24 BIOLOGICAL EFFECTS OF ATOMIC RADIATION 



The mutation rate that would maintain a frequency of 1 % homozygotes against a 5 % selec- 

 tion disadvantage is 5 x 10"*. This is on the assumption of a single locus. It would be reduced 

 tenfold if there were 10 equivalent loci. Accepting provisionally the hypothesis of only one 

 locus, the estimated mutation rate is so high as to suggest that the observed incidence is due 

 to balance of opposed selection pressure (class A) rather than to adverse selection balanced 

 by mutation (class B). If this is the case, the burden of overt schizophrenia is the price 

 society pays for benefits conferred by persons of slightly schizoid type among the heterozy- 

 gotes or the 90% homozygotes that do not break down. The effect of increased radiation 

 would be very slow even if a gene of class B is responsible, if the condition is recessive. The 

 predisposition to manic depressive reaction is treated by Penrose as due to a dominant gene 

 with frequency of heterozygotes of about 0.5% in the population, 14% breakdown of these 

 with loss of fecundity of 10% (s=l/70). This leads to an estimated mutation rate of 

 0.000035. This is considerably lower than for schizophrenia but raises the same question. 



It is thus far from certain what the effect of increased radiation would be in the burden 

 from the major psychoses. It is among the most important problems in the field to obtain a 

 better understanding of the genetic situation, for in neither case is it likely to be as simple as 

 assumed in the above analysis. 



With respect to categories 1 1 and 12 (burden from completely incapacitating conditions 

 that appear at birth or in childhood leading to early death (12) or incapacity of long dura- 

 tion (11)), there is a considerable portion that is certainly nongenetic (birth injury, childhood 

 infection, etc.) but also a well recognized portion due to rare genes, undoubtedly of types BI 

 or B2. The effect of increased radiation on the genetic burden can be estimated more reliably 

 than in any of the other categories though there are still plenty of problems. 



Embryonic and fetal death (category 13) is known to be due often to maternal virus 

 infection and other nongenetic causes. There is no doubt also a genetic component but this is 

 obviously more diflScult to analyze than in categories 11 and 12. This is not of primary im- 

 portance since the genetic burden is less important. 



From this survey it may seem that a minimal estimate of the effects of increased radiation 

 can be obtained for consideration of rare family traits in categories 1 1 and 12 and some (such 

 as Huntington's chorea) in others. The total effect is probably considerably greater and may 

 be enormously greater depending on appraisals of heritability and of the portion of that herit- 

 able in class B, especially Bl in such conditions as poor physical constitution, feeble minded- 

 ness, physical and mental breakdown in maturity. 



