44 SEX-DETERMINATION 



Most of these sex-types have been encountered and have 

 been found to conform with the predictions made. 



Sex-determination would therefore seem to be the end 

 resuh of a quantitative balance between X-chromosomes 

 and autosomes. In Drosophila melanogaster the X is not a 

 determiner of sex but is a differential. The genes that are 

 concerned in sex-determination are scattered irregularly 

 throughout all the chromosomes, sex-chromosomes and 

 autosomes alike. In a general way these genes are to be 

 classified as female- and male-determining and the two 

 types are in a way opposed to each other. In the X the genes 

 for femaleness preponderate over those for maleness so that 

 this chromosome is, on the whole, female-determining. In 

 the second and third chromosomes the male-determining 

 genes preponderate over the female-determining genes and 

 these chromosomes therefore are on the whole male- 

 determining. The fourth chromosome is mainly female- 

 determining. 



*Both sexes are due to the simultaneous action of two 

 opposed sets of genes, one set tending to produce the 

 characters called female and the other to produce the 

 characters called male. These two sets of genes are not 

 equally effective, for in the complement as a whole the 

 female-tendency genes outweigh the male-tendency genes, 

 and the diploid (or triploid) form is a female. When the 

 relative number of the female-tendency genes is lowered by 

 the absence of one X, the male-tendency genes outweigh the 

 female, and the result is the normal haplo-X male. When 

 the two sets of genes are acting in a ratio between these two 

 extremes, as in the ratio of 2X13 sets of autosomes, the result 

 is a sex-intermediate — the intersex.' 



The use of fragments of the X- duplications of various 

 lengths and from different regions of the chromosome and 

 of deficiencies of the X by Dobzhansky and Schultz (1934) 

 provided experimental proof of these assumptions. 



Lepidoptera. Standfuss (1908) crossed Saturnia pyrt and 

 S. pavonia and then back-crossed the F. i males to S. pyri 

 females to get 42 males and 38 'gynandromorphs'. Federley 



