134 Dihybrids, Trihyhrids, and Polyhybrids 



well as between two pairs of autosomes as Miss Carothers dem- 

 onstrated cytologically in the invertebrate animals, Brachystola 

 magna, Arphia 'simplex, and Dissosteira Carolina. In each she 

 found a pair of autosomes which was heteromorphic ; that is, a 

 pair in which one of the homologues was visibly different from 

 its mate. In Arphia the male has one X chromosome and no Y 

 chromosome, and the heteromorphic pair of autosomes consists 

 of a large and a small chromosome. In a large number of 



Wild-Type x Miniature, Purple Miniature, Purple x Wild-Type 



P, (XM) (XM) PrPr x (X/n) Y prpr (Xm) (Xm) prpr x (XM) Y PrPr 



F. (XMKXrn) Prpr + (XM) Y Prpr (XM){Xm) Prpr + (Xm) Y Prpr 



F 



'1(XM)(XM) PrPr normal, red female 3 fllXM) (X ml PrPr normal, red female 



Ml 



\2iXM] iXM) Prpr '' \2[XM) iXm) Prpr 



\l{XM){Xm) PrPr 1 I ^XM) iXm) prpr •• purple 



2{XM){Xm) Prpr 3 |l(Xm) (Xm) PrPr miniature, red 



1{XM]{XM) prpr " purple" *^ 12 (Xm) (Xm) Prpr 



l[XM){Xm) prpr " " " 1 HXm) iXm) prpr - purple •• 



jliXM) Y PrPr " red male o (KXM) Y PrPr normal, red male 

 •^ 12(XM) Y Prpr '^12(XM) Y Prpr 



1 1 iXM) Y prpr " purple " 1 1 (XM) Y prpr - purple " 



fl(Xm) Y PrPr miniature, red male o /KXm) Y PrPr miniature, red •• 

 3 i2(Xm) Y Prpr - " " •^l2(Xm) Y Prpr " " " 



1 KXm) Y prpr " purple " 1 1 (Xm) Y prpr " purple " 



Fig. 45, The Fi and F2 of reciprocal crosses between a gene (miniature 

 wings) in the X chromosome and one (purple eye) in an autosome. Left, 

 wild-type female X miniature, purple male. Right, miniature, purple fe- 

 male X wild-type male. 



anaphases of the first meiotic division of males, the X chromo- 

 some went into the same cell as the large member of the hetero- 

 morphic pair of autosomes about 50 per cent of the time. 



The genetic ratios in a dihybrid when one of the pairs of 

 genes is in the X chromosome are naturally different from the 

 ratios when both pairs of genes are in autosomes. In Drosophila 

 melanogaster the gene for miniature wings {m) is in the X 

 chromosome and the gene for purple eye (pr) is in the auto- 

 some, chromosome II. If a wild-type female, homozygous for 

 both M and Pr, is crossed with a miniature, purple male, all the 

 Fi flies are wild type and the phenotypic ratio in the F2 is 6 

 wild-type females : 2 purple-eyed females : 3 wild-type males : 

 1 purple-eyed male : 3 miniature-winged males : 1 miniature- 

 winged, purple-eyed male. The reciprocal cross gives different 

 results in both the Fi and F2 because of the presence of the 

 sex-linked gene (Fig. 45). 



