438 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



breeding true for a sex-linked character, the female is homozygous 

 for the character in question while the male is heterozygous and incap- 

 able of becoming homozygous. Reciprocal crosses with such a race 

 give unlike results, because the female transmits the character to all 

 her offspring, but the male transmits it to only half his offspring, viz., 

 the females. 



FIG. 91. Sex-linked inheritance of barred and unbarred (black) plumage in 

 poultry. P, parents, barred male, unbarred female; F,, barred males and females; 

 F 2 , males all barred, females in equal numbers barred and unbarred. (After 

 Morgan.) 



2. Poultry type. Another type of sex-linked inheritance exists in 

 which the sex relations are exactly reversed. This was first observed 

 in the moth, Abraxas, but more familiar cases occur hi poultry, for 

 which reason it may be called the poultry type of sex-linked inherit- 

 ance. Here the male is the homozygous sex, the female being hetero- 

 zygous. This condition is found in moths and in certain birds, viz., 



