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 horned, but ram with larger 
horns than ewe. (After Arkell.) 
The results are explicable if a factor difference 
exists that is not in this case carried by 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 F, sheep will be Hhe (hornless) and Hhe 
(horned). If the female is XX and the male XY, 
the F, gametes are as follows: 
Gametes Fie HX —hX 
Gametes Fiz HX — hX — Hy — hY 
