THE MENDELIAN METHOD 



be more accurate, however, to say that it determines whether a 

 fertihzed egg with a normal outfit of the other chromosomes, the 

 autosomes, shall develop into male or female. How the autosomes 

 affect the issue we shall see in a later chapter. 



Sex-Linkage 



Corresponding with this cytologically observed situation is the 

 genetically observed mechanism of sex-linked inlieritance. When 

 a wild type, red-eyed, female fly is crossed with a white-eyed male, 



Fig. 10. — Reciprocal crosses showing the transmission and expression of a gene pair 

 in the X chromosome, having no allelomorph in the Y and with dominance, e.g. 

 white-eye in Drosophila or haemophilia in man. In the black-yellow difference of 

 the cat, the heterozygote is distinguishable as the tortoiseshell female. 



The solid circle below the X denotes the domuiant allelomorph, the empty circle 

 the recessive. The thick outer circle denotes the dominant phenotype. Broken lines 

 show the transmission of the Y. 



all the Fi are red : white is recessive. In the Fg (from crossing brothers 

 and sisters in the F^) we get three red flies to every white. But all 

 the white-eyed flies are males: the gene is sex-linked. 



When the cross is made the other way round, white female being 

 crossed with red male, the sex linkage makes itself apparent in the Fj. 

 As before, the females are red but the males are now white. In the 

 F2 the males as before are half red and half white, but now so are 

 the females too. 



What does this mean? The females in the F^ are the same no 

 matter which way the cross is made. The females must therefore 

 inherit the gene equally from both parents. The males in Fg are all 



48 



