72 



MENDELISM 



CHAP. 



generation was similar ; the rams were horned, and 

 the ewes were hornless. In the F^ generation raised 

 from these F^ animals both horned and hornless 

 types appeared in both sexes but in very different pro- 

 portions. While the horned rams were about three 

 times as numerous as the hornless, this relation was 

 reversed among the females, in which the horned 

 formed only about one-quarter of the total. The 



Dorset Suffolk 

 Ram Ewe 



Suffolk Dorset 

 Ram Ewe 



C? X f 



rf cJ ? 



Fig. i6. 



Scheme to illustrate the inheritance of horns in sheep. Heterozygous males shown 

 dark with a white spot, heterozygous females light with a dark spot in the centre. 



simplest explanation of this interesting case is to 

 suppose that the dominance of the horned character 

 depends upon the sex of the animal — that it is 

 dominant in the male, but recessive in the female. A 

 pretty experiment was devised for putting this view to 

 the test. If it is true, equal numbers of gametes with 

 and without the horned factor must be produced by the 

 Fj ewes, while the factor should be lacking in all the 

 gametes of the hornless Fg rams. A hornless ram, 

 therefore, put to a flock of F^ ewes should give rise to 

 equal numbers of zygotes which are heterozygous 



