CHAPTER X 



SEX 



In their simplest expression the phenomena exhibited 

 by Mendelian characters are sharp and clean cut. Clean 

 cut and sharp also are the phenomena of sex. It was 

 natural, therefore, that a comparison should have been 

 early instituted between these two sets of phenomena. 

 As a general rule, the cross between a male and a female 

 results in the production of the two sexes in approxi- 

 mately equal numbers. The cross between a hetero- 

 zygous dominant and a recessive also leads to equal 

 numbers of recessives and of heterozygous dominants. 

 Is it not, therefore, possible that one of the sexes is hetero- 

 zygous for a factor which is lacking in the other, and that 

 the presence or absence of this factor determines the sex 

 of the zygote ? The results of some recent experiments 

 would appear to justify this interpretation, at any rate in 

 particular cases. Of these, the simplest is that of the 

 common currant moth (Abraxas grossulariata) , of which 

 there exists a pale variety (Fig. 17) known as lacticolor. 

 The experiments of Doncaster and Raynor showed that 

 the variety behaved as a simple recessive to the normal 

 form. But the distribution of the dominants and reces- 



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