92 



White- 

 eyed 



Red- 

 eyed 



^ 



: Red- 

 ^ eyed 



t 



u; + 



+ w 



w m B^ i^ 



l»^3^ 



Fo 



FIGURE 4.1. Sex-linked inheritance of eye color in Drosophila. Reciprocal crosses 

 of white-eyed and red-eyed flies show the pattern of inheritance of a gene carried 

 on the X chromosome. This type of inheritance results from the absence of w and 

 other genes which are similarly inherited from the Y chromosome. The genotype 

 is thus determined by the segregation of the X chromosome after Sturtevant and 

 Beadle, 1939, And Introduction to Genetics, Philadelphia, Saunders, pp. 30-31 I. 



sexes segregate 1:1 for eye color, rather than 3:1. In the reciprocal 

 cross, between a homozygous red-eyed female and a white-eyed male, all 

 Fj progeny are red-eyed, but in the F^ all the females and half of the 

 males are red-eyed. It should be recalled that in Drosophila. the 

 female carries two \ chromosomes, and the male an X and a Y: se.\ deter- 

 mination is a consequence of the segregation of .\ and Y into separate 

 sperm in meiosis. 



Bridges in\estigated a strain of white-eyed females whose progeny 

 did not all follow this pattern. In crosses of these females with red- 

 eyed males, about 4 per cent of the female progeny \\ ere white-eyed like 

 their mothers and about 4 per cent of the males were red-eyed like their 

 fathers. directl\ yiolating the regularities of se.x-Iinked inheritance 

 which had just been established. 



In his doctoral dissertation. Bridges demonstrated that this unusual 

 pattern of segregation resulted from the special chromosome composition 

 of this white-eyed female strain. It had originated by abnormal egg 

 formation in which the t\yo X chromosomes stayed together in meiosis, 

 resulting in the appearance of two unusual classes of eggs, XX and O. 

 This process is called primary nondisjunction. On fertilization, four 

 abnormal classes of progeny are produced: 



