SEX-CHROMOSOMES AND INHERITANCE 201 



are given off by the egg are shown in the next diagram 

 (Fig. 93). One X may go out of the egg, and the other X 

 and the Y stay in ; or one X may stay in the egg and the 

 other X and the Y go out. In these hvo cases, X and X 

 may be thought of as members of a pair that conjugate, as 

 in the normal female, and then separate, and chance alone 

 determines whether the Y stays in or passes out. Again 

 Y may go out of the egg and X and X stay in ; or X and X 



POLAR 

 BODY 



EGGS 



SPERM 



4 



Fig. 93. — Non-disjunction. In the upper part of the figure the four possible modes of 

 elimination of the sex-chromosome from XXY eggs are shown; the results of their fertili- 

 zation by an X-bearing sperm of the male is shown below. 



go out and Y stay in. Here X and Y may be supposed to 

 be members of the conjugating pair, and the free X goes 

 to the same pole as the X that conjugated. 



In the diagram, each of these four types of eggs is 

 represented as fertilized by an X-bearing sperm. In order 

 to make the outcome more apparent the original XXF 

 female may be supposed to have had white eyes (clear 

 X^s) and the male that fertilized her red eyes (here repre- 

 sented by the black X carrying the gene for red eyes). 



Four classes of individuals are expected : (1) Rod-eyed 

 females (XXZ) ; (2) red-eyed females (XX) ; (3) red-eyed 



