394 INTRODUCTION TO CYTOLOGY 



Heteroploidy and Sex. — A valuable clew to the probable mode of 

 action of chromosomes in the determination of sex is afforded by hetero- 

 ploid organisms. It was long supposed that sex in Drosophila, for exam- 

 ple, was a matter of the X-chromosomes alone, one of these causing 

 maleness and two, femaleness. In his researches on heteroploid flies, 

 however, Bridges^^ was able to show that the two sexes are correlated not 

 simply with the number of X-chromosomes present but rather with the 

 numerical ratio between these and the autosomes. Individuals in 

 which the ratio is 1 : 2 (one X to two autosome sets) are male ; those with 

 2:2 or 3:3 are females (diploid and triploid). An intermediate ratio 2:3 

 is correlated with the intersexual state, while flies with the extreme ratios 

 3:2 and 1:3 are " superfemales " and "supermales," respectively. In 

 certain mosaic individuals the monoploid regions with a ratio of 1:1 are 

 female in character, although other known monoploid insects are male. 



The study of sexual and other characters in such heteroploid flies 

 led Bridges to adopt the hypothesis of "genie balance," according to 

 which "each character of an individual is the index of the point of 

 balance in effectiveness of a large but unknown number of genes, some 

 of which have a tendency to change development in one direction and 

 others in the opposite. "^^ With respect to sex itself, it had been suggested 

 by Goldschmidt (1911) that the sex grade is a sort of balance between 

 determiners of opposite tendency. As stated by Bridges, both maleness 

 and femaleness are due to the simultaneous action of two opposed sets 

 of genes, whose influences on the organism are toward maleness and 

 femaleness, respectively. These genes, which may affect other characters 

 also and are regarded as distinct from other genes, must lie in most if 

 not all of the chromosomes of the complement; the X-chromosome is 

 peculiar merely in having a preponderance of genes tending to produce 

 female characters. The balance is such that, if the number of X-chromo- 

 somes equals the number of autosome sets, the female-producing genes 

 in the complement as a whole outweigh in effect the genes tending toward 

 maleness and the individual is consequently female; whereas, if there 

 are only half as many X-chromosomes as autosome sets, the female- 

 producing genes present are insufficient to accomplish this result and the 

 individual is male. Intermediate and extreme ratios would be expected 

 to produce the results actually observed. Since this is precisely what is 

 found in the case of characters other than sex, Bridges concludes that 

 "the conception of the nature and action of genes as gained from the 

 study of non-sexual characters is valid in interpreting sex phenomena." 

 This conclusion is borne out by the fact that in strains showing additional 

 fragments of X-chromosomes, the sexual expression varies with the size 



41 Bridges (1921a, 1922, 1925, 1930, 1932). 



"^ Genie balance is also correlated with metabolic rate and duration of life by 

 Gowen (1931a6). 



