356 AN INTRODUCTION TO MODERN GENETICS 



philia gene seems to remain constant, mutation must replace it as fast 

 as it is lost by the failure of affected males to breed. This gives a muta- 

 tion rate of about i in 50,000 life cycles, which, in terms of life cycles^ 

 is rather higher than is typical for Drosophila genes. 



Matters are quite different for autosomal recessives. Only a very 

 small proportion of carriers are homozygous and show the character 

 (frequencies p^ A A : 2pq Aa : (f ad). Sterilization of the recessive 

 phenotypes can therefore only act very slowly to prevent the appearance 

 of the character in future generations, and it will act the more slowly 

 the rarer the gene is. It was at one time thought that, on account of 

 this, steriUzation could be of very Httle use, since most deleterious 

 characters were thought to be inherited as recessives. It seems, how- 

 ever, that many apparent recessives are really dominants with low 

 penetrance, in which case selection against them would be somewhat 

 more effective. 



Sterilization has been legalized and practised in some countries, the 

 most important being the U.S.A. and Germany. In the U.S.A. the 

 laws have been in force for some years and considerable experience has 

 been gained in their effects. California is the state which has been most 

 active in this respect, some 20,000 steriHzations having been performed 

 up to 1935. Most mental defectives and cases of insanity submit volun- 

 tarily to the operation before leaving institutional care, and a fairly 

 thorough supervision is kept over them after release. It is still uncertain 

 exactly how mental defect is inherited, in so far as it is inherited at all 

 and not environmentally determined, but it seems probable that among 

 the many multiple genes which seem to be concerned (p. 336) some at 

 least are partially dominant, so that selection against them may not be 

 too slow. Moreover, there is a considerable tendency for the feeble- 

 minded to marry each other, and the proportion of feeble-minded genes 

 in the inbreeding groups may be quite high, so that even if they are all 

 recessive selection can be expected to have appreciable effects. The 

 method has, however, not been in use long enough to have produced 

 any marked changes in the incidence of feeble-mindedness in the 

 population. But it seems clear that the operation in itself produces no 

 untoward symptoms in those who submit to it. 



The social dangers of a procedure such as steriUzation should be 



very obvious.. It is, however, a subject on which opinions tend to be 



extreme, and some of its advocates, such as the authors of the model 



sterilization law,^ suggest that it should be applied to some classes of 



^ Quoted in Haldane 1938. 





