FOUNDATIONS FOR SEX 



13 



larger sex combs. Some 95 per cent of the 

 variation in sex comb teeth has been ac- 

 counted for by this equation. 



The above equation results when the ef- 

 fect of the sex chromosomes and autosomes 

 is considered as operating on a simple addi- 

 tive basis. It is interesting to consider these 

 effects on the basis of the ratio of sex chro- 

 mosomes to autosomes as utilized by 

 Bridges. As is customary, the male geno- 

 type is given a weight of 0.50, the intersex 

 0.67, the female 1.00, the superfemale 1.50, 

 and the triploid female 1.00. With these 

 values the data on the sex combs are fitted 

 by the equation 



Sex combs = 13.40 - 6.38 X/A 



The fit of this equation to these data 

 shows control of less of the variation in the 

 sex comb teeth. Only 76 per cent of the 

 variation is accounted for by these methods 

 whereas 95 per cent is accounted for when 

 the effects of the X and A chromosomes are 

 considered as additive. 



If it is agreed that the condition of the 

 sex combs is a good unbiased measure of 

 the degree of sexuality of the Drosophila, 

 it follows that it would be more probable 

 that the genes in the X chromosomes oper- 

 ate additively with those of the autosomes. 



III. Sex Genes in Drosophila 



A. MUT.\XT TYPES 



Bridges' concept of sex determination 

 turned on the action of sex genes located 

 more or less fortuitously throughout the 

 inheritance complex of the species. In Dro- 

 sophila it happens that the major female 

 determining genes seem to be located in the 

 X chromosome and the male determining 

 genes in the autosomes, whereas the Y 

 chromosome seems essentially empty of sex 

 genes. In support of this concept limited 

 data are cited on specific genes affecting 

 the reproductive system or its secondarily 

 differentiated elements and two cases where 

 genes affected the primary reproductive 

 system as a whole. During the interim be- 

 tween 1938 and the present, the numbers 

 of these genes and the breadth of their 

 known effects have been notably increased. 

 Again the genes as a whole affect every 

 phase of sexuality, morphology, fertility, 



and physiology. Single genes may occa- 

 sionally alter both sexes or may frequently 

 affect only male or only female phenotypes. 

 Single genes may appear to influence two 

 or more distinct characteristics observable 

 in the developing flies, although this multi- 

 ple phenotypic expression may go back to 

 a gene action which is controlling a single 

 event in development. Genes affecting the 

 structural development of either male or 

 female organs frequently are accompanied 

 by sterility of various degrees. A very large 

 category of genes is known only through 

 its effects on sterility of either or both 

 sexes. Experience has shown that when 

 properly analyzed anatomic changes are 

 probably basic to the sterility. In this sense 

 genes for sterility should be considered 

 genes for sex characters. Berg (1937) fur- 

 nished data on the relative frequency of 

 sterility mutations in the X chromosome 

 as against those in the autosomes. 12.3 per 

 cent mutations in which the males were 

 sterile were found in the X chromosome 

 against 4.5 per cent found in the second 

 chromosome. These results show that the 

 X chromosome has many gene loci occu- 

 pied by genes capable of mutating to ste- 

 rility genes which affect males. Sterility is 

 also common for the females but requires 

 more testing. The loci for these genes are 

 widely distributed both within and among 

 the chromosomes. 



Genes affecting sex morphology are found 

 in all Drosophila chromosomes. Of 17 

 which have been recently studied; 6 were 

 in the 1st chromosome, 5 in the 2nd, 5 in 

 the 3rd, and 1 in the 4th. Insofar as can be 

 determined these genes are no different than 

 those affecting other morphologic traits. 

 They may be dominant, they may be re- 

 cessive, and a limited number of them may 

 show partial dominance. They affect a vari- 

 ety of sex characteristics and do not always 

 involve sterility of one or the other sex. 

 The loci occupied by these sex genes may 

 have several alleles, some of which may 

 lead to sterility, others not. Most affect 

 characters like size and development of the 

 ovaries, the characteristics of the eggs, duct 

 development, spermathecae, ventral recep- 

 tacles, parovaria, paragonia, sex combs, po- 

 sition of genitalia, and so on. 



