7- 



Heredity and Eugenics 



case the gametes formed by the male would all be X, but 

 those formed by the female would be of two sorts equally 

 numerous, viz., N and N-\- (Fig. 36). Egg N fertilized by 

 sperm N would produce a zygote 2.Y, a male; egg X+ 

 fertiHzed by sperm N would produce a zygote 2X+, a 

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



be approximately equal, but 

 the sex of a particular indi- 

 vidual would depend upon 

 which sort of egg gave rise 

 to it. 



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 Rocks breed true, but when 

 crossed with other breeds, the male proves to be homozy- 

 gous, the female heterozygous in barring. For the male 

 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. Further, the barred individuals in this cross 



Fig. 36. — Diagram of sex determi 

 nation when the female is heterozy 

 gous, the male homozygous. 



