FUTURE OF THE SCIENCE OF BREEDING 171 



might have expected other animals to fall into line with this 

 conception of the nature of the difference between male and 

 female. Curiously enough, however, this is not so. Soon 

 after the discovery and elucidation of sex-linked phenomena 

 in the bird and in the moth a case of peculiar interest was 

 found in the pomace fly (Drosophila). Normally the eye is 

 red, but in certain cultures a few white-eyed individuals 

 appeared. These turned out to be males, and experiments 

 showed that white eye is recessive to red. Mated with red- 

 eyed females the white-eyed males gave only red-eyed off- 

 spring. Subsequently white-eyed females were obtained, and 

 it was found that these, when mated with any red-eyed male, 

 gave red-eyed females and white-eyed males only. As in the 

 poultry, the reciprocal crosses produced an unlike result. 

 But, whereas in the fowls it is the female with the dominant 

 character that is always heterozygous, in Drosophila it is the 

 male. Hence we must infer that here it is the male that is 

 heterozygous for a sex-factor. 



Since the case of the white eye was worked out, the same 

 type of sex-linked inheritance has been demonstrated for a 

 number of other characters in Drosophila, and a careful study 

 of these has led Professor Morgan and his collaborators to the 

 conclusion that they are all contained in the same member of 

 the four chromosomes which are present in this species. For 

 not only do these characters exhibit sex-linked inheritance, 

 but they also show, in the female, that peculiar form of 

 linkage among themselves which we have already described. 



But what is the form taken by the sex-factor itself ? The 

 Drosophila workers consider that the determination of sex is 

 a function of a whole chromosome, viz. the chromosome in 

 which the sex-linked factors reside. From recent cytological 

 work it appears that there is a visible difference in the sexes 

 in respect of one of the chromosome pairs. In the female the 

 two members of the pair are indistinguishable, but in the 

 male one is like those found in the female, while the other 

 presents a slight but definite difference in that one end is 

 bent over in the form of a hook. It is to this pair of chromo- 

 somes that sex-determination is attributed. The straight 



