/I2 MENDELISM chap. 



to marry a normal male than a colour-blind one, 

 we can understand why colour-blindness is so much 

 more common in men than in women. These 

 two different types of sex-limited inheritance lead 

 us to believe that in certain groups of animals, 

 viz. birds and moths, the female is the heterozygous 

 sex, while the converse holds in flies and mammals. 

 Moreover, in Drosophila there is definite evidence 

 derived from the nature and arrangement of the 

 chromosomes in favour of this view. Unfortunately 

 the determination of the relations of the chromo- 

 somes is very much more difficult in the other 

 groups, owing to their greater number and to the 

 differences between them being far less marked. In 

 the currant moth, where, thanks to Doncaster's 

 researches, more is known than in man or in fowls, 

 the normal number of chromosomes in the body 

 cells is 56, and is the same in both sexes. It has 

 not been found possible to identify any given pair 

 as the sex-chromosomes, for there is no distinctly 

 unequal pair in the heterozygous sex, which in this 

 case is the female. But though the numbers of the 

 sex pair may be visibly indistinguishable, it does not 

 necessarily follow that they are qualitatively so. In 

 Drosophila the X and the Y chromosomes are 

 similar in point of size, and it is quite possible 

 that in some other species the highest powers of the 

 microscope may be unable to distinguish between 

 them. It is conceivable that in the group of cases 

 of which the fowl and currant moth are typical, 

 the pair of sex-chromosomes in the female sex may 

 be qualitatively unequal, and of such a nature that 

 while one of them, like the two sex-chromosomes of 



