92 



A MANUAL OF MENDELISM 



But why does none of the cocks get a second non- 

 barring factor ? It can only be that this second non- 

 barring factor is intransmissible to males and therefore 

 always tied up with or coupled with femaleness. The 

 hypothesis now is that the Barred Rock birds which 

 throw barred sons always but non-barred daughters 

 occasionally are of the constitutions — ^the closed bracket 

 being used to indicate the tied factors : 



Males 



MM 

 Bb 



Females 



M 

 B 



The male progeny of such parents must take M from 

 each parent, but, while they may take either B or b 

 from their fathers, they must always take B from their 

 mothers. The female progeny, on the other hand, must 

 always take F and the tied b from their mothers, but, 

 while they must always take M, they may take either 

 B or the untied b from their fathers. The progeny 

 should consist therefore of the four following kinds : 



MM 

 BB 



MM 

 Bb 



M 

 B 



Barred 

 Cocks 



Barred 



Hens 



Unbarred 

 Hens 



Thus the occasional appearance of unbarred hens in the 

 Barred Rock breed is explained. Some of the cocks 

 are hybrids. 



But will this hypothesis, which was first suggested 

 by Mr. W. J. Spillman, stand the test of other cases ? 



