88 HEREDITY AND SEX 



In each combination in the table the character of 

 the plant is determined by the dominant factors, in 

 this case yellow and round, hence : 



9 YR : 3 YW : 3 GR : 1 GW 



This result works out on the assumption that there 

 is independent assortment of the original determiners 

 that entered into the combination. The determiner 

 for yellow and the determiner for round peas are 

 assumed to act independently and to segregate from 

 green and wrinkled that entered from the other parent. 

 The 9:3:3:1 ratio rests on this assumption and is the 

 actual ratio realized whenever two pairs of characters 

 freely Mendelize. 



THE HEREDITY OF TWO SEX-LINKED CHARACTERS 



The inheritance of two sex-linked characters may be 

 illustrated by an imaginary case in which the linkage 

 of the factors to each other is ignored. Then the same 

 case may be given in which the actual results obtained, 

 involving linkage, are discussed. 



The factors in the fruit fly for gray color, G, and for 

 red eye, R, are both sex-linked, i.e. contained in the 

 X chromosome. Their allelomorphs, viz., yellow color, 

 Y, and white eye, W, are also sex-linked. When a 

 gray red-eyed female is mated to a yellow white-eyed 

 male, the daughters and sons are gray-red, GR. Their 

 origin is indicated in the following scheme : 



Gray-red $ G R X GR X 



Yellow-white <? Y W X- ... 



G R X Y W X Gray-red $ 

 G R X . . . Gray-red* 



