SEX-LINKAGE 



19 



(GX)Y 

 gross. $ 



(gX)Y F.2 

 lact. ? 



2. 



heterozygous 



gross. S 



(GX)(gX) 



(GX) (gX) 



(GX)Y (gX)Y 



gross. ^ lact. ? 



lact. c? 

 (gX)(gX) 



(gX) 

 (gX)Y 

 lact. ? 



gametes 



gametes 



(GX)(GX) (GX)(gX) 



homozygous heterozygous 



gross, o gross, o 



lact. ? : 



(gX)Y 



(gX) Y 



(GX)(gX) (gX)(gX) 



homozygous heterozygous 



gross, o gross, o 



3. F.I gross. ? " 



(GX)Y 



(GX) Y 



(GX)(gX) 

 heterozygous 

 gross, o 



4. As 3 above. 



In the domestic fowl the phenomenon of sex-linkage — 

 this association in inheritance of a discernible character and 

 of the character of sex itself — has in recent years formed the 

 basis of a large industry. Day-old chicks, every one of them 

 guaranteed to be a female, are sold by the thousand every 

 year. The seller does not examine their genitalia in order to 

 determine whether the chick is a male or a female; its sex 

 is signalled by its plumage coloration. As an example the 

 plumage characters barred and non-barred may be cited. 

 A non-barred (black) cock mated with barred hens produces 

 barred male and non-barred female offspring. Sons 'take 

 after' their mother, daughters 'after' their father, a 

 phenomenon known as criss-cross inheritance. 



The actual sex-chromosome constitution of the domestic 

 fowl is not yet finally established, the number of chromo- 

 somes is very large and many of them are very small. 

 Assume that it is of the Lygaeus type with female hetero- 

 gamety. Assume further that the genes for the characters 

 barred and non-barred are X-borne and that barred is 

 dominant. A non-barred cock must then have the constitu- 

 tion (bx)(bx) and the barred hen (BX)Y. 



