recessive lethals, there should be, on the average, about 10 lethals per sex cell or, in the average individual, some 20 mutant genes each of which is capable of kıllıng when homozygous This number may be an overestimate, because some-possi bly most-lethals are not completely recessive but have a slightly deleterious effect on heterozygous individuals Lethals therefore do not accumulate in populations up to the frequencies ex pected if they were completely recessive, in studies with popu lations of Drosophila, it is frequently found that lethals are only half as frequent in the population as one would expect from their mutation rate If lethal genes in man behave similarly, there may still be as many as 5 lethals on the average per human sex cell, or some 10 concealed recessive lethals per individual Another approach to an estimate of the genetic load in man utilizes the fact that the incidence of genetic deaths caused by mutation of a given gene is, at equilibrium, equal to the mutation rate and is independent of the kind of harm caused by the mutation If, then, the total mutation rate were known (that is, the proportion of sex cells carrying newly arisen mu tants of any gene), we should have a basis for estimating the collective effect of these mutations on the population Estimates have been made indicating that lethal mutations represent about one fifth of all mutations which can occur at a given gene locus that is, for every mutation to a lethal gene there are four muta tions to genes with minor deleterious effects Admitting that we do not know the average mutation rate to lethal genes, we may guess that it lies somewhere between 0 000,01 and 0 000 001 Assuming that man has some 10,000 genes per gamete, and mul ·COMPX RECESSITE DEFECTS - 115 uplying the mutation rate to lethals by five to obtain a measure of the total mutation rate, we may are at the total effect of mutation on human populations Because of the many uncertainties involved in these calcu fauons, the resulting estimates range from to 50 percent In other words between 5 and 50 percent of the fertilized egg cells in man are "lose" through failure to develop, spontaneous abort clon, miscarnage, neonatal and infant death, partial or complete stenkty, and sarious forms of ill health L S Penrose, an eminent human genetics, has estimated that any one generation of human beings anses from fewer than 50 percent of the indı viduals of the preceding generation The Genetics Commstree of the National Academy of Sciences has estimated that about 2 percent of all children born in the United States suffer disorders af genetic origin Each of these estimates is compauble with our calculations, obviously, then, more reliable data are needed in this area. Incompletely Recessive Defects In the discussion above we have assumed that for the most part deleterous mutant genes are completely recessive to their normal counterparts (alleles), so that heterozygous carrers of these mutants are as fit and vigorous as the normal homozygotes However, as with many assumpuons, this one is not always valid we must now inquire how much violence our conclusions would suffer if we gave it up Tuo possibslicies must be ex amined First, that the defective mutant gene incapacitates het erozygotes as well as homozygotes, though the former less so than the latter, second, that the defective gene makes heterozygoces more vigorous than normal homozygotes which do not carry the gene We will discuss only the first of these possi bilities in the present chapter Suppose that a recessise lethal mutant (that is, one which kills all homozygotes) has a slighe deleterious effect on the health of heterozygous individuals Perhaps the chances of reaching ·I20 GENES IN MENDELIAN POPULATIONS adulthood are only 95 percent as great for heterozygotes as they are for persons free of this lethal, or perhaps heterozygotes produce, on the average, only 95 percent as many children as homo zygous normals do In other words, the heterozygotes have a 5 percent disadvantage in survival, or in childbearing or rearing or in a combination of all these factors We may also use this percent coefficient of reduction of the survival and reproduc tive efficiency to calculate the effect of mutant genes which are not lethal but produce various relatively minor incapacities (that is, semilethal and subvital conditions) Thus a mutant which lowers the fitness of homozygous individuals by 10 per cent may be assumed to lower that of heterozygotes by 005 X 0 10 = 0005 (5 per 1,000, or 05 percent) Considering each kind of defect separately, most recessive defects and hereditary diseases are rare in human populations although in the aggregate they are responsible for much misery As pointed out above, rare recessive mutant genes are carried chiefly in heterozygous individuals Fren if the harm done by such genes to the heterozygote is small, it will have an over whelming effect on the equilibrium frequency of the genes in the population We have seen that a completely recessive lethal arising once in every million gametes would reach an equilib rium frequency of I per 1,000 in the gene pool-one thousand times the mutation rate if, however, this lethal lowered the survival or fertility of heterozygous individuals by 5 per cent it would accumulate until its frequency were only 20 times the mutauon rate-1 in 50 000 Similarly a gene with the same muita tion rate which lowered the survival of homozygotes by i percent (these homozygotes would be 99 percent 'normal ) would reach an equilibrium frequency of 1 per 100 If, on the other hand this same gene lowered the fitness of heterozygotes by 5 percent of i percent, or by 0 05 percent, the equilibrium frequency would be only 1 per 500 The general equation that expresses the relauon ship between mutation and selection for an incompletely retes. sive gene at equilibrium is u = pqhs, where u is the mutation rate, pand q are the frequencies of the normal and mutant genes respectively, h is the fraction of the mutant genes effect which is expressed in heterozygous individuals and s is the effect of ·COMPLETELY RECESSITE DEFECTS 121 the mutant gene on homozygotes Since che mutant gene is rare, p is very nearly equal to 1 Thus g can be shown to equal u/hs (see Figure 13) pri u/hs 02/13 A O prisu/As A @ FIGURE 38 An allustration of the equlibrium that 15 established between mutation and selection in the case of an incompletely recessive gene First we note that the shaded area is much larger than the small square in the lower nght hand corner any apprecable effect of the mutant gene on heterozygous inds viduals will overshadow that on the rare komorygotes the latter may be ignored in our calculations Gene a anses Eromu A at a rate w it is lost from the population at a race which corresponds to hs (the disadvantage of heterozygotes) x 2pq (the frequency of heterozygotes in the population) * ¥ spre me elimination of a heterotygore remoses only 1 a gene) Thus Aspa equals u or sunce p is very nearly 100 4 equals u/hs The gene frequencies of incompletely recessive defects would therefore be lower at equilibrium than if they were completely recessise in the balance sheet we hale been keeping for the population, however, the adverse effect of these genes ·122 • GENES IN MENDELIAN POPULATIONS would be greater than calculated before In fact, the effect of a mutant gene which lowers the viability of heterozygous ind, viduals is about twice as great as that of a completely recessive gene The reason for the larger effect is that the harmful muta tion will be relatively rare in the homozygous condition and will, therefore, exert its effect almost exclusively through heterozygous individuals As we saw above, the equilibrium frequency, q, for a gene which lowers the survival or reproductive ability of heterozygotes equals u/hs The frequency of heterozygotes us, of course 2pq Since q is very small, p. (149) can be consid ered 1 This leaves the frequency of heterozygotes as 2u/hs The amount by which heterozygotes are affected equals hs, and the product of 2u/hs times hs equals 2u The biological meaning of these computations is that an incompletely recessive mutant gene which adversely affects het erozygous individuals lowers the fitness of the population by twice its mutation rate We saw above (page 115) that the number of gencuc deaths produced by a completely recessive deleterious gene is equal to the mutation rate, and is inde pendent of the degree of harm this gene produces This inde pendence of the number of genetic deaths from the degree of harm produced obtains also for incompletely recessive mutants, but here the genetic deaths are twice the mutation rate There appears to be no easy escape, mutations will take their toll How Much Time Is Required to Reach a Genetic Equilibrium? Mutation rates are not necessarily constant through time There is indeed, a danger that mutauon rates may go up in connection with the use or misuse of atomic energy Nor are intensities of selection constant--Modern medicine, for example promises to diminish the selection coefficients of some genetically conditioned defects We have discussed gene frequencies at equilibrium and we have seen how these equilibria arise ·HOW MUCS DE IS HEQURE TO Exca x GERETIC EQUILIBRIUS? . 123 through the opposing forces of mutation and selection the influx of gene mutations into a population and the siphoning off of these mutations by differences--sometimes slight, sometimes marked-in survival and fertility How long does it take for an equilibrium to become established? The answer to this question depends on the nature of the gene under discussion but in general it takes a long time Let us consider once more the example of a recessive lethal that arises by mutation once in every million gametes The equi librum frequency of the gene is 0001 If I were possible to postpone selection until equilibrium had been established, it would require 1 000 generauons or in man some 30 000 years for the frequency of this lethal to increase in a population from percent to Ol percent the equilibrium value Selection o£ course does not wast and so there will be occasional homozy gous individuals formed long before the frequency of this lethal reaches 0 I percent consequently it will take much longer than 30 000 years to attain this frequency Recessie genes with effects less drastic than complete le thality will take even longer to reach their final frequencies We have seen that in these cases the equlbrium frequency 15 equal to Vu7s The quantity s (the rate of selection) is usually greater than u the mutation rate Consider a mutant gene which makes the homozygotes 99 percent normal and which aruses once in every million gametes Its equilibrium frequency will be 001 It would take much longer than 10 000 generations or 300 000 years for this frequency to be reached This is a time Interval comparable to the known span of the existence of mankınd as a species since the first manlike creatures appeared on earth probably less than a million years ago In the case of murauons with dominant or semidominant deletenous effects, equilibria are reached sooner than in the case of recessives Nevertheless a mutant which is lethal when homozygous and which reduces the fitness of heterozygotes by percent would take 20 generations or between 5 and 6 cen furies to reach about half of its final equilibrium frequency A mutant which in the finore of harmime I narrent 5 ·126 .CENES IN MENDELLAX POPULATIONS and that of heterozygotes by one twentieth of this amount would still be far below the equilibrium frequency in 2000 genera trons, or 60,000 years These calculations may seem to indicate that deleterious mutants never reach their theoretical equilibria in human pop ulations However, it is important to keep in mind that at no time has mankind had a set of normal genes free from delererı ous mutations Species--including that of man-do not arise from Adams and Eves, they arise from populations that in one way or another have become separated from the rest of the species Geographical isolation is generally the initial cause of this sep aration An isthmus sinks under the sea, for example, or in following a retreating glacier some members of a species go to one side of a mountain range, of a desert, while the rest go to the other side The array of genes present in these separated populations serves as the foundation stock on which still further genetic changes occur Some of mans geneuc characteristics may have arisen long ago in our prehuman ancestors Indeed the ABO blood types occur not only in man but in anthropoid apes as well, there is every reason to think that they were present in the common ancestors of men and apes, who lived probably several millions of years ago The problems of genetic equilibria and the time required for their establishment become very real when we consider the consequences of the genetic damage radiation may do to human populations Suppose for example, that the use of atomic energy for power will become unnecessary within 5 or 10 generations because of advances in the utilization of solar energy The in creased mutauon rates during these 5 to 10 generations will certainly not suffice to bring every type of deletenous mutation to new and higher equilibrium frequencies We muse empha size, however, that whatever extra mutations will have been induced in manlınd will take their toll through the inexorable process of selection This elimination of mutant genes will not happen in a single generation, it will be strung out through a great many future generations This matter will be discussed 10 more detail in the following chapter ·HOW MUCH TIME IS REQUIRED TO REACH A GENETIC EQUILIBRIUMI . 125 It is important to distinguish between the genctic damage done per generation and the total damage done to 1 population Simply because our generation-or our children s generationmay not suffer greatly does not mean that the total dımage to mankind will be slight We cannot disregard the damage thirt will befall future generations even though our emotional in volvement in these generations is slight We must remember that no matter how long it may take to re establish a genctic equilibrium the total damage done by a given number of mutant genes is determined by that number and that number alone

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·7 Genetic Effects of Radiation on Populations Surveying the Problem We saw in Chapter 5 that exposure of living beings to high energy radiations makes mutations more frequent Since most mutations cause some damage to their carriers increased muta tion means increased damage In Chapter 6 we considered how mutant genes accumulate in sexually reproducing populations and how natural selection removes mutants from such popula Lions We must now put these things together; we shall try to arrive at a reasoned judgment concerning the probable extent of the genetic radiation damage to living populations, particu Jarly to human populations It is probable that man made radiations will be a perma nent feature of the human environment from now on and that, as a result, sustation rates ss man and in other organisms will kenceforth be higher than they were in the past On the other hand, it is conceivable that the use of atomic energy will be merely a transient phase in the history of technology and that substitution of solar energy for atomic energy will make wide- ·128 . GENETIC EFFECTS OF RADIATION ON POPULATIONS spread use of the latter unnecessary some 5 or 10 generations from now If so mutation rates will increase temporarily but will eventually revert to their present values A temporary in crease in mutation rate could also result from a world war in which atomic weapons were used on a large scale thus exposing a large portion of humanity to abnormally intense radiations We must therefore attempt to deal with two kinds of questions (1) the effect of permanently increased mutation rates on human and other populations and (2) the length of time required to cleanse the gene pools of human populations of the additional mutations produced by a temporary increase in mutation rates Our problem is complicated by the fact that as we saw in Chapter 6 different kinds of mutations behave differently in populations Completely recessive mutants which harm homo zygotes but have no effect at all on heterozygotes accumulate in populations to much greater frequencies than do harmful mu tants that are at least partly dominant and injure heterozygous individuals as well Different again is the behavior of mutants that damage homozygotes but are actually favorable (heterotic) in heterozygous condition Mutants with such ambivalent action as we shall see later reach equilibrium levels that are deter mined almost entirely by natural selection and are largely inde pendent of the mutation rate Then there is the time factor The equilibrium frequencies which were discussed in the last chapter are attained by harmful mutants only after an infinite number of generations during which the mutation rates and selection coefficients must remain constant In reality no biological variable remains constant forever We shall be more realistic if we estimate the magnitude of the genetic load in a population after a limited number of generations For this purpose we can use 10 100 and 1000 generations as convenient milestones in the history of the population لا tion when in the United States the thirteen original WIV already established but most of the land west of the Appalachian Mountains was still controlled by the native Indian tribes. In ·SUAVEYDNG THE PROBLEM. 12$ Europe, various absolute monarchs reigned and amused themseltes with their favorites and courtiers One hundred genera- Lions (2 500 years) ago the Old Testament already governed the lites of a small tribe inhabiting the hills of Judea Although the Pyramids were hoary with age, the classical period had not yet dawned in Greece One thousand generations (25,000 years) ago the human species, Homo sapiens, was already in existence, busily shaping stone tools and producing an occasional painter of genius whose caye wall drawings we still admire Let us try to project these time intervals into the future Events of 10 generations ago are capable of producing strong emotional reactions (witness the requirement for joining the Daughters of the American Revolution), we may be seriously concerned about the state of affairs 10 generacions hence. The actions of people 100 generations ago still evaluate as ad mirable or reprehensible But we can scarcely hold men of 1,000 generations ago responsible for their deeds, even though these may possibly have affected us Perhaps men living 25,000 years hence will vsew us as tolerantly-a primitive people not wholly responsıble for deeds committed in ignorance, but a people who on occasion produced a masterpiece of thought or As far as genetic damage is concerned, sankınd is, in the long run, a single population The mutants induced in the sur vivors of Hiroshima and Nagasaki will not be confined to future populations of these cities Because of the mobility of people, some of these mutants have undoubtedly diffused throughout Japan and, since there will always be some migration from country to country, may find their way to populations of all parts of the world No nation should contemplate with equa nimity the possibility of an atomic war, even if that partrcular nation were sane enough not to participate Nor should anyone leel satisfied provided only he is protected from radsatson exposure Although some people, such as radiologists and those mo sandie radioactive materials, are professionally incurring greater radiation exposures than others, mutations induced in them will enter the common gene pool of the entire population. The only fair measure of the extent of genetic radiation damage action ·130 GENETIC EFFECTS OF RADIATION ON POPULATIONS is given by the mean radiation exposure-first of the people of a given country, and then of mankind The future of one mans genes is, ultimately, dependent on that of everyone else's Radiation-induced Recessive Mutants at Equilibrium Consider a Mendelian population which, from a certain time on, is exposed to man made high energy radiations Before that time, when the population was exposed only to back ground radiations, its genes were mutatıng "spontaneously at a certain average rate, for which we have used the symbol u (see Chapter 6) The additional increment of mutation, the magnitude of which will of course depend on the amount of additional radiation exposure, may be designated as u The total mutation rate in the population in question will then be the sum of u and u', this sum we shall write as U. In homozygous condition, recessive mutant genes are likely to produce hereditary diseases, malformations or constitutional weaknesses Suppose that a mutant lowers the fitness of its car riers by a fraction s As shown in Chapter 6 this mutant will accumulate in the population to an equilibrium frequency value of q=Vu/s This equilibrium will be reached when the mutation rate, u, equals the rate of elimination ( genetic death ) of the mutant genes The elimination rate will, then, be sq* (the selection coefficient, s, multiplied by the square of the fre: quency of the mutant gene in the gene pool, q) The increased mutation rate, U, will raise the equilibrium level, this new equilibrium level is easily estimated as VU7s The total rate of genetic death (U/s X s) will now be U instead of u, the in creased genetic damage produced by the increased mutation rate will evidently be U - u = u', the radiation induced increment of mutation All this may still seem too abstract. To illustrate the magni tude of the genetic damage which exposures to radiation may induce, we shall give an example using numerical values ob- ·RADIATION INDUCED RECESSIVE MUTANTS AT EQUILIBRIUM. 131 tained from studies of Drosophila We do not imply that these values are identical with those for man, but they can serve to illustrate the method used in estimating the magnitude of radı ation induced genetic damage which exposures to radiation may produce In man, the genetic damage would, if anything, be greater than in Drosophila Experiments have shown that in Drosophila not treated with any mutation inducing radiations, about 5 sex cells out of every 1,000 contain a newly arisen recessive lethal mutation in a certain chromosome (the so-called second chromosome) The spontaneous mutation rate for this particular chromosome is, thus 0 005, or 5 x 10-3 The mutation rate in the progeny of flies treated with 1,000 r of x rays is almost ten times as large Hence, about 5 chromosomes per 100 will carry a newly arisen lethal the mutation rate per chromosome will be 005, or 5 X 10-2 From other experiments (see Chapter 5) it has been esti mated that the second chromosome of Drosophila contains, roughly, 500 genes (loci) capable of yielding recessive lethal or other deleterious mutants Drosophila also has another chromosome (the third), which contains about as many genes as the second The two chromosomes together have, then, about 1,000 genes capable of mutation Using these figures, we can estimate average mutation rate per gene The spontaneous mutation rate per gene is evidently u = 0005 – 500 = 0.000,01, or 1 X 10-5, the rate after administration of 1,000 r of x rays is V = 005 – 500 = 0000 1, or 1 x 10-4 Since the frequency of induced mutations is proportional to the amount of radiation applied, it follows that the increment of mutation per gene per 1r of radiation (u) is about 0 000,1 – 1,000 = 0 000 000,1, or 1 x 10-7. It is interesting to note that this figure lies in the midrange of several estimates made by Russell and his collaborators for the average mutability of the 7 genes in mice (see Chapter 5, page 86) The values obtained by these workers for 'chronic" radiation were 2 x 10-7 and 3 x 10-7 mutations per gene per Ir for males and females respectively, for "acute" radiation they obtained values of 4 x 10-8 and 5 X 10-8 for females and males the ·252 . GENETIC EFFECTS OF RADIATION ON POPULATIONS respectively The reader will see that we are not being com pletely 'unrealistic" in choosing fruit flies as the starung point of our argumenti Let us now summarize the estimates for Drosophila in the following tabulation Mutation rate, spontaneous, second chromosome 0005, or 5 x 10 Mfutation rate, 1,000 r of radiation, second chromosome 0050, or 5 x 10-> Number of genes second chromosome 500, or 5 X 10- Spontaneous mutation rate, ut, per gene 0 000,01, or 1 x 10-5 Induced mutation rate, u', per gene, per lr 0000,000,1, orl X 10-7 Genes in the second and third chromosomes 1,000, or 1 X 100 Here we are forced to make a step in our calculations which involves the greatest risk of error For reasons discussed in Chapter 5, most genetsc studies have been made with mutations yielding recessive lechal genes But we suspect thar lethal mu tants-that is, absolutely fatal hereditary diseases--represent a minority of all mutations Since the over all fraction of a popu lacion which is lost through death and incapacitation caused by mutation is equal to the mutation rate and is independent of the degree of harm to afflicted individuals (see Chapter 6), it becomes necessary to estimate what fraction of all mutations 11 lethal and what fraction produces less pronounced damage Several geneticists hase estimated that the lethal fraction » about one fifth, and for want of a more reliable figure we shall use this estimate We shall also ignore the fact that Drosophila has another large chromosome in addition to the second and third, this chromosome is insolied in the determination of sex and its behavior in populations is a separate story, which seed not concern us here We have seen that the spontaneous mutanon sate produs ing lethals in Drosophila is u 0000 01 per gene per genera tion For 1,000 genes this means that about 0000 OL X 1,000+ 001, or about 1 percent of the sex cells, carry a newly ansen spontaneous lethal in each generation The incidence of all mutauons as, then, 001 X 5 = 005, or 5 percent per generator.

·BADIATION INDUCED RECESSIVE MUTANTS ON THE WAY TO EQUILIZRVM This means that at equilibrium about 5 percent of the indi viduals in each generation will be subject to genetic death owing to spontaneous mutahon By how much will the rate of genetic death be increased if a population is irradiated? The Genetics Committee of the Nauonal Academy of Sciences has recommended that the mean exposure of sndividuals in human populations should not be allowed to exceed 10 r before the age of 30 years (the age cor responding to about the middle of the chuldbearing period in the population of the United States) In Drosophila the lethal muta tion rate per gene per single roentgen us as we have seen above 0 000 0001 Thus we should multiply by 10 (the number of roentgens to which the population may concesvably be exposed) and also by 1000 (the number of genes) and by 5 (to take into account the deleterious mutants that are not fully lethal) The result is 0005 or 05 percent In other words at equi librium the rate of elimination of harmful mutants (the fre quency of genetic death) in a population continuously exposed to 10 r of man made radiation would be 55 percent instead of 50 percent Is this increase large enough to worry about? A 10 percent increase in the frequency of hereditary alls in the suffering of the families inolved and in the financial burden imposed not only on those families but on the population as a whole is certainly something to consider seriously Radiation induced Recessive Mutants on the Way to Equilibrium The calculations abose tell us what happens to a population exposed to radiation for an infinite number of generations or at least for a number large enough to permit all kinds of mu tants to approach their equilibrium levels He must also con sider the more immediate generation by generation aspect of the problem Let us assume that a population is from a certain time on exposed to a certain amount of man made radiation in etcty generation Let us also assume that this population had reached ·134 - GENETIC EFFECTS OF RADIATION ON POPULATIONS an equilibrium state for spontaneous mutations before the ex posure to radiation began We may, then, limit ourselves to consideration of the effects of radiation induced mutants, which are added to the mutants arising spontaneously Without radi ation, a mutant arising spontaneously at a rate u per genera tion will reach an equilibrium frequency in the gene pool of Vu7s (see page 112) The additional mutations induced by the radiation arise at a rate u' per generation After 1,2 3, n generations of irradiation, there will be lu', 24', 3u', nu' induced mutants added to the gene pool If n 18 small the frequency of the mutant gene in the gene pool in the nth gen eration will be approximately 4n = 9 to nu', or q» Vu75+ nu' The frequency of individuals homozygous for this mutant gene the population will, consequently, be one (Vu/s + nu')" = u/s + 2nu Vüts + (nu')2 Since the quantity (nu ) will generally be small enough to be neglected, the increase in the proportion of homozygous individuals ascribable to the radiation exposure may be adequately estimated as g; – qe 2nu' Vu/s How great an effect will this increased frequency of mutant homozygotes have on the welfare of the population? How many genetic deaths will it cause? This effect is measured as the prod uct of the radiation induced increment of homozygous mutants times the degree to which these mutants are incapacitated by genetically conditioned ill health-(2nu' vu7s)s =anu' vus This formula is worth examining in detail, since it discloses some important facts First of all, notice that the formula con tains the value s, the selection coefficient This means that the damage done by the additional radiation induced mutants after a specified number of generations but before a new equilibrium has been reached is not independent of the degree of harm they produce The more drastic mutants-lethal or semilethal herede tary diseases will cause greater harm than will milder disturbances Notice also that the formula contains the value 1, the spontaneous mutation rate This means that the more frequently the mutation arises spontaneously, the more harmful the radi ation induced increment of mutation will be The reason for this is that most homozygous mutant individuals carrying an induced mutant gene will have a mutant of spontaneous Origin ·RADIATION INDUCED BECESSIVE MUTANTS ON THE WAY TO EQUILIBRIUM . 135 as the other member of their homozygous pair (Homozygous individuals carrying 2 induced mutant genes will have the frequency (nu), which we have neglected as insignificant) Now let us try to translate this algebra into figures whose meaning is easier to visualize Consider recessive mutants which are lethal when homozygous, and which consequently have a selection coefficients - 10 As before, assume that the organism has 1,000 genes capable of producing such lethal mutants, that the spontaneous mutation rate per gene is u = 0.000,01, and that the induced mutation rate per gene per roentgen of radi ation is u' = 0000,000,1 Suppose that, beginning at a certain time, the population is exposed to 10 r of radiation per genera tion, so that the value u' equals 0 000,001 How many additional genetic deaths (lethal homozygotes) will occur in this population 10,100 and 1,000 generations after the beginning of irradiation (n = 10 100 and 1,000)? We may now substitute these values in the formula 2 nu' Vus given above The results are as follows 10 generations 0 000 06 100 generations 0 000 6 1,000 generations 0 006 The estimate for 1,000 generations is obviously not valid, it seerns higher than would be expected when the new equilib rium value (0001) is reached in making our approximations we have neglected the slow but steady elimination of the newly induced lethals from the population Our formula is not good enough to calculate the effects of additional radiation induced lethals for 1,000 generations of continuous irradiation But the estimates for 10 and for 100 generations are reasonably accurate After 10 generations the frequency of genetic deaths increases by 0 006 percent, after 100 generations by 0 06 percent, at equi librium it should be increased by 0 I percent. In other words, after 100 generations of continuous irradiation more than half of the final equilibrium frequency will have been attained Remember, though, that there will be some additional genetic deaths from the very start, one cannot decrease the proportion of homozygous individuals by increasing the frequency a given type of mutant gene As stated above, only about one fifth of all mutations act complete lethals About four fifths range from semilethal ·138 * GESTETIC EFFECTS OF RADIATION ON POPULATIONS only slightly deleterious or nearly neutral conditions Unfortu nately, there is no reliable method of estimating how the focal genetic damage gradually increases after various numbers of generations of irradiation The difficulty is that we have no estimates, even for Drosophila, of just what arrays of the selec. tion coefficient (s) and of the mutation rates (u and ") we are faced with in attempung to encompass all kinds of mutations that may arise We shall, accordingly, confine ourselves to consideration of a single example Suppose that an organism has 1,000 different genes (loci), with an average mutation rate per gene per genera tion four times as large as we have assumed above for lethal mutants (that is, 4 = 0 000,04 and u' = 0000 000,4 per roentgen of radiation, instead of the 0 000,01 and 0 000,000, 1 respectively estimated for lethals) Suppose, further, that the mutations lower the fitness of homozygous individuals by 10 percent (that is the selection coefficient is s :01) We can now use the formula 2nu' Vus for the increase of genetic damage The computation gives the following figures 10 generations 0 000,16 100 generations 0001,60 1,000 generations 0016,00 The increase in the expected frequency of mutant homo zygotes at equilibrium will be about 0004 We see, then, that about 100 generations of continuous irradiation must elapse before a sizable fraction of the increased damage expected at equilibrium is realized Once more our formula proves to be 100 crude for the 1,000 generation interval, our calculations would indicate a greater amount of harm after 1,000 generations than ac equilibrium This, of course, cannot be so, the fault lies in neglecting the term (nu") Radiation-induced Mutants That Harm Heterozygoles Some geneticists question whether mutant heterozygotes are eser completely identical with the “normal” (that is, non ·BADIXTON DIDUCED MUTANTS THAT HAAM HETEROZYGOTES - mutant) homozygotes This is perhaps one of the most important unsoled problems of modern population genetics, and it happens to be critical for an estimation of the genetic radiation damage As shown in Chapter 6, if a mutant gene incapacitates heterozygotes even slightly, its equilibrium frequency in the population is much lower than it would be if the mutant were completely recessive Nevertheless the genetic damage produced by incompletely recessive genes is twice as great as that resulting from complete recessises wich like mutation rates Conversely, of the heterozygous carriers of a mutant gene are more fit than the noncarriers (a possibility to be discussed in Chapter 8), the fitness of the population as a whole may be augmented even though the homozygous condition results in incapacitation or even death What proportion of the mutants arising in man and in other organisms decreases, increases, or leaves unchanged the fitness of heterozygotes can be determined only by future research Such research is urgently needed Let us consider first how murants that injure heterozygous carriers behave m unirradiated and in irradiated populations As before, let u and u' stand for the spontaneous and the in duced mutation rates, V for the sum of the two, s for the loss of fitness in mutant homozygotes and hs for the damage in mutant heterozygotes As shown in Chapter 6 the equilibrium frequency of the mutant gene in the gene pool of a nonirradı ated population will be approximately q u/hs for most likely values of h and s, and the genetic damage caused by this gene will be approximately 2u Irradiation adds u' mutations in cach generation The equilibrium frequency of the mutant gene will, then be U/hs, and the genetic damage 20 It is possible that some of the mutations that arise are completely recessite and others incompletely so The total genetic damage to the popu Jatron caused by spontaneous and induced mutations when all the mutants have reached their equilibrium frequencies will then lie somewhere between U and 20 in our present state of ignorance this is as much as we can say about the situation We must hoxeser, ask how rapidly the new genetic eqm libriz for uncompletely recessile injurious mutants ws}} bc reached in irradiated populations, and low great the genetic damage is likely to be on a generation by generation basis A ·139 GENETIC EFFECTS OF RADIATION ON POPULATIONS I before, suppose that the organismn has 1,000 genes (loca) which mutate spontaneously at a rate u = 0000 01 to a condition which is lethal to homozygotes We have seen above (see page 132) that, if the mutants are completely recessive, the total genetic damage to the population will be 1,000u = 001, or percent We have likewise seen that if the mutant also injures heterozygous carriers, the genetic damage produced at equilib. rum will be about twice as great, 2 X 1,000u = 002, or 2 per cent This doubling of the amount of damage applies for all practical purposes whether the harm to individual heterozygotes 18 great or small Now, without trying to follow the algebra needed to make the calculations, let us consider two numerical examples that will help to visualize the important features of the situation Suppose that the mutant genes which are lethal to homozygotes reduce the fitness of heterozygotes by 5 percent or by 2 percentthat is, that the value h is 005 or 002 At the time irradiation commences, spontaneous mutation has already reduced the the oretically possible optimal fitness by about 2 percent Assume as on page 133, that the population is exposed to 10 r of radi ation per generation, the value which, according to the Genetics Committee of the National Academy of Sciences, should not be exceeded in human populations The increment of radiation induced damage afflicting any one generation, added to the 2 percent (002) damage resulting from spontaneous mutation will be as follows Generations 5 10 25 50 h=005 0 000 46 0 000,80 0001,44 0001,82 0001,98 0 002 00 h=002 0 000 20 0 000 36 0000 80 000128 0001 74 000200 100 Infinity What this tabulation shows is that the greater the degree of injury to heterozygous carriers, the more rapidly the equiliby rium frequencies for incompletely recessive deletenous mulanes ·BECESSIVE MUTANTS PRODUCED AY TEMPORARY BADIATION POSURES 139 are attained in irradiated populations Accordingly, the genetic damage from such incompletely recessive mulants will be felt sooner if the heterozygotes are appreciably incapacitated than if they are not To visualize the situation even more easily, con sider dorinant lethal mutants (s = 1, h = 1) Since such mu tants kill every individual who carries them, all such lethals are eliminated from the population in the same generation in which they appear, and the equilibrium is accordingly, reached in a single generation By contrast, equilibria for completely reces $11e mutants are reached very slowly (see page 135). Recessive Mutants Produced by Temporary Radiation Exposures In the foregoing pages we have considered the radiation damage resulting when every generation of a Mendelian popu lation is beginning at a certain time, exposed to a certain amount of mutagenic radiation We liave tried to estimate the amount of the genetic damage caused per generation under various specified conditions It is pointless to attempt to sim up the total damage produced through all generations given infinite time and an infinite number of generations the total damage will also be infinite Now we shall face a different prob lem-the damage caused in a population exposed to radiation for a limited time only Here we wish to find out not only how much damage will result in any one generation but also what the total damage caused in successive generations will be This type of problem is particularly important in evaluating the genetic damage that would be produced by such a contingency as a nuclear war We concern ourselves with the summation of damage through successive generations because we believe that a person is morally obliged to consider how his actions will affect other human beings even those whom he will never meet in the flesh, If a population is exposed to radiation for a small number of generations only, the number of recessive deleterious manis ·140 . GENETIC EFFECTS OF RADIATION ON POPULATIONS introduced into the gene pool is easy to calculate It is very nearly equal to the number of generations during which the radiation is applied, times the frequency with which mutations are induced in each generation (nu') Here we may use once more the figures given on page 132 If a population is exposed for 10 generations to 10 r of man made radiation, there will be induced 10 X 10 X 10-?, or 10-5 new recessive lethal mutants per gene Since the population contains some similar (allelic) lethal mutants that have arisen spontaneously, a few radiation induced lethals may be eliminated immediately if they happen to form homozygous individuals However, the fraction of new lethals eliminated within a short span of time will be very small the bulk of them will be lodged in the gene pool of the popu lation when the irradiation is terminated (Remember that 10 r of radration applied over a period of 10 generations :]] produce the same number of mutants as 100 r applied to a single generation) How can we measure the total damage caused by the dele terious genes induced by the irradiation? The task happens to be less formidable than it may seem Consider a population in which lethal mutants of each gene locus have reached their equilibrium frequency The recessive lethal mucants of a given gene a, will have, then, a certain frequency, q, and the normal genes, A, will have the frequency p, where þ=1-9 Let us assume that radiation has induced an additional increment , of the lethal genes a, so that the total frequency of a in the gene pool 18 9 +2 (and the frequency of A will make up the re mainder--that is, 1-9 - ?) The individuals bom in this pop ulation will be of three kinds homozygotes for the nonlethal gene A, heterozygotes for the lethal and the nonlethal Aa, and homozygotes for the lethal, aa These last will be eliminated from the population by death The genes carried in the indıvıduals who are eliminated must be considered further The individuals homozygous for the lethal a are again of three kinds those carrying to old lethals those inheriting an old and a new lethal and those homozygous for two 'new' lethals The composition of the population can, then, be represented by the following diagram ·RECESSIVE MUTANTS PRODUCED BY TEMPORARY RADIATION EXPOSURES. 101 A 01 A 01 FICURE 14 The elimination of newly induced te cessive mutant genes from a population Before irradı ation the population contained normal genes (1) and old recessive mutants (a) Following a period of radiation the supply of a mutants is increased by the addition of some newly induced ones (as) If the effect of these mutants both , and an is completely recessive ac genes will be eliminated only through the reduced viability or reproductive capacity of 2,4, and ah, individuals Since the frequency of a, will ordi narily be many times that of Q, after a limited exposure of the population to radiation the really impor tant means of elimination will be by way of all, individuals (shaded in diagram) 4,4, individuals will be extremely rare and may be ignored for purposes of calculation This situation is analyzed in detail in the text (Figure 14) The old lethals are those denoted as ao and the ones as a In general the fraction : will be small compared with q, therefore most of the newly induced lethals eliminated from the population will be those eliminated in association with old lethals Elimination of newly induced lethals by way of induced or new ·GENETIC EFFECTS OF RADIATION ON POPULATIONS aa, individuals is so rare that it can be neglected. Hoxever, some a, lethals will be lost because of the presence of as there fore, we will simply replace these old genes out of our stock of new ones We do this by saying that there are 2qı new lethals lost in each generation, half of these are actually lost in the lethal homozygotes while the other half are lost" because we arbitrarily call gi new lethals 'old' to maintain the proper frequency of the latter If the frequency of induced lethals per gene locus at the termination of the irradiation is i, their frequency in the next generation is 1 --- 201 or :(1 - 29) In the following generation the frequency of s(I -- 29) is reduced to [1(1 ~ 29)) - 29[:(1 - 29)} This is equivalent to 1/1 -- 29)(? - 29), or i(l - 297 In succeeding generations the frequency of these lethals becomes (1 - 29), (1 - 29), (1 - 29)* In each generation, the frequency of indıviduals who are eliminated from the population because they are homozygous for the lethal a equals 29 times the remainıng frequency of the lethal, thus 1 n C ID 1 Generation 2 3 Lethal frequency ( 129) :{} -- 29) (1 - 29)*-- Individuals lost 291 291(1 -- 29) 29:11 -- 29)2 29111 -29)*** The total frequency of individuals lost from the population the course of eliminating the induced lethals is equal to 291 [1 + (1 - 29) + (1 - 29)2 + Since I 29 is less than one, the series within the brackets has a finite sum, esen though the number of terms as infinite This sum is 1/29 Therefore, the sum of the frequencies of eliminated individuals us 291 X 1/29, or simply. Assuming that the population remains fairly constant in size, the number of individuals eliminated in all future generations will be equal to the fraction : times the number of individuals of which the population is composed in a given generation The greater the average number of people in the world, the greater the absolute number of victims of the short term radiation exposure will be their number relaure to the total population will, of course, ·RECESSIYE MUTANTS PRODUCED BY TEMPORARY BADIATIOT EXPOSURSS + 143 gradually decrease as the elimination of the induced mutants progresses What, however, will be the average rate of genetic death per generation? If we limit ourselves to considering a relatively small number of generations, n, we shall obtain a fairly good estimate by dividing the sum of the first n terms of the series above by n Thus sum is 291 (1 (1 - 29)"}/2q, and the frequency of individuals eliminated per generation is accordingly t/n (1 -- (1 – 2g)"), or very nearly 29: (1 -- qn) Since q is gen erally a small number (we have used the value 0 003 in previous examples), the frequency of elimination remains very close to 2q1 for many generations As before, we shall attempt to convey an idea of what these formulas mean in practice by considering a numerical example Assume that the average individual in a populauon is exposed to 10 r of radiation per generation for 10 consecutive generatrons, whereupon the exposure is stopped (this may occur, for instance, af a safe substitute for atomic energy is eventually discovered) During the 10 generations, 10-5 recessive lethal mutants will be induced in every gene capable of such mutation (sce page 140) These induced lethals will eventually be elim inated by the death of their homozygous carriers Assuming that the world population of the human species is stabilized at 3 billion persons (3 x 10”), this would mean 30,000 genetic deaths caused by mutants at each gene locus, or 30 million deaths for 1,000 genes formula 29:(1 Lest 30 million deaths seem fantastic, remember that they will occur over a time interval infinitely long-literally time without end More to the point is how many genetic deaths will be taking place per generation within some foreseeable future, say 20 generations To arrive at this number we may use the qn) given above If the spontaneous mutation rate is close to 0 000010, the value o will be roughly 2003 (the square root of 0 000 010) With = 10-5 and n=20, the result is close to U 000,000 06 for 1,000 genes this becomes U 000 06 Assuming again that mankind is stabilized at some 3 billion persons this would mean some 180,000 genetic deaths per gen eration, or about 6,000 deaths annually for the 600 or so years ·GENETIC EFFECTS OF RADIATION ON POPULATIONS following the cessation of exposure And this would lease about 90 percent of the induced mutants still to be eliminated! Nor is this all. We have seen above, in discussing the effects of continuous radiacion exposure, that mutants which are dele ferious but not lethal to homozygotes are about four times more frequent than the completely lethal mutants, and that they cause, in the aggregate, a greater number of genetic deaths because they, too, will have to be eliminated eventually For deleterious nonlethal mutants, the total number of eliminations, over in finite time, is, analogously with the formula on page 142, equal to 2sq:[1 + (1 - 259) + (1 – 2sq)2 ), or simply: for a small number of generations, the average frequency of genetic deaths per generation will be approximately 2sqi(1. nsq), where s is, as usual, the selection coefficient and n is the number of generations A complication arises because, as pointed out above, we do not know the distribution of the values of s nor the rate of mutation for different mutants Other things being equal, the smaller the value of s (that is, the more nearly normal the individuals who are homozygous for the mutation), the smaller the average effect this gene has on the fitness of the population in any one generation Unfortunately, the mutation rate of these genes is higher than that of lethals We may assume, for example, that there are 1 000 genes, each of which mutates to deleterious alleles for which s equals 0 10 at a rate of 0 0000! per locus Using these assumptions, it is possible to calculate the average effect of these mutations on the population over a period of 20 generations following the cessation of radiation In this case q, the original equilibrium frequency, equals Vuls, or 002 The frequency, i, of mutations induced during 10 gen erations of exposure to 10 per generation would be 4 x 10-5 Consequently, the average effect of these mutations on the population over 20 generations would be 2 X 0 10 X 002 X 4 X 10-5 X [1 – (20 X 0 10 X 0 02)), or 0 000,000,15 For 1,000 genes this amounts to 0000,15 In a world population of three billion persons there would be 3 000,000 000 X 0000,15 = 450,000 persons eliminated in every generation because they are homozygous for these subvital ·SEMEDOMINANT MUTANTS PRODUCED BY TEMPORARY RADIATION EXPOSURES I<5 recessive defects induced by radiation The value 0 10 which we have assumed for s means that the 450 000 persons eliminated account for only 10 percent of all persons threatened with in Capacitation there would be 4 500 000 of the latter in each generation Using 30 years as the length of a generation we can see that each year some 150 000 persons in the world would be carriers of newly induced deleterious mutations permitting them to survive or reproduce only 90 percent as well as their Normal fellow men hence in effect 15 000 persons would be eliminated annually because of these mutant genes Semidominant Mutants Produced by Temporary Radiation Exposures As stated above there is a school of thought in biology which denies the existence of completely recessive genes It believes instead that most mutant genes are really semidominant the effects of a mutant being generally similar in homozygotes and heterozygotes although much stronger in the former than in the latter According to this view every mutant which causes a hereditary disease when homozygous must produce some frail health in heterozygotes Future work may support refute or qualify this extreme point of view nevertheless we should con sider its bearing on the problem of the genetic radiation damage to populations The task is to estimate the consequences of the temporary exposure of a population to mutagenic radiations We shall pro ceed in a manner very similar to that employed above for com pletely recessive mutants The diagram in Figure 15 is analogous to that in Figure 14 and contains identical symbols (A repre senting the normal and a the mutant gene alleles d, and a the mutants produced spontaneously and those induced by radi ation) The shaded part of the diagram indicates the heterozygotes which carry one normal (A) and one radition induced mutant (a) gene If such heterozygotes are less fit than ·148 • GENETIC EFFECTS OF RADIATION ON POPULATIONS Attempts to Detect Genetic Radiation Damage in Man The foregoing pages of the present chapter may have struck the reader as too theoretical We have operated with formulas that may be valid enough in theory, but in attempting to illus trate their meaning by examples we were forced to use numeri cal values derived largely from genetic experiments on Dro sophila Even then we had to confess that these values are at best rough approximations In a way, this situation is not quite so bad as it may seem The basic genetic processes operating in Mendelian populations obey the same rules in all biological species reproducing sexually and avoiding marriage of close relatives If a deleterious recessive mutant arises at a rate u per generation, its equilibrium frequency will be Vu/s in large populations of fles mice, men, or elephants But we have no right to assume without sufficient evidence that the values of U and s and the incidence of recessive, semidominant, and heter otic mutants are the same in Drosophila as in man To under stand the genetics of human populations, research on Drosophila and on man is equally important and indispensable for the results of such research are complementary and mutually en lightening Attempts have been made to detect genetic radiation dam age in human populations By far the most extensive, careful, and costly work of this sort has been carried out under the leadership of J V Neel and W ] Schull on the survivors of the atomic blast at Hiroshima and Nagasakı In all, the newbom children of some 20,000 women and 14 000 men who were suf ficiently close to the blases to receive anywhere from 8 to 2001 of radiation were examined Among 33 181 parturitions in which at least one parent was irradiated, there sucre 516 still born infants The investigators also studied a control sample- 31,559 parturitions in which neither parent was known to have suffered irradiation In this control sample there were 408 sull births The frequency of stillbirths in families of irradiated par ·ATTEMPTS TO DETECT GENETIC RADIATION DAMAGE IN MAN. 113 ents was, accordingly, 164 percent, which is ostensibly higher than the frequency 1 29 percent in the control sample It is tempting to ascribe this increase in the frequency of stillbirths to radiation induced dominant or semidominant deleterious mutants Other explanations are not, however, entirely ruled out not all stillbirths are caused by genes, and the small excess of their frequency among the infants of irradiated parents may be due largely to statistical errors of sampling or other, undetected causes Neel and Schull also record 300 infants born with congenital malformations of various kinds among 33 527 live births to parents of whom at least one was irradiated The Corresponding figure for the control series is 294 infants with malformations among 31,904 births There is no indication that malformations are more frequent in the irradiated series S H Macht and P S Lawrence, and also J F Crow, have utilized another source of data They sent questionnaires to about 6 000 American radiologists and also to those physicians who are not constantly working with x rays or other radiations during the performance of their professional duties The radiolo gists are undoubtedly exposed to many small doses of radiation, although it is not possible to determine just how many roentgens they accumulate during their lifetime A total of 766 abortions and stillbirths and 328 congenital malformations among infants born alive, were recorded among 5461 births in families in which the father was a radiologist in the control series there were 548 abortions and stillbirths and 216 malformed infants among 4 484 births Critical examination of these results shows that there is an ostensible but statistically not significant, increase in the incidence of abortions and stillbirths, but an apparently significant increase in congenital malformations in the offspring of radiologists What conclusions can be drawn from these data? They would surely be inadequate to prove that radiation produces mutations if we did not possess overwhelming evidence of this from experiments on organisms other than man But it is noth ing short of ridiculous to argue, as some people have attempted to do, that radiation may not be mutagenic in man Even wich ·150 GENETIC EFFECTS OF RADIATION ON POPULATIONS the impressively large number of observations reported by Neel and Schull from Japan, one would not expect enough induced dominant and semidominant lethals and sernilethals to yield striking differences in the number of stillbirths and malforma tions between the irradiated and the control series. We need qualitative and quantitative data concerning spontaneous and radiation induced mutations in man But it would be a waste of time, effort, and money to undertake research on man merely to prove that he, just as every other living organism studied, is subject to the mutagenic action of radiation, ·ම Some Unsolved Problems Introductory Remarks A scienust knowvs certain things to be true or at least overwhelm ingly probable, it is his duty to affirm these things even in the face of opposition from those who, owing to ignorance or to some emotional bias wish to deny them Such 1s, for example, the fact that the living world, including man, is a product of evolutionary development We have to maintain that this is so, even though the laws of certain states still prohibit such an affirmation Similarly, we have to assert that high energy radia tions can produce genetic damage in human populations On the other hand, a scientist must in all humility, admit that he does not know the answers to many riddles of nature there is no reason for him to hide his ignorance Nevertheless, when he tries to do his part in educating the public, the temptation arises to oversimplify his story by the omission of difficulties and uncertainties Up to a point, such omissions are justified But there is always a point beyond which simplification borders on falsification In the foregoing chapters we have pointed out time and again that the presently available understanding of population genetics is imperfect and that its inadequacy makes some of 191 ·132 SOME UNSOLVED PROBLEMS our conclusions concerning the genetic damage produced by sa diation highly tentative in the following pages we shall discuss some topics which in our opinion are particularly in need of clarification This should enable the reader to judge for him selt the weakest links in the geneticists conception of what a misuse of radiation may portend for the future of mankind We also hope to call the attention of our colleagues biologises and geneticists to the need for research on these unsolved problems for we believe that the research effort now being exerted in studies of radiation biology and radiation genetics must include an appreciation of their importance Heterosis and Mutation In Chapters 6 and 7 we discussed two kinds of deleterious mutant genes and their effects on the fitness of populations The simplest situation obtains when a mutant harms homo zygotes but has no effect at all on heterozygous carriers The other kind of mutants discussed damage heterozygotes as well as homozygotes though the latter more severely than the former We say that regardless of the severity of the damage produced in afflicted individuals (ranging from complete lethality to only a mild impairment of health) the total damage to the popula tion is approximately the same It may be equal to the mutation rate or to double the mutation rate or it may lie between these values We muse now consider an even more complex but sery Interesting situation of ambivalent mutants-those which im pasr fitness when homozygous but improve that of their fietera zygous carriers That such mutants do occur is suggested on a gigantic scale by hybrid corn For the past quarter of a century the bull of all com planted in the United States and more recently else where has consisted of hybrids between pairs of different lines These heterozygotes are superior in yield to the best open pol linated field varieties of com which were planted before hic invention of hybrid com and its introduction into general use ·FETEROSIS AND MUTATION 153 To see how this hybrid vigor, or heterosis (these terms are syn onymous), operates within populations we may consider a nu merical example before turning to generalization The genes we are to consider lack the dominance and re cessiveness, so we shall designate one form of the gene as a, and its mutant form (allele) as ag The vigorous heterozygotes, 2142 enjoy the higher fitness, while the homozygotes aja, and azd, are, respectively, 80 percent and 20 percent as fit as the heterozygotes What will happen in a population in which both A. and an are represented? We shall state without proof that the frequencies p of and 9 of will be respectively 0 8 and 0 2 in the gene pool of the population Let us then, make a ledger sheet of the fitness of this population It appears as follows Genotype Frequency Fitness Frequency X Fitness p2 = 064 08 0512 2pq=032 10 0 320 92 = 0.04 02 0008 Average fitness of the entire population 0 840 0191 2102 Galiz The most important fact disclosed by this table is that the over all fitness of the population (obtained by summing the products of the fitness of each genotype times its frequency in the population) has the value 0 840 Yet a population contain ing only the better allele, ai, would have a fitness of only 080 (frequency 100 times fitness 08) In other words, the introduction into the population of the mutant allele a, has improved the over all fitness of the population despite the fact that this mutant allele is deleterious and adds to the genetic load of the population by producing a weak, poorly adapted homozygote, Q222 with a fitness of only 0 2 This amounts to saying that a gene which produces a hereditary disease when homozygous may still benefit the population as a whole provided that it is ad vantageous in heterozygous individuals Of course, one may rightly say that an even better population would be one consisting enurely of the heterotic heterozygotes, azaz True enough, and this is precisely what happens ·154 . SOME UNSOLVED FROELES when hybrid corn seeds giving only heterozygous plants are sown The trouble is, however, that a population consisting of vigorous heterozygotes cannot maintain itself by sexual repro duction According to the Hardy Weinberg Theorem, aja, indi viduals would in the very next generation give a progeny con sisting of only 50 percent 0122, 25 percent ajan, and 25 percent of the ill adapted and2 The fitness of such a population would be only 0 75 This is why a farmer must buy his hybrid corn seed for every planting instead of saving his own seed from the pre vious harvest The situation can be generalized as follows Assume that the fitness of the heterozygote Azaz, is 1 00 while the fitnesses of the two homozygotes is (1 - s) for aja, and (1 1) for any Although we cannot give the proof in this book it can be shown that the relative frequencies of the genes as and an establish a stable equilibrium the equilibrium frequency of as being 1/s + t) and that of as being s/(s +t) The letters s and a stand for the selections coefficients discriminating against the homo zygotes The ledger sheet of the fitness of the population is 25 follows Genotype Frequency Filness p>= 12/(5 + 1)2 1-5 аа. 2p9 = 2st /(s + 1)2 1 2,02 q? = s/(5 + 1)2 1-t Average fitness of the entire population Frequency x Fitness 12(1 - 5)/(5 + 12 2st/ks + 1) sa(1 - 1)/(5 + 1) 1 - {st/(3 + 1] If the algebra seems terrifying take note only of the fact that the fitness of the population in which both di and as occur 15 1 – st/(5 + ?) and that this value is larger than both (1 – 5) and (1 - 1) which measure the fitness of a population consisting of 0;&; or a 22 homozygotes only This is true even if the allele as produces, when homozygous, a lethal hereditary disease~that is if t= 1 The population has the highest fitness only if it con tains some 0,02 heterozygotes Furthermore, the fitness of the population is highest at the equilibrium frequencies of the two alleles any deviation from these frequencies (caused for ·HETEROSS AND MUTATION . 15$ example, by an attempt to cull out the less desirable of the two) results in a decrease in the mean fitness of the entire popu lation Suppose that both homozygotes aja, and Azaz are handi capped equally compared to the vigorous heterozygote This means that the coefficients s and t are equal sat Both genes will therefore, have equal frequencies in the gene pool 05 The gene ar will have the frequency t/2 = 05 and a, the fre quency 5/25 = 05 Now suppose that s is much larger than t, so that aja, is nearly as good as the heterotic ajan while azaz is very inferior or even lethal Natural selection will make the popula tion mostly homozygous for 0,4, However-and this is impor tant-the gene az will be retained in the population by natural selection It will be carried chiefly in heterozygotes ajan but some infirm homozygotes a az, will though rarely, be born We have already pointed out above that it is not well known how often mutations arise which possess properties like those of a, in the example just given Some geneticists believe that such heterotic mutants are rare and hence unimportant in our evaluation of genetic radiation damage Other geneticists take seriously the possibility that such mutants may be impor tant Statements in newspapers and magazines have referred to disagreements among geneticists concerning the dangers of genetic damage produced by radiation exposure These state ments are misleading The real differences of opinion concern this problem of superior fitness produced by heterozygosity There is no disagreement about radiation exposure inducing mutations in human populations Nor does any geneticist doubt that mutations are generally harmful at least in homozygous condition One of the authors has obtained recent experimental evi dence that heterotic mutants may be far more common at least in Drosophila than had been suspected previously The essence of these experiments is as follows It had been known previously (see Chapter 6 page 117) that natural populations of Drosophila carry enormous loads of genetic variants which are more or less deleterious if permitted to become homozygous As stated on page 117, virtually all chromosomes in Drosophila populations ·156. SOME UNSOLVED PROBLEMS act as lethal, semilethal, or subvital mutants when carried in double dose in the same individual (that is, if inherited by an individual both from the mother and from the father) Several strains of flies homozygous for such subvital chromosomes were chosen By means of a method which we cannot describe here in detail, the viability of these homozygotes were compared with that of flies which were genetically similar, except that they had one of the two chromosomes of the pair treated with a small (for Drosophila) dose of x rays (500 T) The viability of the filles carrying an irradiated chromosome appeared slightly but sig nificantly higher than that of the nonitradiated controls Should these experimental findings be confirmed, it would follow that mutations tend frequently to produce hybrid vigor in heterozygous condition This might require a revision of the assumption that mutation is almost always harmful not only to homozygotes (which remains true) but also to heterozygotes (which is now in doubt) Some inflow of new mutations may well be necessary to maintain a variety of genes in the gene pool of a Mendelian population Indeed, except for mutation, all known processes that operate in natural populations tend to pro duce genetic uniformity and homozygosis The process of muta tion is, then, two-faced as far as the fitness of the population as concerned Afutant homozygotes may suffer, but the genes te sponsible may be retained in the population because their heter ozygous carriers are, in fact, the really normal induiduals Genetic Structure of Natural Populations: Classical Hypothesis This possibility brings us face to face with a sery basic question What is a 'normal' man? What, indeed, is a 'normal' Drosophila, or a normal' representative of any biological species? In eseryday social intercourse with people are consider as normal those persons who are free of gross bodily defect and who act in accordance with standards of behavior regarded 35 acceptable in our society A normal cat or mouse, or Drosophila or com plant is one which, in appearance and in behavior, does ·137 GENETIC STRUCTURE OF NATURAL POPULATIONS CLASSICAL HYPOTHESIS . not differ greatly from other cats, or mice, or Drosophila, or corn plants which we have seen before It must also be able to live in the environments in which other normal representatives of its species live, and it must successfully reproduce its kind And yet there is some diversity among normal people Neither are all normal cats exactly alike, nor all normal Dro sophilae How much individuality is compatible with 'normal ity, and what is the source of the differences we observe among individuals? More than 2000 years ago Plato tried to answer these questions He thought that individual humans were all more or less imperfect imitations of an ideal Man, just as individual horses or trees were more or less garbled copies of an ideal Horse or an ideal Tree The ideal prototypes, according to Plato, were eternal and unchanging spiritual entities of ineffable perfection and beauty, their earthly replicas were all defective, but their shortcomings might be diminished in striving to approach the Ideal Although no modern geneticist takes seriously Plato's in genious conception of the relation between heavenly perfection and terrestrial failing, the Platonic mode of thought is deeply ingrained in Western culture and perhaps in 'normal human nature everywhere In some branches of biological research it has not inconsiderable advantages The poet Goethe devised an ideal 'protoplant, which was helpful in understanding the in finitely diverse forms of living plants, and others have used ideal structural plans, schemes, and prototypes to understand and describe the external and anatomic organization of diverse animals The great work of several generations of zoologists and botanists in building a rational classification of all living crea tures was doubtless assisted by the concept of an ideal plan of organization of living bodies The same line of thought can be used to construct a model of the genetic structure of living populations This model is referred to as the classical hypothesis According to the classical hypothesis there emerged, in the evolutionary process, a constellation of genes-a genotype-which makes its carriers live successfully in a certain environment. Thus, the genotype of the human species makes us fit to live in the human environment, the cats genotype makes cats adapted to the feline environment;

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·Glossary ALLELE A variant form of a gene CELL LETHAL A genetic change is capable of killing an individual cell possessing it even if this cell is surrounded by others not carrying this change CENTROVERE A specialized region of a chromosome which is instrumental in the movement of the chromosome during cell division CHROMOSOME A small body found within a cell nucleus which contains many genes As a general rule one can say that every species possesses a characteristic number of chronno somes DEOXYRIBONUCLEIC ACID A complex substance composed of fibrous molecules containing phosphoric acid sugar pu fines and pyrımadines it is believed to be the genetic material that controls the composition and structure of protein molecules DNA See Deoxyribonucleic acid DOMINANT MUTANT An altered form of a gene the effects of which are expressed in heterozygous individuals Partial domitance is used to describe genes whose effect in heterozygous condition is not idenucal with that in homozygous condition complete dominance means that the homo. zygotes and heterozygotes are indistinguishable 197 ·198 GLOSSARY DOUBLING DOSE The dose of radiation required to induce the same amount of gene mutation as that which arises spon taneously without radiation GAMETES Specialized cells that are capable of giving rise to new individuals Sperm are male gametes, eggs, female gametes GENE LOCUS A region of the chromosome where genes are found which have characteristic effects on their carriers Thus, at one spot on one chromosome in the fruit fly, one detects gene changes that lead to white, apricot, cherry, and eosin colored eyes, one infers that at this spot there is a gene governing eye color and that this gene can change in various ways and produce alleles which give the colors mentioned GENE MUTATION The transformation of a gene from one form (allele) to another Spontaneous mutations are those which occur for unknown reasons, induced mutations are those brought about by certain physical or chemical agents GENETIC EQUILIBRIUM Stability of the frequencies of various genes in a population, established by opposing tendencies such as formation by mutation and elimination by selec tion GENCTIC VARIABILITY A property of populations dependent upon the existence of a variety of different forms of many genes It leads to hereditary differences between indi viduals GENOTYPE The sum total of all hereditary factors carried by an individual An individuals genotype is determined at fer tilization in turn, it limits the variety of gametes an andı vidual can produce HETEROSIS The possession by heterozygous individuals of cer tain characteristics (especially in relation to growth or vitality) in excess of the two corresponding homozygotes HETEROZYGOTE An individual carrying two different alleles at a given gene locus, hence, an individual capable of pro- ·SLOSSARY 199 ducing two kinds of gametes containing different alleles of that gene HOMOZYGOTE An individual carrying two identical alleles at a given locus hence an individual capable of producing but one type of gamete as far as that gene is concerned LETHAL A genetic change that can produce death of an indı vidual Recessive lethals are gene mutations two of which result in death dominant lethals are genetic changes that result in death even though they may be present only in Single dose PHENOTYPE The observable characteristics of an individual determined by his genotype and by the succession of en vironments in which he developed RADIOACTIVE SUBSTANCES Substances that emit mutagenic radi ations spontaneously RECESSIVE MUTATION An altered form of a gene whose action can be masked in heterozygous individuals This can be qualified as in incompletely recessive if the effects of the mutant allele are not completely masked in heterozygous condition ROENTGEN (R) The unit of measurement of x rays and other radiation One roentgen is defined as the amount of radi ation which produces 2 082 X 10' 1on pairs per cubic cen timeter of air.

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·Suggestions for Further Keading It may well be that some readers of this book will wish to have more information than is found therein regarding some aspects of the problems with which it deals We are giving below a few references to sources that may be used for this purpose Fundamentals of genetics may be found in any of the numerous available textbooks, such as ALTENBURG E, Genetics, Rev ed New York Henry Holt and Co, 1957 SINNOTT, E B, LC DUNN, and TH DOBZHANSKY, Principles of Genetics, 5th ed New York McGraw Hill Book Co, 1958 SRB, A M and R D OWEN, General Genetics San Fran Cisco WH Freeman and Co, 1955 STERN, C, Principles of Human Genetics San Francisco WH Freeman and Co, 1949 For more information concerning the physical nature of radiations and of atomic energy, see the short but informative book by HECHT, SELIG, Explaining the Atom New York Viking Press, 1954. TOI ·zoz . SUGGESTIONS FOR FDRTHER READING Discussions of the mutational loads in human populations, and of the effects of radiation exposures on these loads, have been prepared by committees of several public bodies, such as Report of the United Nations Scientific Committee on the Efects of Atomic Radiation New York The United Nations The Biological Effects of Atomic Radiation, Summary Re ports Washington, DC National Academy of Sci ence, National Research Council The Hazards to Man of Nuclear and Alled Radiations London H M Stationery Office The Effect of Radiation on Human Heredity World Health Organization (Available through Columbia University Press, International Documents Service, New York) The Effects of Atomic Weapons Washington, DC Super intendent of Documents, U S Government Prinung Office For a most thorough discussion of the genetic consequences of the accumulauon of deleterious mutations in human popu lations, see MIULIER, H J, 'Our Load of Mfutations American Jour nal of Human Genetics, Vol 2 pages 111 176 (1950) Contrasting opinions concerning the wisdom of continued production and testing of nuclear weapons have been ably presented and argued by the following LAPF, R. E and J SCHUBERT, Radiation What It Is and How ll Affects You New York Viking Press 1957 PAULING, LINUS, No More Ilar! New York Dodd Mead and Co, 1958 TELLER, E and A L LATTER, Our Nuclear Future Facts, Dangers, and opportunities New York Criterion Books, 1958 ·Index adaptation, 54, 55 alleles 27. 160 166 American Indians, 108 anemia Mediterranean, 26 antibiotics, 42, 47, 48 Atomic Age, 3,4 Atomic Energy Commission, 69, 180 atoms, 58-61 color blindness, 27 Cordeiro, A R, 164 corn, hybrid, 152, 154 Cosmic rays, 68, 69 Crick, F HC, 23 Crow JF, 149 culture, 14, 193 Curie, P and M., 59 background radiation, see radiation death, genetic, see genetic death bacteriophage, 42 de Vries, H, 35 balance hypothesis, 159 166 diseases, hereditary, 19 21, 26, 37. beta rays, 64, 65 50, 51, 66, 111 115, 120, 150, blood types, 18, 33, 35, 49, 52, 53, 153, 158, 166, 167 107, 108, 124, 176, 177 DNA see nucleic acid Bohr, N.8 dominant genes, 28, 31, 36, 105, 128, Brues, A.M ,91 145 147, 169 173 doubling dose, 83 81 cancer, 1. 8, 10, 45, 53, 77, 91, 94 97, Drosophila, 35, 3745, 80 81, 82, 83, 185 84, 85, 131-132, 155 156, 170- Carter, T C., 85 173, 174 cell divisions 23 dwarfs, see chondrodystrophic dwaris centromere, 44 cesium 137, 71 74 ecology, 168, 169 Chetverikov,SS, 55 egg cells, 20 22, 32, 174, 175 chlorophyll, 7 Einstein, A.S chondrodystrophic dwarts, 35, 36, environment, 13, 1521, 79, 157, 158 50, 59 enzymes 26 chromosome breakage, 44,91 100 equilibrium, genetic, 109 113, 115, chromosomes, 21, 26, 80 120-123, 125, 130, 133 139, classical hypothesis, 156-159 163 165 ·204. INDEX erythroblastosis, 52, 53 Lawrence, P.S, 149 evolution, 32, 188 199 lethal dose, 96-98 extinction, 166-177 lethals, 1941, 46, 50, 52, 54, 80, 82, eye color, 28-29, 31, 103-105 94, 111, 112, 114-120, 129, 131- 135, 140-142, 170-174 fallout, 10, 67, 71 74,186 leukemia, 10.91 Fermi, E, 3 Lewis, E. B,91 fertilization, 21, 27, 28 fuels, 4,6 Macht, S H, 149 malaria, 18 gametes, 21, 32 malignancy, see cancer gamma rays, 63-65, 67, 87, 173 meiosis, 23 25, 31, 32,99 gene loci, 28 Mendel, G, 29, 101 gene pool, 103-106 Mendelian populations, see popula. genes, 25-32 tions genetic code, 34 Mendel's laws, 27 S2 genetic death, 113 116, 129, 130, 138- mice, 85, 86, 88, 98 135, 143, 163, 166 mitosis, 2$ genetic equilibrium, see equilibrium Morgan, T H , 35 genetic load, 55, 116-125, 158, 159 Muller, H J, 38-40, 80-85,91 genetic radiation damage, see radi. mutation, 27, 38-55, 159 ation damage mutations, chromosomal, 43-45 genetics, 13 frequencies of, 3643, 81 86, 109 genotype. 16, 17, 20, 21, 22 113, 115, 118, 122, 127, 128, Glass, H B 176 132 Goethe, W, 157 induced, 79 100 gonad, 24 sofnatic, 89 91,94 100 Gustafsson, A, 48 spontaneous, 32 55, 83, 84, 130. 132 Haldane, J BS, 37 useful, 47-49 half life, 71 Hardy Weinberg theorem and equi- Nagasakı, 5, 129, 148 librium, 105, 106, 112, 154 National Academy of Scences, 9, hemoglobin, 26, 27 119, 133, 138, 174 hemophdia, 36, 37, 58 nature-nurture problem, 1719 heterosis, 128, 152 156, 162 165, 182 Neel, J V, 148, 149 heterozygotes, 28, 31, 119, 136-139, neutrons, 61, 65 153, 155 Niagara Falls, 5 Hiroshima, 3, 129, 148 nucleic aads, 21 23, 25, 34, 35, 65, homozygotes, 28, 31, 119, 155 66, 79, 94, 95, 100 hydroelectric power, 4-6 nucleotides, 54 Ikhnaton, 57 Oenother2.95 Olurer, C. PAS Itano, HA, 26 Os encompensation, 176, 177 isotope, 65, 76 ·Russell W L 85 86 88 131 Patterson JT., 90 Paul ng L 26 Peking Van 2 Schull W J. 148 149 Penrose L.S 119 Schweitzer A 186 phenotype 17 segregation 31 105 P geods, 168 selection coell acnt of 113 115 Plato 157 natural 110 115 12$ 158 polygenes 47 solar energy 7 populat ons, Mendelian 101 123 Solon LR. 69 130 165 species 121 129 167 168 Prometheus, 13 spectrum 62 sperm 2029 32 94 174 175 r-un ts, 67 81 89 84 87 starlings 102 radiat on acute 88 ster lity 93 97 113 background 67 70 74 76 83 stillbirths 148 149 130 strontium 90 71 72 74 98 chron , 88 radiation burns, 8 target theory 89 radiation damage genetic, 8 9 14 thalassemia 26 68 70 77 96-100 127 150 Timofeeff Ressousky N W., 81 152 158 166-179 181 185 tuberculosis 184 189 191 twins 15 32 phys ological 8 83 96-100 dat on exposure 159 150 United Nations 8 68 70 76 rad ation phys a 57 77 University of Chicago 3 rad auon sickness, 45 66 83 96-100 uranium 61 65 radiation syndrome see radiation sckness variation 15 34 radiologists, 149 radium 60 65 69 98 Watson JD 23 random mat ng 107 108 weapon tests 10 67 71 74 186-188 recess ve genes 28 31 36 105 117 World Health Organizations 8 128 133 139 145 163 recomb nat on 31 32 x rays 61-64 67 7477 80 81 87, Roentgen W K 61 93 98 zygote 23 Roosevelt, FDS