12 



BIOLOGIC BASIS OF SEX 



tive system for both the males and females 

 takes its origin from a disc at the posterior 

 extremity of the larvae. The testes or 

 ovaries are only brought together with the 

 secondary reproductive tracts during late 

 development in the pupae, whereas the sex 

 combs retain their separate development. 

 The striking action of chromosome balance 

 in sex determination is that all of these 

 organs and others are jointly affected and 

 simultaneously develop directly into either 

 the male or female sexual types. Of these 

 characteristics, the sex combs are particu- 

 larly trustworthy indices of maleness. In 

 the male, these combs are heavily sclero- 

 tized and pigmented. They have 9 to 13 

 rather blunt teeth on their margins. The 

 normal females lack these sex combs en- 

 tirely. The intersexes have well developed 

 combs, whereas the triploid females and 

 superfemales lack them. This all-or-none 

 situation resembles that observed in the 

 rest of the block of sexually differentiating 

 characteristics found in normal males or 

 females, save that in the intersexes develop- 

 ment may result in the organs of either sex 

 appearing in the primary or secondary re- 

 productive systems. The all-or-none char- 

 acter of the sex combs may, however, be 

 bridged in that the appearance of certain 

 mutations leads to the production of these 

 combs in all of the different sexual types. 

 The combs differ in size, in thickness and 

 length of the teeth, and their number. The 

 sex combs meet the conditions of a quanti- 

 tative character which may be counted or 

 measured in unbiased units. Thus in the 

 case of sex combs, it is possible to obtain 

 quantitative information on the effects of 

 clu'omosomal changes on the expression of 

 this form of sexuality. 



TABLE L2 



Mean sex conih teeth for flies having (lijj'erent X and 



Two gene mutations in D. melanogaster 

 have aided in this search. The first is the 

 dominant gene, Hr, which causes the male 

 reproductive tract to be added to that of 

 the female when this gene is heterozygous 

 (Gowen, 1942 j . The extensive effects of this 

 gene on the whole sex determining system 

 have been described by Gowen (1942, 1947) 

 and Fung and Gowen (1957a). The second 

 gene is that of Sturtevant's (1945) trans- 

 former, tra, which operates on the female 

 phenotype to convert it to that which cor- 

 responds to the male in having sex combs, 

 full male reproductive system including 

 testes, while still leaving the female charac- 

 teristic body size. These genes are located 

 in the third chromosome of D. melanogaster. 

 Crosses between them show allelomorphic 

 effects. Combinations of these genes in con- 

 junction with different chromosomal ar- 

 rangements make possible a series of differ- 

 ent sex types which are distinguishable 

 from ordinary males and females (Gowen 

 and Fung, 1957). As some of these types 

 bear sex combs, a quantitative character is 

 furnished in the variation of the sex comb 

 teeth which may be used as an impersonal 

 measure of departure of these types from 

 ordinary males or females. The groups hav- 

 ing particular interest are those carrying 

 the Hr gene in heterozygous condition with 

 the X and A chromosomes having various 

 numbers. The mean numbers of sex comb 

 teeth for these various groups are shown in 

 Table 1.2. 



Analysis of these data for the contribu- 

 tions made by the sex chromosomes and 

 autosomes to the numbers of teeth found on 

 the sex combs of these different genotypes 

 shows the following relation. 



Sex combs == 12.82 - 3.42 X + 0.89 A 



From this equation it is seen that the X 

 chromosome has four times as much effect 

 on lowering the number of teeth on the sex 

 combs as a set of autosomes. The direction 

 of effect is, as would be expected, increasing 

 the number of X chromosomes tends toward 

 making the individual more female-like in 

 that the sex combs become smaller and less 

 l)ronounced. Increasing the number of au- 

 tosome sets on the other hand tends to push 

 the indiviihial toward the male type with 



