n] Structural Characters: Animals 35 



several distinct ways. It may affect mainly the anterior portion, or the 

 posterior. The split combs of established breeds have possessed ordinary 

 dominance; but a form of posterior splitting somewhat like that of the F^ 

 from Breda x Single occurred apparently as a mutation among extracted 

 singles, and exhibited a curious genetic behaviour suggesting irregularity of 

 dominance (20, pp. 108 and 113). 



47. The normally webbed feathers are dominant to 

 the peculiar feathers of the Silky fowl. 



48. Extra toe is usually dominant to the normal four- 

 toed condition, but exceptions occur. 



This irregularity of dominance is exhibited by all cases of polydactylism 

 yet studied in birds or mammals. It seems to be a property of certain 

 strains. Some families run a perfectly regular Mendelian course, others 

 contain members with only the normal four toes, which are yet capable of 

 transmitting the extra toe. The numbers in such families are not favour- 

 able to the suggestion that the irregularity is caused by a definite disturbing 

 factor. 



49. Crest is dominant to no crest. 



F z may contain individuals with crests far larger than those of the 

 parent crested breed, a fact which suggests that in breeds with small crests 

 (e.g. Silky) the full development of the crest is kept in check by some other 

 factor. 



50. Feathered leg partially dominates over clean leg. 



Both Hurst (156) and Davenport found dominance very irregular. F^ 

 is intermediate, and traces of leg-feathering are occasionally seen in the 

 offspring of clean-legged birds. 



51. "Frizzling," or turning back of the feathers, is 

 dominant to the plain straight feathers of the normal. 



52. Normal size of feathers on the hocks, or tibio-tarsal 

 region, is dominant to elongation of these feathers to form 

 quills the " Vulture-hock " of fanciers. 



53. Muff, or tuft of feathers at sides of the bill and 

 throat, as in Faverolles, is dominant to no muff, as in ordinary 

 breeds. 



54. Imperfect development of coccyx and tail-feathers 

 with absence of tail, as in " Rumpless " fowls, is dominant 

 to the normal development of those parts. 



This interesting case was investigated by Davenport (101). It is 

 exactly comparable with that of the Manx Cat. 



32 



