102 



CHAPTER 14 



FIGURE 14-2. Normal {left) and triploid {right) D. melanogaster /t^/Ha/fi". {Drawn by E. M. Wallace.) 



So far we have described only two sex 

 types in Drosophila. Occasionally, however, 

 individuals occur which have over-all an inter- 

 mediate sexual appearance, being both male 

 and female in certain respects. Such sexual 

 intermediates are called intersexes, and are 

 sterile. Intersexes are relatively frequent 

 among the progeny of certain females. 

 These females are slightly larger than normal 

 females (Figure 14 2) and when examined 

 cytologically prove to be friploids, individ- 



uals having each chromosome threefold (Fig- 

 ure 14-3, where X's are filled in while auto- 

 somes are not and the Y is represented by a 

 broken line). How can we explain the high 

 proportion of intersexes in the progeny of 

 these triploid females? Let us examine the 

 process of meiosis in such individuals for 

 clues to an explanation. 



When triploids undergo meiosis, bundles 

 of three homologous chromosomes (//•/- 

 valents) may be formed at synapsis. This is 



SUPERFEMALE 



TRIPLOID 



SUPERMALE 



FIGURE 14-3. Chromosomal complements of the sexual types found among the 

 progeny of triploid females of D. melanogaster. 



,.^«s«lil^Wfc^ 



FEMALE 



INTERSEX 



MALE 



