20 



SEX-DETERMINATION 



Non-barred ^ 



(bx)(bx) 



(bx) 



(BX)(bx) 



heterozygous 

 barred (J 



Barred ? 



(BX)Y 



(BX) Y 



(bx)Y 



non-barred 



$ 



gametes 



The day-old chick that will develop into a barred- 

 plumaged bird has a white spot on the top of the head* 

 those who will not be barred when adult lack this spot. The 

 barred birds are male and can be separated from the females 

 among day-old chicks. 



The sons of this mating are barred because to be males 

 they must possess two X-chromosomes, and because one of 

 these must come from their barred mother who has only 

 one to offer, one carrying the dominant barred gene. The 

 daughters of this mating are non-barred because to be 

 femal^ they must receive their Y from their mother and 

 their X from their father who has only one kind of X to 

 offer, an X carrying the recessive gene for the non-barred 

 character. 



If the reciprocal cross is made and a barred cock is mated 

 with non-barred hens all the F.i, males and females alike, 

 are barred. In the F.2 there appear on the average in every 4 

 3 barred and i non-barred. Among the barred there are two 

 males and one female in every three and all the non-barred 

 are females. 



Barred (^ 

 (BX)(BX) 



(BX) 



(BX)(bX) 



heterozygous 



Barred (^ 



(BX) (bX) 



(BX)(BX) (BX)(bX) 



homozygous heterozygous 



Barred (^ Barred c? 



Non-barred $ 

 (bX)Y 

 (bX) Y 



(BX)Y 

 Barred ? 



P.I 



gametes 

 F.I 



(BX) Y gametes 



(BX)Y (bX)Y F.2 

 Barred ? non-barred ? 



The recessive character of the grandmother is exhibited 

 by none of her sons or daughters and only by 50 per cent. 



