Experiments with Poultry 129 



This idea of combined dominance was first suggested by Bate- 

 son and Punnett to explain the walnut comb of certain hybrids. 

 Thus when rose comb and pea comb are crossed, walnut comb 

 results. The gametes produced by these hybrids are of four 

 types and in equal numbers ; namely, single, rose, pea, and wal- 

 nut, giving when inbred 9 walnut (rose-pea), 3 rose, 3 pea, i sin- 

 gle comb. The interpretation offered by Bateson and Punnett 

 for these facts is that the characters of the original parents with 

 rose and pea combs are rose and no pea, and pea and no rose. 

 The contrasted characters are rose and absence of rose, pea and 

 absence of pea. When rose and pea bearing gametes meet, the 

 walnut comb is produced. The results follow the rule for two 

 characters. In other words, rose and pea combs are not them- 

 selves contrasted characters, but the allelomorph of each is its 

 absence. The authors point out, however, the danger involved 

 of making general assumptions of this sort when two characters 

 meet. 



The nostrils of the Minorca are slitlike, and this dominates 

 the wide-open nostril of the Polish, but the dominance is imper- 

 fect in the first hybrids. In the next generation, F^^^ the split 

 nostril is present in 21 per cent, but even in this generation the 

 high or dominant nostril is frequently imperfect. 



The Polish fowls have a large cerebral hernia on the top of the 

 head, covered by a hardened layer of outer brain coat or dura 

 mater, and by the skin. The Minorca breed has a normal head. 

 In the hybrids, F-^, not a single case of hernia occurred, but 

 most of them showed evidence of their mixed ancestry in the pres- 

 ence of a frontal eminence. In the second hybrid generation, 

 Fgj the hernia reappears again in 23.5 per cent of cases — • 

 a close agreement with Mendelian expectation. 



A crest is present in all the hybrids, but always reduced in size. 

 ''The crest is dominant, but dominance is imperfect." The 

 crest is larger in the female hybrids, as it is in the female PoHsh 

 breed. In the second hybrid generation the crest was absent in 

 about 30.7 per cent. 



Davenport concludes that in this cross the dominance is incom- 



