104 



CHAPTER 8 



rare: almosl all Drosophila found in nature 

 arc homozygous tra 



So far we have described only two sex 

 types in Drosophila. Occasionally, how- 

 ever, individuals occur which have, overall, 

 an intermediate sexual appearance: that is. 

 they arc both male and female in certain 

 respects. Such sexual intermediates, called 

 intersexes (see Figure 8-2). are sterile. 

 Intersexes arc relatively frequent among the 

 progeny oi' triploid (3N) females (whose 

 chromosomes at mitotic mctaphase are dia- 

 gramed in Figure 8-3; X chromosomes arc 

 represented by filled-in blocks, autosomes 

 h\ blanks, and the Y by a broken line). 



Some o\ the gametes of triploid females 

 are haploid and some diploid; still others 

 contain one. two, or three nonhomologs with 

 or without a haploid set. Whereas haploid 



eggs produce normal males and females 

 when fertilized by sperm from a normal 

 male, diploid eggs produce triploid females 

 when fertilized bj X-bcaring sperm. Diploid 

 eggs produce XX Y individuals with three 

 sets o\ autosomes, however, when fertilized 

 by Y-bearing sperm. Some intersexes have 

 this chromosomal constitution; other inter- 

 sexes carry three autosomal sets and XX — 

 one X derived from an egg containing two 

 autosomal sets and the other from an X- 

 bearing sperm. 



Close observation reveals two additional 

 sex types among the progeny of triploid 

 Drosophila (Figures 8-2, 8-3). These do 

 not appear as intersexes but as sterile "super- 

 sexes" — one type, called a superfemale, 

 shows characteristic female traits even more 

 strongly than does the normal female, the 



SUPERFEMALE 



i^^fl !&'??■>■ 



SUPERMALE 



FEMALE 



INTERSEX 



MALE 



FIGURE 8-3. Chromosomal complements of the sexual types found among 

 the progeny of triploid females of D. melanogaster. 



