474 HEREDITY AND SEX 



from an abnormality of an antler to an abnormality of a testis. 

 If a merino male lamb be castrated the adult is hornless like the 

 female. We are led to the idea that what is actually inherited 

 may be in many cases common to the two sexes, but is capable of 

 masculine or feminine expression according to the liberating 

 stimuli which activate it. 



(d) Of much interest in this connection is the occurrence of what 

 are called " sex-limited characters." Colour-blindness in man- 

 kind is a familiar example. It is much commoner in men than 

 in women. But the colour-blind man with a quite normal wife 

 does not have colour-blind children. His sons are normal and 

 his daughters apparently normal ; but the condition is trans- 

 mitted through the daughters to half their sons. 



In Plymouth Rock poultry with alternate light and dark 

 bars on the feathers, the barred character illustrates sex-limited 

 inheritance. When a male is crossed with a non-barred breed, 

 the offspring are all barred, whether male or female. This means 

 that the male Rock is homozygous, that all his germ-cells bear 

 the determinant of the barred character. When a female Rock 

 is crossed with a non-barred breed, the offspring are half-barred 

 (the males) and half non-barred (the females). This means 

 that the female Rock is heterozygous as regards barred-ness, that 

 half of her germ-cells have and half have not the determinant 

 of the barred character. But there is the further point that in 

 her germ-cells there is some linkage between male-producing 

 and the barred character, between female-producing and the 

 absence of the barred character. 



Another instance may be given. When Dorset sheep, horned 

 in both sexes, are crossed with Shropshire sheep, hornless in both 

 sexes, horns occur on the male offspring, but not on the female. 

 The horn-producing character is dominant in the male sex, 

 recessive in the female. W T hen the hybrids are interbred, their 

 progeny — the F 2 generation — include hornless males and horned 

 females— both breeding true — as well as horned males and 



