22 BIOLOGICAL EFFECTS OF ATOMIC RADIATION 



B. Mechanism of equilibrium primarily that of opposition between pressure of recurrent mu- 

 tation and adverse selection. 



1 . Adverse effect in heterozygote. 



2. Adverse effect in homozygote. Complete recessive. 



Alleles of the A class have an effect on variability far out of proportion to the frequency 

 of occurrence of mutations since they tend to be held at high frequency. Thus if we take 

 s = 0.04 as the average selective disadvantage of type B heterozygotes and v = 4x 10 " as the 

 average mutation rate, the mean gene frequency at equilibrium is v/s = 0.0001. If type A 

 mutation can also occur at these loci but with an average rate of only 4x 10", 99.99% of 

 the mutations that occur are of type B. Nevertheless if the type A mutations reach equilibrium 

 with a mean gene frequency of 0.10 because of the postulated opposed selection pressures, 

 there will be one thousand of them for each deleterious B gene in the population and they 

 could be responsible for most apparent genetic defects. Changes in dosage of ionizing radia- 

 tion have no appreciable effect on the incidence of this sort of gene defect. It is the price the 

 population must pay for the advantages of these same genes in other individuals. 



In the case of deleterious mutations of class B, on the other hand, an indefinitely con- 

 tinued excess dosage of radiation tends to bring about ultimately a corresponding percentage 

 increase in the frequency at which the effects of all such mutant genes are manifested in the 

 population. The mode of approach to equilibrium is very different for mutations with hetero- 

 zygous deleterious effect and for those that are completely recessive. In the former, the devia- 

 tion from the new equilibrium tends to be (1 — s)" of its initial value in n generations, where s 

 is the selective disadvantage. Thus in the case of a dominant lethal with complete penetrance, 

 the full effect occurs in the first generation (and full recovery occurs in the first generation 

 after cessation of the excess radiation). With s = 0.10, it takes about 7 generations to go half 

 way to the new equilibrium, and with s = 0.01, about 69 generations for this to happen. Re- 

 covery after cessation occurs in the same way. In the case of a completely recessive mutation, 

 the approach to the new equilibrium is excessively slow (2 Vus of the ultimate effect per 

 generation at first, where u is the mutation rate and s is the selective disadvantage). Thus a 

 recessive lethal (s= 1 ) arising at the rate 10 ^ per generation goes only 0.6% toward the new 

 equilibrium in each early generation. The recovery rate is a little more rapid at first but still 

 exceedingly slow. With respect to the next thousand years, the mutations due to excess radia- 

 tion that will cause appreciable damage are those of class B with selective disadvantage of 

 the heterozygote greater than 0.01. 



We need not pause long on mutations that merely cause differences within categories 1 

 to 4, as far as damage to society is concerned. There are no doubt many types of mutation 

 that are undesirable from the personal standpoint but these are likely to be held at low fre- 

 quencies by adverse marriage selection. 



The extreme diversity of physical and mental types that can find a satisfactory niche in 

 modern civilization provides an enormous amount of buffering against defects that would 

 have been damaging in primitive man. Conditions that incapacitate for some ways of making 

 a living may make no difference in others. Some conditions that would have been highly 

 damaging once can be corrected at little cost in modern civilization (e.g., visual defects that 

 can be corrected by glasses). Medical advance is continually reducing the amount of damage 

 from many conditions. This does not mean that natural selection is ceasing to operate. It is 

 merely being redirected into the channels most significant today. 



