SEX DETERMINATION 



249 



in different parts of the body differs in respect to sex-linked 

 characters, a sex-mosaic results known as a gynandromorph. 

 Morgan and Bridges have made an exhaustive study of such 

 mosaic individuals found in their cultures of Drosophila. 

 One of the simplest types, a bilateral sex-mosaic, is shouTi in 

 Fig. 153. The right half of this fly shows male characters, 

 viz., shorter wing, black-tipped abdo- 

 men, sex-comb on first leg. The left 

 side of the fly shows the contrasted 

 female characters. The right eye was 

 also white, a character inherited in the 

 single X -chromosome derived from 

 the white-eyed mother of the fly. The 

 left eye was red resulting from the 

 presence (in the female part of the 

 body) of an X-chromosome bearing 

 red-eye, derived from the red-eyed 

 father, which is dominant over the 

 white-eye borne by the X-chromosome 

 furnished by the mother. 



Three different explanations have 

 been offered in recent years for the 

 origin of sex-mosaic insects. These are 

 expressed diagrammatically in (Fig. 

 154). The first. A, was offered by 

 Boveri. It suggests that an egg which 

 has undergone maturation, and which 

 accordingly retains a single X-chro- 

 mosome may, on account of delayed 

 fertilization, undergo nuclear division 

 before fertilization is complete, so that 

 it becomes binucleate before fusion of egg and sperm nuclei 

 has occurred. The sperm now fuses with one of the egg's two 

 nuclei. That nucleus and its descendants will be 2X (female), 

 but the unfertilized nucleus, if it develops by itself will be 

 X (male). A body mosaic as to sex will result, part male, 

 part female. The case shown in (Fig. 153) could be accounted 



Fig. 153. A sex-mosaic, or gynan- 

 dromorphic, Drosophila. The right 

 half of the body shows male char- 

 acters, viz. comb on first leg, short 

 wing, and black-tipped abdomen. 

 The left half of the body shows 

 female characters, viz. long wing 

 and light-tipped abdomen. Note 

 also that the right eye was white, 

 the left eye red. See text. (After 

 Morgan.) 



