Sex Determination 



105 



PHENOTYPES 



figure 8-4. Sex index and 

 sexual type in D. mclano- 

 gaster. 



other type, supermale, shows characteristic 

 male traits even more strongly than does 

 the normal male. Chromosomally, the 

 superfemale contains two sets of autosomes 

 and three X's; the X's are derived from an 

 egg which carries one set of autosomes plus 

 XX and is fertilized by an X-carrying sperm. 

 The superfemale usually dies before adult- 

 hood (see p. 97). The supermale con- 

 tains three sets of autosomes plus XY; the 

 chromosomes are derived from an egg which 

 carries two sets of autosomes plus X and is 

 fertilized by a Y-bearing sperm. 



What conclusions can we draw about sex 

 determination from a knowledge of the chro- 

 mosomal composition of different sex types 

 in Drosophila? 3 Since we know that genes 

 in the X and in the autosomes are sex-deter- 

 mining, let us refer to Figure 8-4, which 

 tabulates the number of X's and sets of auto- 

 somes present for each sex type and also the 

 ratio of X's to sets of autosomes — a numer- 

 ical sex index. This index ranges from 0.33 

 for supermales to 1.5 for superfemales. 

 Note that an index of 0.50 makes for male 

 and that adding a set of autosomes can be 



3 The following is based upon work of C. B. 

 Bridges. 



NO. X NO. SETS OF 



CHROMOSOMES AUTOSOMES 



(A sets) 



SEX INDEX 



No. X's 

 No. A sets 



1.5 



1.0 



1.0 



1.0 



1.0 



0.67 



0.50 



0.33 



interpreted as creating more maleness, pro- 

 ducing the supermale. When the sex index 

 is 1 .0, essentially normal females are pro- 

 duced, indicating that the female tendency 

 of one X overpowers the male tendency of 

 one set of autosomes. But if the index is 

 between 0.50 and 1.00, intersexes are pro- 

 duced, indicating, by the same line of rea- 

 soning, that the effect of two X's is partially 

 overpowered by the extra autosomal set 

 present. Finally, when the sex index is 1.5, 

 the female tendency of the X's becomes so 

 strong that superfemales result. 



These results strongly suggest that sex de- 

 termination is due to the balance of genes 

 located in the X on the one hand, in the 

 autosomes on the other. According to this 

 view, only the balance of the genes involved 

 is important, so that a sex index of 1 .0 

 should (and does) produce a typical female, 

 whether the individual is diploid (2X + 2 

 sets of A), triploid (3X -f 3 sets of A), or 

 tetraploid (4X -f 4 sets of A). Individuals 

 that contain haploid (IX -f- 1A set) sections 

 have been found and, as expected from their 

 sex index of 1.0, these parts were female. 

 Since all known facts support the exact cor- 

 respondence between chromosomal constitu- 

 tion and sexual types, we can accept chromo- 



