MENDELISM 61 



have all walnut combs ; but the second generation, 

 instead of giving 3 walnut : 1 single, as might have 

 been expected, is composed of walnut, rose, pea, and 

 single-combed individuals in the ratio of 9:3:3:1. 

 Further, when rose and pea are crossed, the first hybrid 

 generation has walnut combs. From this it was at 

 first thought that walnut was the hybrid between rose 

 and pea, and that the folio-wing generation would give 

 the ratio 1 rose : 2 walnut : 1 pea. This, however. 



Fio. 9.— Types of Fowls' Combs. A, Pea ; B, Rose ; C, Single ; 

 D, Walnut. (After Punnett.) 



did not occur. Instead, the ratio was again 9 walnut : 

 3 rose : 3 pea : 1 single. 



What is the explanation of these rather remarkable 

 results ? It is assumed that the rose comb is due to a 

 dominant factor A. The pea comb is determined by a 

 separate factor B, also dominant to its absence. The 

 important point now is that A and B themselves do 

 not form a pair of unit characters. Each is dominant 

 to its absence, but the two have nothing to do with 

 each other so far as segregation goes. If A and B are 

 both absent, the fowl has a single comb. If both are 



