58 THE THEORY OF THE GENE 



genes for yellow enables us to follow the genetic history 

 of the attached X's when such a female is bred to a nor- 

 mal wild type male with gray wings. For example: the 

 diagram (Fig. 37) shows that two kinds of eggs are ex- 

 pected after the maturation division: one egg retains the 

 double yellow X, the other egg retains the Y-chromosome. 

 If these eggs are fertilized by any kind of male, prefer- 

 ably by one whose X-chromosome contains recessive 

 genes, four kinds of offspring should be produced, two of 

 which die. The two that survive are a double XXY female 

 that is yellow, like her mother, and an XY male that is 

 like his father with respect to his sex-linked characters 

 because he gets his X from his father. 



This result is exactly the reverse of what happens 

 when a normal female with recessive genes is fertilized 

 by a different kind of male, and the apparent contradic- 

 tion is understandable, at once, on the assumption of 

 attached X-chromosomes. A cytological examination of 

 these double X females never fails to show two X's 

 attached to each other. 



