HEREDITY OF SEX 257 



contain a heterotropic chromosome. At first sight 

 this necessary supposition of a selective fertilization 

 presented itself to Wilson as a serious difficulty. He 

 points out, however, that the experiments of Cuenot 

 with yellow mice* afford perfectly independent evidence 

 of the actual occurrence of a selective fertilization of 

 this kind in a particular species of animal (and similarly 

 Wilson's observations lend a welcome confirmation to 

 Cuenot's conclusions from his experiments). 



In mice Cuenot found that on crossing together 

 heterozygous yellows CYCG x CYCG, he obtained no 

 pure dominant yellows CYCY such as were to be 

 expected ; and this in spite of the fact that CYCG x 

 CGCG gave equal numbers of yellows and greys 

 (showing that the CYCG individuals were giving off 

 equal numbers of yellow- and of grey-bearing gametes). 

 Cuenot explained his unusual result by supposing that 

 yellow-bearing spermatozoa unite only with grey- 

 bearing eggs, and vice versa. On this explanation, one 

 might at first sight expect to get a proportion of two 

 yellows to one grey instead of nearly three to one, 

 which was the ratio actually observed, since the 

 fertilization yellow by yellow fails altogether. But 

 Wilson points out that, since spermatozoa are in great 

 numerical excess as compared with eggs, it will be 

 possible for all the Y-bearing eggs to be fertilized by 

 G-bearing spermatozoa, as well as half the G-bearing 

 eggs by Y-bearing spermatozoa, thus bringing the pro- 

 portion of yellows to greys more or less nearly up to 

 three to one. 



* See p. 195. 



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