duction. Linkage of characters is probably more common 

 than sex-hnkage where the characters lie in the sex chromo- 

 some. 



Instances of sex-linked inheritance have already been 

 discussed, consequently only a brief treatment will be given 

 of linkage of other factors. 



Morgan says: "When factors lie in different chromo- 

 somes they give the Mendelian expectations; but when 

 factors lie in the same chromosome they may be said to be 

 linked, and they give departures from the Mendelian ratios. 

 For example. Morgan crossed a gray vestigial-winged male 

 Drosophila with an ebony long-winged form. The F, were 

 all gray long-winged flies, and the Fo gave the normal Men- 

 delian ratio of 9 :3 :3 : L In this case the factors lay in differ- 

 ent chromosomes. 



But when he crossed a black vestigial-winged male fly 

 with a gray long-winged female form, the F, were all gray 

 long-winged flies. When, however, it is known that the 

 factors black and vestigial lie in the same chromosome and 

 the factors gray and long lie in another chromosome, it will 

 be clear that the cells of the Fi carry two kinds of chromo- 

 somes and that its gametes should be bv and GL for each 

 sex. The Fo should be expected to give black vestigial and 

 gray long in the ratio of 1:3 according to the following 



diagram : 



Male 



Gametes 



bv 



GL 



= female gametes 



bv 



GL 



Actual breeding results ,however, included in addition 

 a small percentage of black long and gray vestigial forms. 

 This departure is explained if the female gametes are bv, 

 GL, bL, Gv, according to the following diagram: 



bv GL bL Gv = Female 



Male 

 Gametes 



I- 



%l 



122 



