( 'hapter *8 



SEX DETERMINATION 



Drosophila 



In Chapter 7. it was mentioned that the 

 ordinary Drosophila melanogaster female is 

 3AA + XX and the male. 3AA + X + Y. 

 One cannot decide the chromosomal basis 

 for sex determination from these facts alone, 

 however, since two variables are involved, 

 the X's and the Y. Is the male a male be- 

 cause he has a Y, because he has only one 

 X, or because he has both one X and one 

 Y? 



By knowing the sex of flies that carry — 

 besides two sets of autosomes — either XXY 

 (female). XX YY (female), or XO (male), 

 we can see that the Y is not sex determining 

 in this organism. (As has been indicated 

 in Chapter 7, the Y is necessary for fertility. 

 XO males having nonmotile sperm. ) 



Knowing that sex in Drosophila is cor- 

 related with the chromosomal alternatives of 

 XX versus X, one can ask: What is the 

 detailed genetic basis for sex in terms of 

 genes located in the X chromosome? The 



data so far presented can be interpreted to 

 mean that only a single pair of genes (in the 

 case of XX) or a single gene (in the case 

 of X ) is the total genetic basis for sex de- 

 termination. There are several implications 

 in this interpretation. The X-linked sex 

 gene need not have an alternative allele if 

 the presence of one such gene produces one 

 sex, and the presence of two, the other sex — 

 dominance not being involved. It can also 

 be claimed that the Y carries no allele for 

 this sex gene. Two additional assumptions 

 must be made, however, in Drosophila and 

 in other species having heteromorphic sex 

 chromosomes, to correlate the genetics with 

 the cytology of sex. 



1. That the sex gene must be located in 

 a region of the X which distinguishes 

 X from Y cytologically. 



2. No chiasma may occur between X and 

 Y within this cytologically different 

 segment. 



These postulates are necessary to preserve 

 the exact correspondence between the mor- 

 phology of the X and its sex gene content. 

 Consequently, even though a chiasma occurs 

 between the X and Y in a segment which 

 they share (for example, both carry an 

 allele of bobbed), the resultant strands that 

 appear cytologically as X will carry the sex 

 gene, whereas those that appear as Y will 

 not. These requirements are reasonable 



A. PHENOTYPiC RESULTS 



B. GENOTYPIC EXPLANATION 



