1 90 



COMPARATIVE PHYSIOLOGY 



pj 



Ft 



by genetical factors for which one sex is heterozygous, so that 

 a I : I sex ratio is maintained by the normal consequences of a 

 homozygous-heterozygous mating. Since in this case maleness 

 is the state associated with the single condition and femaleness 

 with the duplex state as regards the sex-linked genes, the 

 male may be represented symbolically as F/ and the female as 

 FF, using the symbol F for that which determines femaleness. 

 This type of sex-linked inheritance occurs in most insects 

 and in mammals ; and for reasons 

 given later may be anticipated to 

 occur in practically all higher bi- 

 sexual animals except birds and 

 lepidoptera (moths and butterflies). 

 The phenomenon of sex-linked in- 

 heritance was first discovered in the 

 latter group by Doncaster (1906). 

 A variety of the currant moth 

 Abraxas grossulariata is distin- 

 guished by the pale colour of the 

 wings as lacticolor. If a lacticolor 

 female (wings of a pale cream tint) 

 is crossed with the normal dark- 

 winged (grossulariata) male, all the 

 offspring of both sexes are of the 

 grossulariata type : in the F.2 there 

 is a 3:1 ratio of grossulariata to 

 lacticolor, but all the males are of 

 the grossulariata type. In the reciprocal mating the grossu- 

 lariata female can only transmit the grossulariata pattern to 

 her sons ; all the female offspring are of the lacticolor type. 

 When the F.i are mated inter se^ equal numbers of lacticolor 

 and grossulariata males and females are produced (diagram). 

 Here the female moth is constitutionally simplex with 

 respect to the sex-Hnked genes. This is the exact reverse of 

 the state of affairs in sex-linked characters in Drosophila. 

 Femaleness is associated with the simxplex condition of genes 

 which in the duplex condition give rise to maleness. 



The predetermination of sex by genetic factors does not 



Fig. 42. — Sex-linked inherit- 

 ance in Drosophila. 



