SEX IN ANIMALS 



549 



tration may be illustrated by a few typical cases. In sheep there are 

 breeds which are hornless in both sexes, hornless in the female and 

 horned in the male, and horned in both sexes. When both sexes are 

 horned there are usually rather striking differences in the horns, 

 those of the male being the better developed. Fig. 215 shows this for 

 Dorset sheep which have been used in such experiments. The effect of 

 castration in such breeds has been studied by Marshall. He finds that 

 as a result of castration of males in those breeds in which males only are 

 horned, the horns fail to develop. In breeds in which both sexes are 

 horned castrated males develop horns like the females. 



The above effects are of interest when taken in connection with studies 

 of inheritance of horns in sheep. Thus Wood has studied crosses between 



FIG. 215. Dorset sheep showing the differences in development of horns in the two sexes. 



(After Shaw and Hetter.) 



Dorset and Suffolk sheep. The latter are hornless in both sexes. The 

 F\ of this cross consists of horned rams and hornless ewes. The F 2 con- 

 sists of horn and hornless individuals in both sexes in the approximate 

 ratio: 



3 horned d" : 1 hornless cf : 1 horned 9 : 3 hornless 9 . 



Bateson and Punnet assume that the horned breeds are of the constitu- 

 tion HHXX, female, and HHXY, male; and the hornless, hhXX, female, 

 and hhX Y, male. The FI then consists of HhXX, hornless females, and 

 HhXY, horned males. Here they make the assumption that one dose 

 of the factor H results in the production of horns in the male on account 

 of the sex relations, but in the female two doses are necessary for the 

 development of horns. This hypothesis does not account for those 

 breeds which are horned in the male and hornless in the female, but it 

 is not absolutely necessary that it should. We see here, however, a 

 basis for the modifications of horns following castration for evidently 

 the factor complex for the horned condition reacts differently in male and 

 female. 



