So far as I know there are no thoroughly worked out cases of the inheritance of such differences in man or in mammals, but in the case of certain races of birds I have been able to show both by breeding tests and by castration experiments that glandular differences are inherited according to the Mendelian scheme.
Fig. 10. Above (A) normal adult hen-feathered Campine cock. Below (B) castrated cock about one year after operation. The castrated bird has developed the secondary sexual characters of cock-feathering.
There is a race of fowls known as Campines in which there are two kinds of males, hen-feathered males and cock-feathered males. If the hen-feathered male is castrated, the new feathers that develop are the long feathers of the cock-feathered male, [Fig. 10]. In another race of fowls, Sebright bantams, only the hen-feathered males are known. If these are castrated, the new feathers that develop are the long feathers characteristic of all other races of poultry, [Fig. 11].
Fig. 11. To left (a) hen-feathered Sebright cock. To right (b) castrated Sebright cock that has developed characteristic cock-feathering.
If the Sebright male is out-crossed to a hen of another breed in which only cock-feathered males occur, it will be found that all the first generation males are hen-feathered. If these are now bred to their sisters there are produced, in the second generation, three hen-feathered males to one cock-feathered male, showing that the difference between the two races is inherited, [Fig. 12].