98 



Genes in the X and Y Chromosomes 



term is descriptive but it is unnecessary and in a sense might 

 be considered misleading. The only fundamental difference be- 

 tween this and other crosses is that the Y chromosome lacks 

 the locus under consideration, for genes which are present are 



Parents 



Pi gametes 



Miniature Female 



Wild-Type Male 



F^ 



Fi gametes 



Wild-Tj^je Females 



X X 



Miniature Males 



XY 



XX XX XY XY 



Miniature Wild-Type Miniature Wild-Type 

 Female Female Male Male 



Fig. 33. Diagram of sex-linked inheritance in a cross between a minia- 

 ture-winged female and a wild-type male of Drosophila melanogaster. This 

 is the reciprocal of the cross in Fig. 32. The Fi consists of wild-type 

 females and miniature males and the F2 of equal numbers of wild-type 

 females, miniature females, wild-type males, and miniature males. The 

 relationship of the Pi and Fi generations has resulted in the term ''criss- 

 cross" inheritance for this cross. 



distributed and behave in exactly the same fashion as genes 

 located in autosomes. 



ZW and ZO Sex Linkage. In animals with the Abraxas and 

 similar types of sex inheritance, the results obtained in the Fi 

 and F2 will be the reverse of those in the XY and XO types. 

 One of the best-known examples is the barred gene in Plymouth 



