202 



PHYSICAL BASIS OF HEREDITY 



females {XXX) that die, and (4) red-eyed males (XY). 

 The last are exceptional, since white-eyed females nor- 

 mally never produce anything but white-eyed sons. Here 

 the exceptional male is due to an egg without an X, being 

 fertilized by a "female-producing" (or Z-bearing) sperm. 

 The three X individuals have never been found, and 

 undoubtedly die, presumably from too many X's. The 

 remaining red females are of two kinds, one normal XX 



POLAH 

 BODY 



EGGS 



SPERM 



WHITE d 

 s 



WHITE d 

 6 



WHITE 9 (^EPT'ON) 

 7 



DIES 



Fig. 94. — Non-disjunction. In the upper part of the figure the four poBsible modes 

 of elimination of the sex-chromoaome from the XXY eggs are shown, and the results of 

 their fertilization by a Y-bearing sperm of the male is shown below. 



and the other (XXY), which is expected to repeat the 

 exceptional behavior of her mother. In fact, this is what 

 she does. 



In the next diagram (Fig. 94) the fate of the same four 

 kinds of eggs is shown if they are fertilized by a Z-bearing 

 sperm. Four classes of individuals are expected (5) white 

 males (XYY) ■ (6) white males (XY) ; (7) white females 

 (XXY) ; and (8) YY individuals. No individuals having 

 the last make-up have ever been found, and there can 

 be no doubt that an individual without at least one X dies. 

 The white-eyed females are exceptional, since white-eyed 



