94 



< HAPTER 7 



\ V (barred >, producing X B Y (barred 

 9 ) and X'.V (barred ) in F,; the re- 

 ciprocal mating of .VY (barred V ) by 

 \ \ (nonbarred ; ) produces VY (non- 

 barred 5 ) and V V (barred ) in F, 

 ( Figure 7-4. A-l and B-l ). 



Support for die existence ol sex chromo- 

 somes may be sought from etiological ob- 

 servations. If the gene content of the X and 

 > is different as in Drosophila and poultry. 

 the cytological appearance of the two kinds 

 oi sex chromosomes might also be different, 

 i Note, however, that the preceding explana- 

 tion of sex-linkage was made independently 

 of any cytological expectation.) 



In Drosophila it is found cytologically 

 (Figure 7-5) that three of the four pairs of 

 chromosomes seen at mitotic metaphase cor- 



respond in the male and the female, homo- 

 logs being \er\ similar morphologically. In 

 the female the homologs of the fourth pair 

 are also morphologically similar. In the 

 male, however, only one member of this 

 pair looks like its homologs in the female; 

 its partner's morphology is distinctly differ- 

 ent. Thus, the distinctive cytological ap- 

 pearance 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 

 homolog in the male is then called the X and 

 is present twice in the female. The reverse 

 cytological picture is observed in poultry; 

 here the homologs are similar for each pair 

 of chromosomes in the male, whereas the 

 female has one heteromorphic pair; that is, 



A 



Nonbarred ^ 



Barred f~Y?~) 

 Barred QO 



x Barred 



B 



/-V^ P, Barred V/ x Nonbarred r^ 



a"a" 

 99 



Barred 



Nonbarred 



b 

 X Y 



A- 



V x X X (J 



B 

 X Y 



2 



B-l 



b b * 



X x O 



B b 

 X X 



x%99 



B b 

 X X 



X Y 



do' 

 99 



FIGURE 7-4. Phenotypic (A and li) and genotypic (A-J and B-l) results of reciprocal 

 nuttings involving barred and nonbarred feathers in chickens. 



