v] MENDELIAN HEREDITY 67 



As in plants, several of these cases are not in 

 any way connected with domestication, and the wide 

 diversity of species and characters in which Mendelian 

 inheritance has been discovered shows that the phe- 

 nomena are not rare or exceptional, but universally 

 distributed. 



It has been mentioned that of a pair of allelo- 

 morphic characters, one is regarded as containing some 

 factor absent from the other, and it may be well to 

 give an example of the kind of evidence that leads 

 to this conclusion. In fowls there are three chief 

 forms of comb; 'single' with a median serrated 

 ridge, 'rose' with a broad upper surface covered 

 with papillae, and 'pea' with a shape consisting 

 essentially of three parallel low ridges. Rose and 

 pea each behave as dominants to single, but when 

 rose is crossed with pea a fourth type, 'walnut' 

 results, which in the adult is swollen and dimpled, 

 and, in the young at least, is crossed by a transverse 

 band of bristles. In the Malay breed such ' walnut ' 

 combs breed true, but when made by crossing ' rose ' 

 by 'pea,' and mated together, the resulting chicks 

 appear in the ratio of 9 walnut, 3 rose, 3 pea, 1 single. 

 The appearance of singles in the F 2 generation from 

 pure rose by pure pea is explained by the ' presence 

 and absence ' hypothesis. Rose (R) and pea (P) are 

 each allelomorphic with their absence (r, p). A rose- 

 combed bird is thus Bp, and a pea-combed rP, 



52 



