VARIATIONS IN POTENCY 



In merino sheep the male has well-developed 

 horns but the female is hornless ; yet if the male 

 is castrated early in life no horns are formed. 



When a breed of sheep horned in both sexes, 

 such as the Dorset, is crossed with one horn- 

 less in both sexes, such as the Shropshire, horns 

 are borne by the male but not by the female 

 offspring. Both sexes, however, are heterozy- 

 gous in horns, as is shown by their breeding 

 capacity. For in F 2 occur both horned and 

 hornless individuals in both sexes. The horn- 

 less males and the horned females prove to 

 be homozygous, but the horned males and the 

 hornless females may be either heterozygous 

 or homozygous. Accordingly the character, 

 horns, behaves consistently as a dominant char- 

 acter in one sex, but as a recessive in the 

 other. Further, the presence of the male sex- 

 gland in the heterozygote raises the potency of 

 the character, horns, from semi-potent to uni- 

 potent, as the result of castration shows. 



It is impossible to be certain that in a horn- 

 less race the character horns has been wholly 

 lost. It may merely have fallen so low in 

 potency that under ordinary conditions it pro- 



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