SEX 



99 



birds zvere always females (cf. Fig. 21). And in 

 this respect all the F a males behaved alike, whether 

 they were from the Silky hen or from the Silky 

 cock. We have, therefore, the paradox that the F 

 hen, though herself deeply pigmented, cannot trans- 

 mit this condition to any of her offspring when she 

 is mated to the unpigmented Brown Leghorn, but 

 that, when similarly mated, the F X cock can transmit 

 this pigmented condition to a quarter of his female 



(Brown Leghorn) 



Silky 



(BrownLegh.)^ X ? 



X 9 (Brown Leghorn) 



9 9 



T 1 1 1 



FIG. 21. 



Scheme to illustrate the result of crossing F t birds (e.g. Brown Leghorn X Silky) 

 with the pure Brown Leghorn. 



offspring though he himself is almost devoid of 

 pigment. 



Now all these apparently complicated results, as 

 well as many others to which we have not alluded, 

 can be expressed by the following simple scheme. 

 There are three factors affecting pigment, viz. (i) 

 a pigmentation factor (/*) ; (2) a factor which 

 inhibits the production of pigment (/) ; and (3) a 

 factor for femaleness (F), for which the female birds 

 are heterozygous, but which is not present in the 

 males. Further, we make the assumptions (a) that 

 there is repulsion between F and / in the female 

 zygote (Fflt), and (#) that the male Brown Leghorn 



