74 



Heredity and Eugenics 



Female 





Male 



barring B, the male of pure race is plainly BB and every 

 sperm is B. But the female clearl}- contains only one B 

 and cannot be made to contain two. Perhaps a second B 

 is kept out by some structural clement, X, the distinctive 

 structural element of the female incHvidual. Then the 

 eggs will be of two sorts: B and A' (Fig. 37). Since the 

 sperms are all B, the first t\']:)e of egg when fertilized will 



contain BB, a homozy- 

 gous barred individual 

 and a male, since it 

 lacks A'; the second 

 type will contain BX, 

 a bird heterozygous in 

 barring, and a female, 

 since it contains A". 

 This agrees with the 

 experimental result. 



A heterozygous 

 barred male will form 

 two kinds of sperm, 

 only one of which will 

 contain B. If such a 

 male be mated with a barred female, four sorts of zygotes 

 should result as follows: 



Gametes of heterozygous barred male = B and — 

 Gametes of barred female = B and A" 



Zygotes = i> • B (homozygous barred male); B ■ —(heterozygous 

 barred male), B • X (barred female), and — • A' (non-barred female). 



The observed result of this cross accords fully with the 

 foregoing expectation. 



The sex-limited inheritance of barring in fowls may be 

 explained, as we have just seen, on the assumption that the 



(3e) 



l-iG. 37. — Diagram of sex-limited inheritance 

 wlien tlie female is a lieterozygote, as in l:)arrcd 

 fowls. X, female sex determiner; B, barring. 



