Basis of Sex Determination 207 



The experiments were carried further. No white-eyed 

 females had appeared thus far. On the same assump- 

 tions of the relation of the X, X, and Y chromosomes to 

 the heredity of sex as well as to eye colour it was 

 possible to predict under what conditions and in which 

 proportions white-eyed females should arise. Thus if 

 a red-eyed female of the Fj generation (a cross between 

 white-eyed male and normal female) be mated with a 

 white-eyed male the result should be an equal number 

 of white-eyed males and white-eyed females if the 

 chromosome theory of sex determination were correct. 

 The reasoning would be as follows : 



The red-eyed female, having the chromosome con- 

 stitution XX should form two kinds of eggs in equal 

 numbers with the constitution X and X; the white- 

 eyed male having the chromosome constitution XY 

 should form two kinds of spermatozoa X and Y. The 

 following four types of individuals must then be pro- 

 duced in equal numbers : 



(i) XX, (2) XX, (3) XY, and (4) XY. 



In this case (2) must give rise to white-eyed females 

 and (4) to white-eyed males, while (i) must give rise 

 to red-eyed females and (3) to red-eyed males. Hence 

 white-eyed males and females and red-eyed males and 

 females are to be expected in this case in equal num- 

 bers, and this was actually observed. 



The numerical agreement in this and the other 



