UNIT-CHARACTERS OF POULTRY 147 



^ merely for lack of black pigment. Yellow varieties are im- 

 perfectly recessive to black in crosses, the ancestral pattern 

 usually resulting in Fi. Blue is a heterozygote between black 

 and splashed white (an impure sooty strain of white). It is 

 unfixable. 



A color pattern of fowls, not ancestral in origin, but domi- 

 nant in crosses is found in breeds with barred plumage, such 

 as the Dominique and the barred Plymouth Rock. Its 

 inheritance is sex-linked. It may be transmitted through 

 white breeds, as for example the white Leghorn. 



A black pigmented skin associated with black bones is 

 found in certain strains of fowls, e. g,, silkies. This is domi- 

 nant over normal (white or yellow) skin. 



Several morphological variations of the plumage are in- 

 herited as unit-characters. Thus, the possession of a topknot 

 or crest (usually associated with cranial hernia) is an im- 

 perfectly dominant character; frizzled (twisted) feathers are 

 dominant over normal feathers; silky feathers (devoid of 

 barbules) are recessive to normal feathers (with barbules). 

 An extra or fifth toe (due to a divided hind toe) is an imper- 

 fectly dominant character found in Houdans and Dorkings. 

 The comb is also a highly variable character. Single comb is 

 the form found in Gallus hankiva and in the connnoner breeds 

 of poultry. It consists of a high serrated ridge. Pea comb 

 is a dominant variation from this ancestral form in which the 

 comb is lower and broader, without distinct serrations but 

 with two low lateral ridges in addition to a chief central 

 ridge. It is found in Indian Games and the Brahma breeds. 

 Rose is another form of comb, likewise dominant over single. 

 It consists of a broad flat comb with numerous papillae not 

 arranged in distinct rows. A cross of rose with pea produces 

 a peculiar type of comb known as walnut, which is found in 

 the Malay breeds. When produced by crossing, it does not 

 breed true without fixation, but in F2 gives rise to walnut, 

 rose, pea, and single comb in the ratio, 9:3:3:1. Evidently 

 walnut in such cases is due to the joint action of two domi- 

 nant factors (R and P) which act separately in pea-combed 



