86 



CHAPTER 12 



F^ Nonbarred Q x Barred 

 Barred (J O 



99 



Barred 



c/. 



B 



Barred Q x Nonbarred 



Barred (30 

 Nonbarred 



c/ 



99 



A-l B 



b p) B B/-^ 



P, XYVx XX*^ 



X Y ^^ X Y 



FIGURE 12-5. Phenotypie {A and B) and genotypic (A-l and B-1) results 

 of reciprocal matings involving barred and nonbarred feathers in chickens. 



B p. b b^ 



P, XYVx XX^ 



^^ 



99 



B b 

 X X 



The genotypes of the bird crosses are, on 

 these hypotheses, X''Y (nonbarred 9 ) by 

 X^X^ (barred (f) producing X^Y (barred 9 ) 

 and X^X* (barred cf) in Fi; the reciprocal 

 mating of X«Y (barred 9 ) by X''X'' (non- 

 barred cf ) produces X'-Y (nonbarred 9) and 

 X^X* (barred d") in Fi, all as observed 

 (Figure 12-5, A-l and B-1). 



You may be wondering if any clues can be 

 obtained, from cytological observation of 

 chromosomes, for the absence of alleles on 

 the Y which are present on the X, both in the 

 case of Drosophila and of poultry. It would 

 be reasonable to expect, if the gene content 

 for X and Y is so different, that this might be 

 reflected in a difference in the cytological 

 appearance of the two kinds of sex chromo- 

 somes. Note, however, our explanation of 



sex -linkage has been made independently of 

 any cytological expectation. 



It is found cytologically (Figure 12-6) in 

 Drosophila that three pairs of chromosomes 

 are the same in the male and the female, 

 homologous chromosomes being very similar 

 morphologically. In the female the homo- 

 logs of the fourth pair are also similar mor- 

 phologically, but in the male, while one 

 member of the fourth pair is just like the 

 homologs of the fourth pair in the female, its 

 partner chromosome has a distinctly different 

 morphology. Thus, the distinctive cytological 

 appearance of this last chromosome is con- 

 sistent with the genetic expectation for a Y 

 chromosome, being present once in the male 

 and not at all in the female. The other homo- 

 log in the male is then called the X, and is 



