104 



CHAPTER 14 



3 egg, carrying one set of autosomes plus XX, 

 which was fertilized by an X-carrying sperm. 

 These usually die before adulthood (see 

 p. 90). Supermales prove to contain three 

 sets of autosomes plus XY, derived from a 

 type 3 egg, carrying two sets of autosomes 

 plus X, which was fertilized by a Y-bearing 

 sperm. 



What conclusions can we come to, relative 

 to sex determination, from a knowledge of 

 the chromosomal composition of ditTerent 

 sex types in DrosophikP. •' Since we already 

 know that genes in the X and in the auto- 

 somes are sex-determining, let us tabulate for 

 each sex type the number of X's and sets of 

 autosomes present. This is done in Figure 

 14-5, which gives also the ratio of X's to sets 

 of autosomes, as a kind of sex index. These 

 sex indexes range from 0.33 for supermales 

 up to 1.5 for superfemales. Note that an 

 index of 0.50 makes for male, and that addi- 

 tion of a set of autosomes can be interpreted 

 ^ Based upon work of C. B. Bridges. 



as making for more maleness, so that the 

 supermale is produced. When the sex index 

 is 1 .0 essentially normal females are produced, 

 as if 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 produced because, on this line 

 of reasoning, the effect of two X's is partially 

 overpowered by the extra autosomal set pres- 

 ent. Finally, when the sex index is 1.5 the 

 female tendency of the X's becomes so strong 

 as to make superfemales. 



What we have postulated here is that sex 

 detei niination is due to the balance of genes 

 located in the X on the one hand and those 

 in the autosomes on the other. According 

 to this view, only the balance of the genes in- 

 volved is important, so that a sex index of 

 1.0 should (and does) produce typical female, 

 whether the individual is diploid (2X + 2 

 sets of A) or triploid (3X + 3 sets of A). 

 Individuals carrying 4X + 4A sets (being 

 tetraploid) and parts of individuals that car- 



FIGURE 14-5. Sex index and sexual type in D. melanogaster. 



SEX INDEX 



No. X's 

 No. A sets 



1.5 



1.0 



1.0 



1.0 



1.0 



0.67 



0.50 



0.33 



