18 



BIOLOGIC BASIS OF SEX 



The sections of the chromosomes added 

 were derived from previous translocations 

 generally to the 4th chromosome and were 

 of varying lengths determined through cy- 

 tologic and genetic study. Summaries of 

 these comparisons are found particularly in 

 the papers of Pipkin (1940, 1947, 1959). In 

 their search for a major female sex factor 

 in the X chromosome, Patterson, Stone and 

 Bedichek (1937), Patterson (1938), Pipkin 

 (1940), and Crow (1946) finally were un- 

 able to show that any single female sex de- 

 terminer, located in the sex chromosome, 

 was of primary importance to sex. Evidence 

 for multiplicity of genes with a bias toward 

 female determination was found by Dob- 

 zhansky and Schultz (1934) and by Pipkin 

 (1940). They were able to transform dip- 

 loid intersexes into weakly functioning hy- 

 potriploid females by the addition of long 

 fragments of the X chromosome to the 

 2X + 3A intersex complement. Short sec- 

 tions of the X chromosome in some cases 

 shifted the sex type in the female direction. 

 Pipkin found that additions of short X 

 chromosome sections, to the 2X -|- 3A chro- 

 mosome sets, although covering in succes- 

 sion the entire X chromosome, were insuffi- 

 cient to make the flies other than of the 

 intersexual type. Longer and longer frag- 

 ments from either the left or right end of 

 the X chromosome caused a qualitatively 

 progressive shift toward femaleness. 

 Weakly functional hypotriploid females re- 

 sulted when either right- or left-hand sec- 

 tions of two translocations with t-lz (17) 

 and Iz-v (W13) breaks were present in the 

 2X + 3A chromosome complement. These 

 duplication intersexes possessed 1 or 2 sex 

 comb teeth when reared at 22 °C. and up to 

 5 well developed teeth when reared at 18°C. 

 These facts give support to the multiple 

 sex gene theory of Bridges or at least that 

 quantitative differences in sex potencies 

 exist within the X chromosome. This con- 

 clusion was further strengthened by the hy- 

 pointersexes lacking a short portion of one 

 of their two X chromosomes although pos- 

 sessing three of each autosome, inasmuch 

 as these types were shifted strongly in the 

 male direction. These studies of the X cliro- 

 mosonie show that several parts of this 

 chi-omosomc are concerned witli female dif- 



ferentiation and that the effects are irregu- 

 larly additive. 



Similar search of the autosome II and 

 III for genes of male potency showed that 

 small shifts in the male direction were 

 found in hyperintersexes for several short 

 regions of chromosome III but for none of 

 chromosome II (Pipkin, 1959, 1960). Tl^ree 

 slightly different right-hand end regions of 

 chromosome III produced the largest shifts 

 in the male direction in hyperintersexes, 

 but no increase in number of sex comb 

 teeth. These changes were comparable with 

 those produced in the female direction by 

 the addition of very short sections of the 

 X-chromosome to the 2X + 3A intersex 

 complement. On the other hand, none of the 

 seven different hypointersexes lacking a 

 short section of the 3rd chromosome from 

 the 2X + 3A complement showed a shift in 

 the female direction. This is rather surpris- 

 ing as hypointersexes for two short regions 

 in the X chromosome were shown by Pip- 

 kin (1940) to shift the sex type in the male 

 direction as was to be expected. From these 

 results Pipkin (1959) derives the conclu- 

 sion that 3rd chromosome aneuploids as 

 well as those of the 2nd chromosome and 

 X chromosome support the deduction that 

 dosage changes of portions of the X chro- 

 mosome are more powerful than dosage 

 changes of portions of either of the large 

 autosomes in affecting sex balance. This 

 view receives further support through 

 changes of size and number of sex comb 

 teeth as observed in the chromosomal types 

 carrying the gene Hr and reviewed earlier. 



Influence of the Y chromosome on sexual 

 differentiation has generally been ruled out 

 as XO nondisjunctional flies are male al- 

 though they are sterile (Bridges. 1922). 

 Similarly Dobzhansky and Schultz (1934) 

 ruled out the Y chromosome as an effective 

 influence on sex types of triploid intersexes 

 of D. melanogaster since the mean sex type 

 of Y -(- 2X 4- 3A intersexes did not differ 

 significantly from the sex tyyies of siblings 

 2X + 3A. " 



The steps taken in the studies of chro- 

 mosome IV are of interest. Dobzhansky 

 and Bridges in 1928 concluded that the 4th 

 chromosomes i^lay no part in sex determina- 

 tion in D. melanogaster. The evidence was 



