NON-DISJUNCTION 



25 



Classes 3, the trisomic red-eyed female, and 8, the double 

 Y lacking an X, do not appear. 4, the exceptional red-eyed 

 male, has its origin in an X from the father and a Y from the 

 mother. 7, the exceptional white-eyed female, derives both 

 X's from the mother. She is a female because she is XX. 



The exceptional white-eyed females (7) when mated with 

 normal red-eyed males always yield the same kind of ex- 

 ceptional offspring; the exceptional red-eyed males (4) 

 always behave as normal red-eyed males, they have a per- 

 fectly normal sex-chromosome constitution even though the 

 X and the Y are derived from the wrong parents. There are 

 two kinds of white-eyed males (5 and 6), one perfectly 

 normal, the other cytologically exceptional and therefore 

 producing exceptional gametes in its turn. 6 should be able 

 to yield an XXY female by fertilizing an X-bearing egg. It 

 does. The XXY female has been identified both genetically 

 and cytologically. Sex-determination would therefore seem 

 to be an affair of the sex-chromosome combination, no 



