RESULTS OF CASTRATION IN DUCKS. 5 1 



Brahma female seems to be transmitted to the female offspring 

 only, without regard to whether the Brahma is mother or father 

 (Davenport). But there are some possible complications result- 

 ing from the particular matings made, which make this point 

 uncertain. Thus far, however, the study of sex limited inherit- 

 ance including the invisible factor "D" described by Bateson 

 for the Brown Leghorn and the castration experiments on ducks 

 all point in the same direction. 



Some of the possibilities in the mode of inheritance of sexually 

 dimorphic plumage are as follows : 



1. In the female we may assume that the plumage occurs as a 

 typical Mendelian heterozygote of male color and female color.^ 

 the former being considered recessive. The male, then, must be 

 a homozygous recessive. But such assumptions do not agree 

 with the results of breeding experiments as mentioned above 

 for they show that in the female, a color factor occurs in only 

 half her gametes. 



2. Among the various other ways of representing the mode of 

 inheritance of sexually dimorphic plumage, the following seems 

 the nearest approach to our present knowledge. Assume that 

 the male is homozygous for sex, the female heterozygous; that 

 the male color (or color factor) mc also is homozygous in the male, 

 but in the female heterozygous, the other element of the pair 

 being an invisible modifier, M, which always couples with female- 

 ness; or, assume that the male color always couples with male- 

 ness. Then the male is mc mc, gametes all mc, the female is 



mc M, gametes mc, M. The breeding experiments mentioned 

 d^ 9 cf 9 



above afford the reasons for assuming that the male is homozy- 

 gous for sex, and the female heterozygous, and for the assumed 

 couplings. The castration experiments give the reason for as- 

 suming a male color and a modifier. The expression, male color, 

 however, is used in a purely descriptive sense. I do not wish to 

 be understood as giving it any special significance. 



3. By the additional assumption of selective fertilization, the 

 male can be shown to be heterozygous instead of the female, 



IE. g.. male Brown Leghorn color and female Brown Leghorn color as such. 



