14 



BIOLOGIC BASIS OF SEX 



Altlioiigh Drosophila is the leader in fur- 

 nishing types for analyses of the inherit- 

 ance basis for sexuality, it is well known 

 that similar conditions exist in other forms 

 of life. 



B. MAJOR SEX GENES 



Major sex genes affecting the dichotomy 

 of the sexes have appeared among the ob- 

 served mutational types. Sturtevant (1920a, 

 1921 j in Drosophila simulans isolated a 

 gene in its second chromosome which when 

 homozygous could convert diploid females 

 into intersexuals and render XY males 

 sterile. Phenotypically these intersexuals 

 were female-like in that they lacked sex 

 combs and had 7 dorsal abdominal tergites, 

 ovipositor of abnormal form, 2 sperma- 

 thecae, and lacked the penis. They were 

 male-like in having first genital tergite al- 

 though abnormal in form, lateral anal 

 plates, claspers, black pigmented tip to the 

 abdomen. The gonads were rudimentary. 

 The gene was recessive and, as expected, 

 showed no effect on D. simulans X D. mel- 

 anogaster hybrids. 



In 1934 a new intersexual type was ob- 

 served by Lebedeff in D. virilis. Intensive 

 study of this type showed that it depended 

 on a fairly complex inheritance. A 3rd chro- 

 mosome recessive gene ix™ at 101.5 con- 

 verted the XX females into sterile males, 

 showing only one noticeable female charac- 

 teristic, the presence of a rudimentary 5th 

 stcrnite. Sterility could be accounted for, 

 even though the internal genitalia were 

 completely male, by the small size of the 

 testes and the degeneration of the germ 

 cells largely at the spermatocyte stage. Ob- 

 servable effects of this gene appeared only 

 in females, XX + 2A. Genetic modifiers 

 were isolated which acted on the ix"yix'" 

 complex. Other gcnotyi)es were derived 

 through recombinations of these genes. The 

 resulting phenotypes showed a greater vari- 

 ation of the male-female mosaics. One of 

 these types was sterile but fully hermaph- 

 roditic having a complete set of external 

 and internal genitalia of both male and fe- 

 male. Genetically, this type was homo- 

 zygous for the ix'" gene but in addition luid 

 both a previously found dominant scmisu})- 

 pressor and also a second semisuppressor. 

 Another line having the ix'" gene homozy- 



gous had a full set of male organs but there 

 were rudimentary ovaries attached to the 

 testes. This line showed the dominant semi- 

 suppressor as the restraining element on the 

 developmental pattern of the ix'" gene. An- 

 other type was separable in that it was still 

 more female-like, yet had male external 

 genitalia and rudimentary testes. This type 

 was the result of the homozygous ix'" genes 

 operating in conjunction with 3 different 

 semisuppressors. Extensive embryologic 

 studies were interpreted as indicating that 

 gonads, ducts, and genitalia had started as 

 in females with the XX constitution. 

 Shortly thereafter male organs appeared as 

 new outgrowths from the same imaginal 

 disc. The development of the two sets of 

 organs was then simultaneous but still de- 

 pended on the gene pools present in the 

 particular strain. 



Another intersexual type appeared in D. 

 pseudoobscura as a mutation, presumed due 

 to a single dominant gene (Dobzhansky 

 and Spassky, 1941). In a series of cultures 

 having "sex ratio," two cultures gave 234 

 females, 7 males, and 266 intersexes. The 

 females' progeny transmitted only the nor- 

 mal condition to their Fs progeny. The 

 males were sterile presumably because they 

 lacked a Y chromosome. The intersexes 

 were also sterile so that further study of 

 the genetic condition became impossible. 

 The evidence, however, is interpreted as 

 showing that the intersexes were trans- 

 formed females which had inherited a domi- 

 nant gene governing this condition. The 

 intersexes were characterized by two sets 

 of more or less complete genital ducts and 

 external genitalia but only one pair of gon- 

 ads. One set of ducts and genitalia was al- 

 most always more female-like and the 

 other more male-like. Sex combs were pres- 

 ent, the distal comb had 2 to 4, and the 

 proximal 4 to 6 teeth, compared with the 

 normal male number of 4 to 7 on the distal 

 and 6 to 9 on the proximal comb. Body de- 

 velo{)ment was more like that of the female. 

 Cytologic examination showed two of the 

 intersexes had (wo ()\-aries each. One of 

 these two had a rudimentary testis. The 

 chroinosoiiu' complement was 2X + 2A. 



A dominant gene which causes intersex- 

 iiality in diploid females of D. virilis was 

 estal")lished by Briles. Stone (1942) located 



