RELATING TO SECONDARY SEXUAL CHARACTERS. 27 



if the mutation were autosomal. 1 In consequence the recessive mutant 

 would have a greater chance of being observed and selected if it 

 appeared in a sex chromosome. But dominant sex-linked characters, 

 however, have the same chance as dominant autosomal ones and the 

 question turns therefore on the kinds of characters shown in the cross. 



The first indication of sex-linkage in fowls was furnished by evidence 

 that Spillman published in 1903 on information supplied by poultry- 

 men information that has been proven subsequently to have been 

 accurate. Spillman pointed out clearly the similarity between the 

 facts he quoted and the then known cases of sex-linkage in the canary 

 and in the currant moth. The case referred to by Spillman was a cross 

 between Barred Plymouth Rock and Black Langshan. Goodale and 

 I repeated the cross, using both Plymouth Rock and American Dom- 

 inques, publishing the results in 1912. In addition to the FI results 

 evidence was obtained for the F 2 generation. The theory was also 

 tested by back-crossing. The results of such a cross that are typical 

 for all cases of the sort are briefly as follows : Plymouth Rock cock by 

 Langshan hen gives F! barred sons and barred daughters. These 

 inbred give F 2 barred cocks and barred and black hens (2:1:1). 



In the following schemes the sex chromosomes are represented by Z 

 and W, while the exponents stand for the factors involved, viz, B for 

 barred and b for not-barred, which here means a black bird. 



Birrrsdo 



zV 



Darredc? Barredc? Barredj DlacKj 



In the reciprocal cross, a black cock was mated to a barred hen. 

 The sons were barred, the daughters black (Fi). These inbred gave 

 (F 2 ) barred males and females, black males and females in the ratio 

 of 1 :1 :1 :1. The chromosome scheme of inheritance is as follows: 



z B z b -ft zV 



Barred^ Black d* Barred^ Blacky 



1 If the recessive mutation occurs first in the Z chromosome of an egg of the female it will not 

 appear in the next generation; then if it has passed into a male, half his daughters will show it. 

 The single factor-pair involved is carried by the sex chromosomes Z Z. 



