188 Peculiar Pigmentation of the Silky Fowl 



pigmented F^ % as well as for the unpigmented. But wlien the F^ ^ 

 was crossed with a Brown Leghorn % about one in eight of the offspring 

 were deeply pigmented and these were always females. To assist the 

 reader in following this somewhat complicated case we append a rough 

 scheme. It will be understood of course that the scheme gives no 

 indication of the proportions in which the various classes are produced, 

 neither for the moment do we attempt to differentiate between the 

 various grades of pigmentation other than the fully pigmented state. 



We may now state briefly the interpretation to which our various 

 experiments have led us. We consider that three factors are involved 

 of which two are directly concerned with the degree of pigmentation. 

 These are (a) a pigmentation factor (P), and {^) an inhibition factor (/) 

 which can prevent the full development of the pigmentation'. The 

 various grades of pigmentation met with depend upon the various 

 compositions of the zygotes in regard to these two factors ; e.g. a bird 

 of the constitution PPii will be fully pigmented, a bird of the con- 

 stitution Ppli will be slightly pigmented, while birds of the constitu- 

 tion ppll, ppli, or ppii will be unpigmented (see also p. 200). 



The third element with which we are concerned in these experi- 

 ments is sex. Here we have made certain assumptions. We regard 

 the female as differing from the male in possessing a special element, 

 F, of which the hereditary behaviour is like that of any other 

 Mendelian factor. Moreover we con,sider that the female is always 

 heterozygous for this factor so that the zygotic constitution of a female 

 is Ff while that of a male is ff. Further we suppose that in such 

 zygotes as are heterozygous for both F and / there occurs a repulsion 

 between these two in gametogenesis so that F and / do not pass into 

 the same gamete. We may allude to the cases of the inheritance of 

 the lacticolor variety of Ab7-axas grossulariata- and of the red eye of 

 cinnamon canaries^ in which similar phenomena can be shown to follow 

 the same system of descent. 



It must be expressly stated that the suggestion that females are 

 heterozygous for femaleness is offered without prejudice as to the 

 possibility that males may also be heterozygous in maleness. The 

 systems followed by the descent of colour-blindness'' in Man and by 



' The condition of the gamete from which either or both of these factors are absent we 

 shall denote in the conventional way by the use of the corresponding small letters p and i. 

 - Doncaster, L., Reports to the Evolution Committee of the Royal Society, iv. 1908. 

 3 Durham, F. M., Reports to the Eroluliou Committee of the Royal Society, iv. 1908. 

 * Mendel's Principles, 2nd imp., 1909, p. 195, note. 



