Genetic Loads and Their Population Effects 



225 



mately 10 to 400 mutations per million peo- 

 ple. How much modification does this figure 

 now need in order to bring it up to date? 

 Before accurate estimates of germ-line mu- 

 tational damage due to fallout can be ob- 

 tained, many factors need be taken into ac- 

 count, among them: 



1. Carbon- 14, whose long half-life was 

 not considered in this report 



2. The changes in rate of testing (accord- 

 ing to the United States Atomic En- 

 ergy Commission, in 1958 alone the 

 amount of fallout-producing radioac- 

 tive material in the stratosphere was 

 doubled by the numerous test explo- 

 sions of nuclear weapons conducted by 

 the United States and the U.S.S.R.) 



3. The unequal distribution of fallout in 

 different parts of the world 



4. Reduction in decay taking place in the 

 stratosphere since fallout is descending 

 faster than expected 



5. Changes in the nature of bombs tested 

 and in the location of the test sites 



6. The decrease in exposure as a result of 

 the test ban treaty. 



Each month brings more of the data re- 

 quired to estimate the fallout risk to the 

 germ cells. Apparently, the possible dam- 

 age has been underestimated. In 1953 the 

 International Commission on Radiological 

 Protection recommended — and various U.S. 

 Government agencies adopted — 80 units as 

 the maximum permissible concentration of 

 strontium-90 in food. In 1958 the Com- 

 mission recommended this maximum be 

 lowered to 33 units, and the new value has 

 subsequently been employed as a guideline 

 by the U.S. Government. 



In principle, exposure to man-made radia- 

 tion undoubtedly produces point mutants in 

 the somatic and germ lines of man, but this 

 possibility is not easy to demonstrate in prac- 

 tice principally for two reasons: The first is 

 that the expected point mutants are not 



qualitatively different from those which oc- 

 cur spontaneously; the second is that the 

 quantitative effect, although large in total, 

 is small enough in any one generation to 

 be masked by the general variability of hu- 

 man genotypes and environment. Through 

 the use of improved statistical methods, 

 however, the evidence that radiation has pro- 

 duced such genetic effects is becoming in- 

 creasingly strong. On the other hand, clear 

 proof that radiation can cause structural 

 changes in human chromosomes does exist. 

 With the recent perfection of cytological 

 methods for studying human chromosomes 

 and the evidence that aneusomy is a rela- 

 tively frequent event in oocytes, it is likely 

 that additional data will be forthcoming 

 about the numbers and kinds of gross chro- 

 mosomal mutations which different types 

 and doses of radiation can induce in man. 



In discussing the genetic effects of low 

 radiation doses, we recognized a danger 

 which is not likely to be calamitous to the 

 human gene pool; however, the very high 

 radiation doses from a nuclear war could be 

 disastrous, for if the whole body receives 

 500 r in a short period of time, the chance 

 is 50% that the affected person will die in 

 a few months. If the person survives this 

 period, his life expectancy is reduced by 

 some years, probably because of somatic 

 mutations, and children conceived after ex- 

 posure will be handicapped by many detri- 

 mental mutants. It is even possible, but 

 not probable, that in a nuclear war enough 

 radiation would be released to destroy the 

 human species. 



Finally, it should be realized that we are 

 being constantly exposed to man-made mu- 

 tagenic chemical substances. Although it 

 is very probable that we are getting fewer 

 germ-line mutations from chemical sub- 

 stances than from radiation, more somatic 

 mutants may be produced by chemical sub- 

 stances than by our present exposure to 

 radiation. 



