Chapter 14 



SEX DETERMINATION (II) 



WE HAVE already seen in the 

 last Chapter that the geno- 

 type can be the principal fac- 

 tor in the determination of sex. It was found, 

 in these cases, that sex can be correlated with 

 the chromosomal alternatives of XX versus 

 X, or Y versus no Y. The question now 

 raised is, what is the detailed genetic basis for 

 sex in terms of genes located in the X and Y 

 chromosomes? The data so far presented 

 have been analyzed as though a single pair 

 of genes (in the cases of XX vs. X) , or a single 

 gene (in the cases of Y vs. no Y), was the 

 total genetic basis for sex determination. 

 There are several implications of this interpre- 

 tation. The sex gene found in X chromo- 

 somes need have no alternative allele, the 

 presence of one such gene producing one sex 

 and the presence of two the alternative sex, 

 there being, of course, no dominance involved. 

 So, in Drosopliila, for example, where an X 

 may be considered to carry a gene producing 

 femaleness, the presence of one such gene 

 causes differentiation to proceed toward 

 femaleness but stops at a point we label male, 

 whereas two such alleles cause differentiation 

 to proceed further toward femaleness whose 

 end point we call female. In those cases 

 where XX vs. X is sex determining and the Y 

 though present has no influence in this regard, 

 it must also be assumed that the Y carries no 

 allele for the sex gene. However, two addi- 

 tional assumptions must be made in such 

 cases in order to correlate the genetics with 

 the cytology of sex. First, the sex gene must 

 be located in a region of the X, which is 

 100 



cytologically different in appearance from the 

 corresponding region of the Y, and which is 

 used to distinguish X from Y cytologically. 

 Second, no chiasma may occur between X 

 and Y within this cytologically different seg- 

 ment. These postulates are necessary to pre- 

 serve the exact correspondence between the 

 morphology of the X and its sex gene content. 

 The consequence of these hypotheses is that 

 even though chiasmata form between the X 

 and Y, the resultant strands that appear X 

 cytologically will carry the sex gene while 

 those that appear Y will not. These require- 

 ments are not unreasonable in view of the 

 known fact that synapsis does not occur 

 between cytologically different regions in 

 homologous chromosomes, and that, in the 

 absence of synapsis, a chiasma cannot form. 



Consider next the implication of the Y or 

 no Y sex determining mechanism in human 

 beings and mice. In this case the Y must 

 carry a gene for maleness in that portion 

 which makes it cytologically unique; this 

 gene is absent from the X and has no alterna- 

 tive allele that produces femaleness. If you 

 admit that the presence of the gene for male- 

 ness on the Y makes for male in these cases, 

 what genetically is responsible for the fe- 

 maleness produced in the absence of this 

 gene? There may be some genetic factor 

 present, other than in the Y chromosome, 

 which determines femaleness. There may 

 exist, therefore, some genetic factor in these 

 cases which makes for femaleness but whose 

 female tendency is overcome by the presence 

 of the sex gene for maleness whenever a Y 

 chromosome is present. 



Let us consider certain crosses with D. 

 melanogaster,^ where it is found that for each 

 100 eggs laid, instead of approximately 50 

 forming males and 50 forming females, about 

 75 are males and 25 females (Figure 14-lA). 

 Since just as many eggs become adult in this 

 unusual case as in a strain giving a normal 

 ratio of sexes, the abnormal results cannot 

 1 Based upon work of A. H. Sturtevant. 



