72 



Heredity and Eugeiiies 



case the gametes formed by the male would all be A", but 

 those formed by the female would be of two sorts equally 

 numerous, viz., N and .A^+ (Fig. 36). Egg A^ fertilized by 

 sperm A^ would produce a zygote 2.V, a male; egg N-\~ 

 fertilized by sperm N would produce a zygote 2iV+, a 

 female. Hence, here as in other animals, the sexes would 



be approx imately equal, but 

 the se x of a particular indi - 

 vidual would depend upon 

 which~sortr^of egg gave rise 

 toilTT^^ 



Upon the existence, as in 

 the foregoing cases, of an 

 unpaired or odd structural 

 element in the egg, may per- 

 haps depend the explanation 

 of a curious sort of heredity 

 known as sex-limited heredity. 

 Everyone who knows any- 

 thing about poultry is ac- 

 quainted with the popular 

 American breed called barred 

 Plymouth Rock. In this 

 breed the feathers are marked with alternate bars of darker 

 and lighter black. Pure barred R ocks breed^t rue, but when 

 crossed with other breeds, th e male proves to be homozy - 

 gous, the female heterozygous in barrin g. For the male 



Fig. 36. — Diagram of sex determi- 

 nation when the female is heterozy- 

 gous, the male homozygous. 



Rock crossed with a non-barred breed produces only barred 

 offspring in both sexes, but the female Rock crossed with 

 the same non-barred breed produces offspring approxi- 

 mately half of which are barred, the other half being 

 non-barred. P'urther, the barred individuals in this cross 



