150 DISTRIBUTION OF THE CHROMOSOMES 



results showed that the exceptions could be explained, 

 if, occasionally, the two X chromosomes failed to 

 disjoin in the reduction division, both passing out of 

 some of the eggs of the Avhite-eyed mother into the 

 polar body, or, conversely, both remaining in the egg. 

 If the two white-bearing X's should remain in the egg 

 then such an egg fertilized by a Y sperm would give 

 rise to a white-eyed daughter. Likewise the no-X 

 egg fertihzed by the X sperm of a red-eyed male 

 would give a red-eyed son. The white daughters 

 would, as just shown, contain two X's and one Y 

 chromosome, unlike ordinary daughters, which con- 

 tain two X's only. Since in these females there are 

 three sex chromosomes instead of a pair, at the 

 reduction division two must pass into one cell and 

 one into the other. This division might take place 



XY X Y XX 



in four ways: -^j ^y' XX ^^^ "^ (representing 



the egg below and the polar body above in each 

 case). The first two types of reduction, depending 

 on a more symmetrical pairing of the chromosomes, 

 might be more frequent than the other two types. 

 There would then be four types of eggs — a large 

 number of X and XY eggs, and a few XX and Y eggs. 

 Let us suppose that an XXY white female is mated to a 

 red male. The progeny produced by the X bearing 

 sperm would be: 



(1) (2) (3) (4) 



)a MY XM XY 



O red O red missing (5 red 



