. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. SEX IN ANTMALS 549 tration may be illustrated by a few typical cases. In slieep there arc 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
. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. SEX IN ANTMALS 549 tration may be illustrated by a few typical cases. In slieep there arc 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 Heller.) Dorset and Suffolk sheep. The latter are hornless in both sexes. The Fi of this cross consists of horned rams and hornless ewes. The F2 con- sists of horn and hornless individuals in both sexes in the approximate ratio : 3 horned cf : 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 hhXY, 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 followin
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