EELATING TO SECONDARY SEXUAL CHARACTERS. 27 



if the mutation were autosomal,^ 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-hnked 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-Unkage in fowls was furnished by evidence 

 that Spillman published in 1903 on information suppUed by poultry- 

 men — ^information that has been proven subsequently to ha,ve 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, pubUshing the results in 1912. In addition to the Fi results 

 evidence was obtained for the Fa 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 Fi barred sons and barred daughters. These 

 inbred give F2 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. 



Barredif BlEick^ 



zV zV 



'" Barred J Banjedj 



■ft ft zV zV 



E3aiTBd(? Barred*? Barredj Qaekj 



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

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

 (F2) barred males and females, black males and females in the ratio 

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



Btackd" Barredj 



f. zV Z'W 



' Barred^ Blacky 



T^t tt zV zNv 



BaiTed(f Black (? Barred^ Black j 



' 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. 



