412 



EVOLUTION, GENETICS, AND EUGENICS 



eyed female, derived from such an experiment as that just described, 

 and breeding her to a normal red-eyed male. The Fj hybrids will be 

 white-eyed males and red-eyed females, the two eye colors simply 

 changing sexes. This is explained by the fact that females always in- 

 herit an X-chromosome from their fathers, while males always get their 

 X-chromosome from their mothers. We speak of this phenomenon 



Flies 



CliTomosomes 



5X0 : 5 XX 

 <? ? 



X 1^ 9 



M m X 



Parents 

 Gametes 



Fi 



Gametes 

 Fz 



Fig. 8i. — Reciprocal cross to that shown in Figure 8o. Parents, red-eyed 

 male and white-eyed female; Fi, white-eyed males and red-eyed females ("criss- 

 cross inheritance" — Morgan); Fa equal numbers of red-eyed and white-eyed 

 individuals of both sexes. The distribution of the sex chromosomes is shown at 

 the right, as in Figure 8o. {From Conklin, after Morgan.) 



as crisscross inheritance. There are many evidences that, in general, 

 daughters inherit more largely from fathers and sons from mothers, 

 and it is probable that the mechanism of this condition is like that 

 just described. But to continue the reciprocal-cross experiment to 

 the Fj generation, let us breed together the males and females of Fj. 

 The result will be exactly like the F^ of the previous experiment: red- 

 eyed males and females in equal numbers (Fig. 8i). 



The type of sex linkage which we have just described lor Dro- 

 sophila and which also prevails in man has come to be called the 



