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 two 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 



POLAB 

 BODY 



EIGGS 



SPERM 



4 



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

 elimination of the sex-chromosome from XX Y 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 XXZ 

 female may be supposed to have had white eyes (clear 

 X'5) 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) Red-eyed 

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



