IX 



SEX 



85 



from the 3 : I ratio obtained in F^, as well as from 

 the fact that the Fj birds mated with golds give 

 equal numbers of golds and silvers. 



(2) Femaleness is dominant to malencss, the female 

 being heterozygous for a factor, F, which is not 

 present in the male. This assumption is in accord- 

 ance with the fact that the sexes are produced in 

 equal numbers. The female must be supposed to 

 produce equal numbers of female-producing eggs 

 containing F, and of male- 



Silver 



6 



Gold 



e? 



I 



<S (S 



Fig. 18 



1 

 f 



--F„ 



Scheme illustrating the cross between a 

 silver cock and a gold hen. Black 

 sex signs denote gold, and light ones 

 denote silver birds. Light sex signs 

 with a dot in the middle denote 

 heterozygous silvers. 



producing eggs which do 

 not contain the factor. 

 Since the male does not 

 carry F, this factor cannot 

 occur in any spermato- 

 zoon. Every female, on 

 this view, receives the 

 factor for femaleness from 

 her mother. 



(3) The silver hen is 

 always heterozygous for 

 the silver factor, S, as 

 well as for that for femaleness, F. Moreover, it 

 must be supposed that when she forms eggs some 

 form of repulsion occurs between the two factors, of 

 such a kind that they will not enter into the same 

 gamete. The eggs of such a bird must be supposed 

 to be of two kinds and of two kinds only, viz. Fs 



and fS. 



If we make these assumptions, the experimental 

 facts are open to a simple explanation, as shown 

 graphically on Plate V. The original silver male 

 was homozygous for S. As he cannot contain F 



