110 SEX INHERITANCE 



the inheritance of a secondary sexual character 

 (for both sexes of the Suffolks are hornless and both 

 sexes of the Dorsets are horned), but we are study- 

 ing a single factor difference between the two 

 breeds in question, a difference that is not sex- 

 linked, but, is sex-limited in development. Horned 

 Dorsets crossed to hornless Suffolks give horned 

 sons and hornless daughters. If these are inbred 

 they give three horned males to one hornless male, 

 and three hornless females to one horned female. 



Fig. 36B. — Dorset sheep, both sexes hurue'd, Ijut ram witli hirger 

 horns than ewe. (After Arkcll.) 



The results are explicable if a factor difference 

 exists that is not in this case carried b}^ the sex- 

 chromosomes, and if in the male one gene for horns 

 suffices to produce horns, while in the female two 

 genes for horns are necessary to produce horns. 

 For example, the horned breeds carry the genes 

 HH, and the hornless breeds their allelomorphs hh. 

 The Fi sheep will be Hh? (hornless) and Hhcf 

 (horned). If the female is XX and the male XY, 

 the Fi gametes are as follows: 



Gametes Fi 9 HX — hX 



Gametes ¥,^ HX — hX — Hy — hY 



