GENERAL, TOPICS IN INHERITANCE. 



Of the varietal characteristics, the positive or patent characteristics almost 

 always are dominant, white plumage forming an occasional exception. On 

 the other hand, phylogenetically old characters are not more apt to be domi- 

 nant than "new" ones. Some evident sports, such as crest, frizzling of 

 feathers, unlimited growth of tail, and black skin (of Silky), are dominant. 

 Other sports hernia, shortened premaxillary, silkiness, and rumplessness 

 are recessive ; the novelty or antiquity of the characteristic has nothing to 

 do with its dominance. Dominance of a character in hybridization is deter- 

 mined by the same causes as determine the appearance in the race of a posi- 

 tive variation. A progressive variation, one which means a further stage in 

 ontogeny, will be dominant ; a variation that is due to abbreviation of the 

 ontogenetic process, which depends on something having dropped out, will 

 be recessive. 



This conclusion, however satisfactory, must be regarded as tentative. It 

 is doubtful if it is of general validity ; for while long tail and crest feathers 

 are dominant in poultry, long hair (equally due to prolonged life of the fol- 

 licle) is recessive in mammals (Castle, 1903 ; 1905, pp. 64-67, 73-74; Hurst, 

 1904). White is usually recessive to pigment in flowers and mammals, but 

 it is usually dominant over pigment in poultry. It is still too early to regard 

 the conclusions expressed in the last paragraph as anything but an hypothesis. 



While dominance and recessiveness are typically found in Mendelian in- 

 heritance, yet they may be absent even in cases when segregation of charac- 

 teristics occurs in the second hybrid generation. Thus, the barred offspring 

 of the black-and-red Tosa fowl and the white Cochin throw in the F 2 gener- 

 ation 25 per cent black and red and 25 per cent white, but the remainder, 

 like all of Fj, is barred with white, and no one can .say which plumage color 

 is dominant. The same is true of some black-and-white barred hybrids. It 

 is also true of hybrids between single and V comb. The phenomena of domi- 

 nance and recessiveness do not always accompany segregation/' 



Another modification of the law of dominance and recessiveness must be 

 recognized, namely, that they are by no means always complete. Even in 

 the first hybrid generation the dominant characteristic is more or less inter- 

 mediate. The antagonistic characteristics a and a' of the two parent types 

 are not only united in the zygote, but they pass in the development of the or- 

 ganism into all the tissues of the body, and particularly into the cells out 

 of which the organ A is developed. The dominant characteristic, a, and 

 the recessive characteristic, a', each works to determine the quality of the 

 organ A. If a dominates, it is because it is more active than a' . It does 

 not dominate by excluding a'. Sometimes, as in the case of barred feathers, 

 it appears that a and a' in ontogeny alternate in their activities. The cells 

 of a certain zone of the feather manufacture only black pigment ; in the next 

 zone black is wanting ; tnen comes a zone of black, and so on, in many repe- 

 titions. Dominance as contrasted with recessiveness is a matter of degree 

 and not of kind. 



^ 



