28 SEX-DETERMINATION 



determined by planned experimentation. Gynandromorph- 

 ism is not uncommonly encountered in this form. 



Morgan and Bridges (1919) showed that in these Droso- 

 phila melanogaster gynandromorphs whereas the trans- 

 mission of the autosomal characters was not affected, both 

 male and female parts displaying them equally, that of the 

 characters the genes for which were X-chromosome-borne 

 was disturbed. It was possible therefore to conclude that in 

 such a gynandromorph the male and female regions differed 

 from each other in respect of the X-chromosome content of 

 the nuclei of their component cells. 



A lateral gynandromorph with a white-eyed side display- 

 ing the male characterization and a red-eyed side exhibiting 

 the female characterization can be explained as follows: 



White eyed ^ x Red eyed $ Parents 



(wX)Y (WX)(WX) 



(WX)(wX) the gynandromorph in its 



beginning 



The gynandromorph started its life as an XX individual, 

 being the result of the fertilization of an t^g with an 

 X-chromosome carrying the dominant gene for red-eye by 

 a spermatazoon with an X-chromosome carrying the reces- 

 sive gene for the white-eye character. At the first division of 

 this fertilized tgg when each of these X's splits longi- 

 tudinally and when normally each of the nuclei of the two 

 resulting daughter cells receives two daughter X's, one 

 paternal and one maternal in origin, one of these daughter 

 X's — the maternal one carrying the dominant red-eye gene 

 — became excluded from one of the daughter nuclei. The 

 sex-chromosome constitution of the two daughter nuclei 

 therefore came to be: 



(wX) : (WX)(wX) 



Then if from each of these cells there arose the tissues of one 

 lateral half of the body and if the single X constitution 

 equals maleness and the XX constitution leads to the 

 development of femaleness, a *half sider' should result, one 

 side being male and white-eyed, the other female and red- 

 eyed. 



