SEX DETERMINATION 223 



as that between the ''realizers" and the alternative reaction systems, 

 A and G in Correns's notation, which we shall discuss again later. The 

 theoretical possibility of such a distinction does not, of course, by any 

 means prove Goldschmidt's point that there is only one F gene in the 

 X chromosome of Drosophila. In fact it seems much easier to interpret 

 the results of Dobzhansky and Schultz,^ who studied the effect of 

 duplicated fragments of the X on intersexes, by admitting their sug- 

 gestion that the fragments really changed the time of the switch-over 

 and therefore contained true sex genes affecting the production of the 

 male and female substances. The effects of the fragments were of a 

 fairly low order, however, as they had Httle or no effect on the sexual 

 differentiation of normal males and females, which have probably got 

 rather more margin of safety than the intersexes; it is possible there- 

 fore that there are one or more intense sex genes in some other part 

 of the X. The location of male factors in the autosomes has been very 

 litde studied in D. melanogaster, but in D. simulans, Sturtevant^ has 

 found a factor in the Ilird chromosome which has a considerable effect 

 in the male direction, even converting normal females into intersexes. 



3. Other Organisms 



In certain other organisms, the differential sex factor does seem to be 

 a single gene. Thus in the fish Lebistes,^ there is normal crossing-over 

 between the X and tlie 7, so that there cannot be a large number of 

 sex factors in the X as they would be separated by crossing-over and 

 the mechanism disrupted. But even here there are minor factors which 

 influence sex determination, since Winge^ has been able to select a race 

 which was homozygous for the normal sex-differential gene and thus 

 all XX and by rights female, but in which the sex determination was 

 taken over by another gene pair whose effects are normally obscured by 

 the rest of the genotype. This was heterozygous in the female, although 

 the original race had male heterogamet}\ 



A case of sex determination depending on a single differential gene 

 pair has been produced artificially in maize, which is normally monoe- 

 cious or hermaphrodite. The two races described by Emerson^ depend 

 on the factors barren-stalk, which suppresses the formation of female 

 flowers, and tassel-seed, which converts male flowers into female ones. 

 One race contains the allelomorphs barren-stalk- 1 and tassel-seed-2, 



^ Dobzhansky and Schultz 1934, cf. Patterson, Stone, and Bedichek 1935. 



^ Sturtevant 1921. ^ Winge 1923. 



* Winge 1932. ^ Emerson 1932, Jones 1934. 



