434 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
only in male individuals. But if the recessive sex-linked character is 
introduced by the female parent, it appears in F, in male individuals 
but in F, in both sexes. 
Suppose now a cross is made between two races, each of which 
possesses a different sex-linked recessive character, as for example 
white eye and yellow body. (See Table II, p. 436.) If the white-eyed 
Flies Chromosomes 
XO XX Parents 
fe) g 
3 
><] Gametes 
xX xX F 
2X%0:2XX F1 
Fic. 87.—Sex-linked inheritance of white and red eyesin Drosophila. Parents 
white-eyed male and red-eyed female; F;, red-eyed males and females; F;, red- 
eyed females and equal numbers of red-eyed and white-eyed males. A black 
X indicates an X chromosome bearing the gene for red eye, a white X bears white 
eye. 4) indicates that X is wanting; in recent publications Morgan replaces it 
by Y. (From Conklin, after Morgan.) 
parent is a female, there will be produced white-eyed males in F, 
and white-eyed flies of both sexes in F,. But the male parent being 
yellow, there will be no yellow flies produced in F, and only yellow 
males in F,. In the reciprocal cross (yellow femaleX white-eyed 
male) yellow males will be produced in F, and yellow flies of both sexes 
in F,, while white-eyed flies will not appear until F, and then only in 
the male sex. In either of the reciprocal crosses we expect the pro- 
duction in F, both of yellow-bodied males and of white-eyed males. 
