Genetic Loads and Their Population Effects 



223 



GENETIC 

 LOAD 



Spontaneous 



Radiation 

 Exposure 



GENERATIONS 



figure 16-3. Genetic load and exposure to 

 radiation. 



It is clearly important to learn in detail 

 the genetic effects of high-energy radiation 

 to which human populations are being sub- 

 jected either purposely or circumstantially. 

 In order to make the best evaluation, we will 

 need to know much more about: the dis- 

 tribution of the energy of various radiations 

 in tissue; the exact amount of gonadal ex- 

 posure to radiations of different types; the 

 detriment of the induced mutants in hetero- 

 and homozygous conditions; the persistence 

 of mutants; and the different types and the 

 frequencies of mutations that each kind of 

 radiation produces in different stages of male 

 and female gametogenesis. 



In the last respect, it is necessary to de- 

 termine for various types of mutations, the 

 relative mutagenicity of a concentrated dose 

 and one given in a protracted manner. It 

 is also necessary to learn as accurately as 

 possible the mutability of spermatogonia and 

 oocytes, because these are the stages in 

 which the human germ cells producing the 

 next generation remain for the longest pe- 

 riod of time. It is suggested that the largest 

 number of germ-line mutations occurs in 

 oocytes. Because spermatogonia are con- 

 stantly dividing, mutants producing a detri- 

 mental effect may be selected against so that 

 they are reduced in frequency by the time 

 gametes are formed; the human female, how- 

 ever, is born with all, or almost all, her 



future gametes already in the oocyte stage 

 so that there is no parallel mitotic selection 

 in this germ line. Not only do oocytes fail 

 to undergo mitosis, but they remain rela- 

 tively inactive for decades before becoming 

 ova; as oocytes age during this period, they 

 become disproportionately sensitive to spon- 

 taneous mutation (at least to factors leading 

 to aneusomy). 



Although at present we do not have as 

 much information about any one of these 

 factors as we would like, available informa- 

 tion along these lines already gives us ap- 

 proximate answers (see the references at the 

 end of this chapter). It should be noted, 

 therefore, that all values given in the dis- 

 cussion below may be in error by as much 

 as a factor of two or more. 



It has been a practice to discuss the germ- 

 line effect of radiation in terms of the amount 

 of increase any particular exposure would 

 produce in our spontaneous mutation fre- 

 quency. The general impression is held that, 

 as a species, man is fairly well adapted to 

 his spontaneous mutation rate, and that if 

 this rate is doubled it will not threaten his 

 survival. Accordingly, the question be- 

 comes, how much man-made radiation would 

 produce as many mutations as occur nor- 

 mally? A United Nations report calculates 

 that about 30 rads (roughly equal to 30 r) 

 is sufficient to double the human sponta- 

 neous mutation rate — the frequency per gen- 

 eration. This amount is called the doubling 

 dose. In a population of one million peo- 

 ple, one rad delivered to the gonads, or sex 

 organs, of each person is calculated to pro- 

 duce between 100 and 4,000 mutants which 

 could be transmitted to future generations. 

 Thus, one rad of gonadal exposure for one 

 generation will result in the birth of 100 to 

 4,000 people with new heterozygous mu- 

 tants. Affected descendants will occur for 

 many generations, since only a small por- 

 tion of the genetic deaths from these mutants 

 will occur in the first generation. These 



