Part 4

A second possible explanation is that there is present within the cells themselves some process that tends to repair damage to the genes and to counteract mutations. It might be a slow-working, laborious process that could keep up with the damage inflicted at low dosage rates but not at high ones. High dosage rates might even damage the repair mechanism itself. That, too, would account for the fewer mutations at low dosage rates than at high ones.

To check which of the two possible explanations was nearer the truth, Russell performed similar tests on female mice. In the female mouse (or the female human being, for that matter) the egg cells have completed almost all their divisions before the female is born. There are only so many cells in the female gonads that can give rise to egg cells, and each one gives rise to only a single egg cell. There is no possibility of damaged egg cells being drowned out by floods of undamaged ones because there are no floods.

Yet it was found that in the female mouse the mutation rate also dropped when the radiation dose rate was decreased. In fact, it dropped even more drastically than was the case in the male mouse.

Apparently, then, there must be actual repair within the cell. There must be some chemical mechanism inside the cell capable of counteracting radiation damage to some extent. In the female mouse, the mutation rate drops very low as the radiation dose rate drops, so that it would seem that almost all mutations might be repaired, given enough time. In the male, the mutation rate drops only so far and no farther, so that some mutations (about one-third is the best estimate so far) cannot be repaired.

If this is also true in the human being (and it is at least reasonably likely that it is), then the greater vulnerability of our genes as compared with those of fruit flies is at least partially made up for by our greater ability to repair the damage.

This opens a door for the future, too. The workings of the gene-repair mechanism ought (it is to be hoped) eventually to be puzzled out. When it is, methods may be discovered for reinforcing that mechanism, speeding it, and increasing its effectiveness. We may then find ourselves no longer completely helpless in the face of genetic damage, or even of radiation sickness.

On the other hand, it is only fair to point out that the foregoing appraisal may be an over-optimistic view. Russell’s experiments involved just 7 genes and it is possible that these are not representative of the thousands that exist altogether. While the work done so far is most suggestive and interesting, much research remains to be carried out.

If, then, we cannot help hoping that natural devices for counteracting radiation damage may be developed in the future, we must, for the present, remain rigidly cautious.

Conclusion

It is unrealistic to suppose that all sources of man-made radiation should be abolished. The good they do now, the greater good they will do in the future, cannot be abandoned. It is, however, reasonable to expect that the present Nuclear Test Ban Treaty will continue and that nations, such as France and China, which have nuclear capabilities but are not signatories of the Treaty will eventually sign. It is also reasonable to expect that X ray diagnosis and therapy will be carried on with the greatest circumspection, and that the use of radiation in industry and research will be carried on with great care and with the use of ample shielding.

As long as man-made radiation exists, there will be some absorption of it by human beings. The advantages of its use in our modern society are such that we must be prepared to pay some price. This is not a matter of callousness. We have come to depend a great deal for comfort and even for extended life, upon the achievements of our technology, and any serious crippling of that technology will cost us lives. An attempt must be made to balance the values of radiation against its dangers; we must balance lives against lives. This involves hard judgments.

Those working under conditions of greatest radiation risk—in atomic research, in industrial plants using isotopes, and so on—can be allowed to set relatively high limits for total radiation dosages and dose rates that they may absorb (with time) with reasonable safety, but such rates will never do for the population generally. A relative few can voluntarily endure risks, both somatic and genetic, that we cannot sanely expect of mankind as a whole.[9]

From fruit fly experiments it would seem that a total exposure of 30 to 100 rads of radiation will double the spontaneous mutation rate. So much radiation and such a doubling of the rate would be considered intolerable for humanity.

Some geneticists have recommended that the average total exposure of human beings in the first 30 years of life be set at 10 rads. Note that this figure is set as a _maximum_. Every reasonable method, it is expected, will be used to allow mankind to fall as far short of this figure as possible. Note also that the 10-rad figure is an _average_ maximum. The exposure of some individuals to a greater total dose would be viewed as tolerable for society if it were balanced by the exposure of other individuals to a lesser total dose.

A total exposure of 10 rads might increase the overall mutation rate, it is roughly estimated, by 10%. This is serious enough, but is bearable if we can convince ourselves that the alternative of abandoning radiation technology altogether will cause still greater suffering.

A 10% increase in mutation rate, whatever it might mean in personal suffering and public expense, is not likely to threaten the human race with extinction, or even with serious degeneration.

The human race as a whole may be thought of as somewhat analogous to a population of dividing cells in a growing tissue. Those affected by genetic damage drop out and the slack is taken up by those not affected.

If the number of those affected is increased, there would come a crucial point, or threshold, where the slack could no longer be taken up. The genetic load might increase to the point where the species as a whole would degenerate and fade toward extinction—a sort of “racial radiation sickness”.

We are not near this threshold now, however, and can, therefore, as a species, absorb a moderate increase in mutation rate without danger of extinction.

On the other hand, it is _not_ correct to argue, as some do, that an increase in mutation rate might be actually beneficial. The argument runs that a higher mutation rate might broaden the gene pool and make it more flexible, thus speeding up the course of evolution and hastening the advent of “supermen”—brainier, stronger, healthier than we ourselves are.

The truth seems to be that the gene pool, as it exists now, supplies us with all the variability we need for the effective working of the evolutionary mechanism. That mechanism is functioning with such efficiency that broadening the gene pool cannot very well add to it, and if the hope of increased evolutionary efficiency were the only reason to tolerate man-made radiation, it would be insufficient.

The situation is rather analogous to that of a man who owns a good house that is heavily mortgaged. If he were offered a second house with a similar mortgage, he would have to refuse. To be sure, he would have twice the number of houses, but he would not need a second house since he has all the comfort he can reasonably use in his first house—and he would not be able to afford a second mortgage.

What humanity must do, if additional radiation damage is absolutely necessary, is to take on as little of that added damage as possible, and not pretend that any direct benefits will be involved. Any pretense of that sort may well lure us into assuming still greater damage—damage we may not be able to afford under any circumstances and for any reason.

Actually, as the situation appears right now, it is not likely that the use of radiation in modern medicine, research, and industry will overstep the maximum bounds set by scientists who have weighed the problem carefully. Only nuclear warfare is likely to do so, and apparently those governments with large capacities in this direction are thoroughly aware of the danger and (so far, at least) have guided their foreign policies accordingly.

SUGGESTED REFERENCES

Books

_Radiation, Genes, and Man_, Bruce Wallace and Theodosius Dobzhansky, Holt, Rinehart and Winston, Inc., New York 10017, 1963, 205 pp., $5.00 (hardback); $1.28 (paperback).

_Genetics in the Atomic Age_ (second edition), Charlotte Auerbach, Oxford University Press, Inc., Fair Lawn, New Jersey 07410, 1965, 111 pp., $2.50.

_Atomic Radiation and Life_ (revised edition), Peter Alexander, Penguin Books, Inc., Baltimore, Maryland 21211, 1966, 288 pp., $1.65.

_The Genetic Code_, Isaac Asimov, Grossman Publishers, Inc., The Orion Press, New York 10003, 1963, 187 pp., $3.95 (hardback); $0.60 (paperback) from the New American Library of World Literature, Inc., New York 10022.

_Radiation: What It Is and How It Affects You._ Ralph E. Lapp and Jack Schubert, The Viking Press, New York 10022, 1957, 314 pp., $4.50 (hardback); $1.45 (paperback).

_Report of the United Nations Scientific Committee on the Effects of Atomic Radiation_, General Assembly, 19th Session, Supplement No. 14 (A/5814), United Nations, International Documents Service, Columbia University Press, New York 10027, 1964, 120 pp., $1.50.

_The Effects of Nuclear Weapons_, Samuel Glasstone (Ed.), U. S. Atomic Energy Commission, 1962, 730 pp., $3.00. Available from the Superintendent of Documents, U. S. Government Printing Office, Washington, D. C. 20402.

_Effect of Radiation on Human Heredity_, World Health Organization, International Documents Service, Columbia University Press, New York 10027, 1957, 168 pp., $4.00.

_The Nature of Radioactive Fallout and Its Effects on Man_, Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, 85th Congress, 1st Session, U. S. Government Printing Office, 1957, Volume I, 1008 pp., $3.75; Volume II, 1057 pp., $3.50. Available from the Office of the Joint Committee on Atomic Energy, Congress of the United States, Senate Post Office, Washington, D. C. 20510.

_Genetics, Radiobiology, and Radiology_, Proceedings of the Midwestern Conference, Wendell G. Scott and Evans Titus, Charles C. Thomas Publisher, Springfield, Illinois 62703, 1959, 166 pp., $5.50.

Articles

Genetic Hazards of Nuclear Radiations, Bentley Glass, _Science_, 126: 241 (August 9, 1957).

Genetic Loads in Natural Populations, Theodosius Dobzhansky, _Science_, 126: 191 (August 2, 1957).

Radiation Dose Rate and Mutation Frequency, W. L. Russell and others, _Science_, 128: 1546 (December 19, 1958).

Ionizing Radiation and the Living Cell, Alexander Hollaender and George E. Stapleton, _Scientific American_, 201: 95 (September 1959).

Radiation and Human Mutation, H. J. Muller, _Scientific American_, 193: 58 (November 1955).

Ionizing Radiation and Evolution, James F. Crow, _Scientific American_, 201: 138 (September 1959).

Motion Pictures

_Radiation and the Population_, 29 minutes, sound, black and white, 1962. Produced by the Argonne National Laboratory. This film explains how radiation causes mutations and how these mutations are passed on to succeeding generations. Mutation research is illustrated with results of experimentation on generations of mice. A discussion of work with fruit flies and induced mutations is also included. This film is available for loan without charge from the AEC Headquarters Film Library, Division of Public Information, U. S. Atomic Energy Commission, Washington, D. C. 20545 and from other AEC film libraries.

The following films were produced by the American Institute of Biological Sciences and may be rented from the Text-Film Division, McGraw-Hill Book Company, 330 West 42nd Street, New York 10036.

_Mutation_, 28 minutes, sound, color, 1962. This film discusses chromosomal and genetic mutations as applied to man. Muller’s work in inducing mutations by X rays is described.

These three films are 30 minutes long, have sound, are in black and white, and were released in 1960. They are part of a 48-film series that is correlated with the textbook, _Principles of Genetics_, (fifth edition), Edmund W. Sinnott, L. C. Dunn, and Theodosius Dobzhansky, McGraw-Hill Book Company, 1958, 459 pp., $8.50.

_Mutagen-Induced Gene Mutation._ The narrator of this film is Hermann J. Muller, who won a Nobel Prize in 1946 for his work in the field of genetics. The measurement of X-ray dose in roentgens and the dose required to double the spontaneous mutation rate in _Drosophila_ and mice are discussed. The magnitude and meaning of permissible doses of high-energy radiation are discussed. Other mutagenic agents (ultraviolet light and chemical substances) are discussed, concluding with comments on the importance of gene mutation in the present and future.

_Selection, Genetic Death and Genetic Radiation Damage._ The narrator of this film is Theodosius Dobzhansky, the coauthor of this booklet. Genetic death is discussed in detail, as are examples of how genetic loads are changed subsequent to radiation exposure. While it is generally agreed that the great majority of mutants are harmful when homozygous, more evidence is needed about the beneficial and detrimental effects of mutants when heterozygous. In the case of sickle cell anemia, heterozygotes are adaptively superior to normal homozygotes. This makes for balanced polymorphism, by which a gene is retained in the population despite its lethality when homozygous because of the advantage it confers when heterozygous.

_Gene Structure and Gene Action._ The lecturer of this film is G. W. Beadle of Cornell University. The Watson-Crick structure of DNA is discussed in terms of mutation. Several tests of the chain separation hypothesis for DNA replication are described (experiments with heavy DNA, radioactive chromosomes, and the replication of DNA in vitro). This working hypothesis is presented: The coded information in DNA is transferred to RNA, which serves as a template for polypeptide synthesis.

PHOTO CREDITS

Dr. Asimov’s photograph by David R. Phillips, courtesy _Chemical and Engineering News_

Page

4 James German, M.D. 6 Bausch & Lomb, Inc. 12 James German, M.D. 20 Indiana University 24 Robert C. Filz, Air Force Cambridge Research Laboratories 25 J. K. Boggild, Niels Bohr Institute, Copenhagen University 26 Brookhaven National University 28, 31 Herman Yagoda, Air Force Cambridge Research Laboratories 41 Oak Ridge National Laboratory

Footnotes

[1]For more detail about cell division, see _Radioisotopes and Life Processes_, another booklet in this series.

[2]This is more commonly known as “Mongolism” or “Mongolian idiocy” though it has nothing to do with the Mongolian people.

[3]Actually, all waves have some of the characteristics of particles and all particles have some of the characteristics of waves. Usually, however, the radiation is predominantly one or the other and little confusion arises under ordinary circumstances in speaking of waves and particles as though they were separate phenomena.

[4]For more about this subject, see _Radioisotopes in Industry_ and _Radioisotopes in Medicine_, companion booklets in this series.

[5]For more about this subject, see _Fallout from Nuclear Tests_, another booklet in this series.

[6]For details on _somatic_ effects of radiation, see _Your Body and Radiation_, a companion booklet in this series.

[7]Estimated average exposures to the gonads, based on 1963 report of Federal Radiation Council.

[8]One thousandth of a rem.

[9]Nevertheless, it should be pointed out that the precautions taken in the atomic energy industry are such that absorption of radiation is not as severe a problem as one might suspect. Fully 95% of those engaged in this work receive less than 1 rem a year. Only 1% receive more than 5 rems.

UNITED STATES ATOMIC ENERGY COMMISSION

_Dr. Glenn T. Seaborg, Chairman_ _James T. Ramey_ _Dr. Gerald F. Tape_ _Dr. Samuel M. Nabrit_ _Wilfrid E. Johnson_

_ONE OF A SERIES ON UNDERSTANDING THE ATOM_

Nuclear energy is playing a vital role in the life of every man, woman, and child in the United States today. In the years ahead it will affect increasingly all the peoples of the earth. It is essential that all Americans gain an understanding of this vital force if they are to discharge thoughtfully their responsibilities as citizens and if they are to realize fully the myriad benefits that nuclear energy offers them.

The United States Atomic Energy Commission provides this booklet to help you achieve such understanding.

Edward J. Brunenkant Director Division of Technical Information

This booklet is one of the “Understanding the Atom” Series. Comments are invited on this booklet and others in the series; please send them to the Division of Technical Information, U. S. Atomic Energy Commission, Washington, D. C. 20545.

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NUCLEAR POWER AND MERCHANT SHIPPING PLUTONIUM OUR ATOMIC WORLD NUCLEAR ENERGY FOR DESALTING CONTROLLED NUCLEAR FUSION WHOLE BODY COUNTERS PLOWSHARE POPULAR BOOKS ON NUCLEAR SCIENCE SNAP, NUCLEAR SPACE REACTORS NUCLEAR REACTORS ATOMS, NATURE, AND MAN MICROSTRUCTURE OF MATTER SYNTHETIC TRANSURANIUM ELEMENTS COMPUTERS RESEARCH REACTORS GENETIC EFFECTS OF RADIATION POWER FROM RADIOISOTOPES NONDESTRUCTIVE TESTING RARE EARTHS FOOD PRESERVATION BY IRRADIATION FALLOUT FROM NUCLEAR TESTS RADIOACTIVE WASTES RADIOISOTOPES IN INDUSTRY ATOMS AT THE SCIENCE FAIR RADIOISOTOPES AND LIFE PROCESSES ATOMIC FUEL ATOMIC POWER SAFETY DIRECT CONVERSION OF ENERGY CAREERS IN ATOMIC ENERGY RADIOISOTOPES IN MEDICINE ACCELERATORS NUCLEAR TERMS, A BRIEF GLOSSARY NEUTRON ACTIVATION ANALYSIS ATOMS IN AGRICULTURE POWER REACTORS IN SMALL PACKAGES

Single copies of any booklet may be obtained free by writing to:

USAEC, P. O. BOX 62, OAK RIDGE, TENNESSEE 37830

Requests for more than three titles generally can not be honored.

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Printed in the United States of America

USAEC Division of Technical Information Extension, Oak Ridge, Tennessee September 1966

Transcriber’s Notes

--Retained publication information from the printed edition: this eBook is public-domain in the country of publication.

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