CHAT. X 



SEX 



99 



9 



females about half were heterozygous and the other 

 half were homozygous. A comparison of Fig. 27 

 with Fig. 18 on p. 85 makes it clear that the case 

 of Drosophila is closely comparable with that of the 

 Sebright bantams, except that the sexes, in respect 

 of the factor showing sex - limited inheritance, are 

 apparently reversed. Whereas in fowls it is the 

 female which must be supposed to be heterozygous 

 both in sex and in the sex- 

 limited factor, in Drosophila 

 it is the male. This view is 

 confirmed by the results of 

 the reciprocal cross, viz. red- 

 eyed male x white - eyed 

 female. In order to make 

 this cross it was first neces- 

 sary to obtain the white-eyed 

 female. This was done by 

 mating the F^ female with a 

 white -eyed male. Such a 

 cross is comparable to that 

 between a heterozygous silver 



cock and a gold hen. Just as in the Sebrights this 

 mating produces equal numbers of golds and silvers 

 in both sexes, so in Drosophila it gave in both sexes 

 equal numbers of red- and of white-eyed flies. 

 Having obtained the white -eyed female it was 

 possible to make the reciprocal cross. It resulted 

 in offspring of two kinds only, viz. red-eyed females 

 and white - eyed males. As in the Sebright, the 

 sons take after the mother and the daughters after 

 the father. The outstanding difference between the 

 bantams and the flies is that in the former the 



a white -eyed male Drosoiihila 

 and a red-eyed female. Black 

 sex signs denote white eye, and 

 light ones denote red eye. Light 

 sex signs with a dot in the middle 

 denote heterozygous flies. 



