Different Types of Sex Determination 471 



no female determiners, could not show relative sexuality or inter- 

 sexuality'. But haploids may show intersexuality: the male haploid 

 must be derived from a more or less hermaphroditic diploid of the 

 t)-pe Witschi has estabHshed for frogs, MMFF\ F^ meaning an 

 X-chromosome with a weak F factor. Haploids, then, would be either 

 MF with F > M = 9 , or MF^ with M > F^ = 5 . In both cases, 

 shifts (relative sexuality) would be possible by means of modifiers or 

 different valencies of F and M. The diploid phase would be hermaph- 

 roditic or almost so, and liable to easy genetic shift, just as in frogs. 

 Both sexes of the haploid would contain both F and M factors with 

 the F/M balance deciding the sex. 



Hartmann derived (many papers and reviews quoted in 1943) a 

 different interpretation from strict adherence to Correns' formulation. 

 I once called this naive and primitive, meaning that it tries to explain 

 everything within the limits of elementary Mendelism. Correns first 

 found (1907) that in crosses between monoecious and dioecious 

 forms of Melandrium, sex behaves like a Mendelian backcross, which 

 had been known before to zoologists from the work on sex-Hnked 

 heredity and the X-chromosome mechanism. At that time the cytologi- 

 cal meaning of sex-linked heredity was not yet knowni; so the back- 

 cross explanation could not be brought in line with the chromosomal 

 mechanism. Thus the formula FF = 9 and FM vdth M epistatic = S 

 was developed. However, since it was known that females could de- 

 velop male characters, another set of genes had to be invented, the 

 A and G groups of genes, for every character which could be sexually 

 dimorphic; the role of F and M came to be decisive in determining 

 whether A or G became more active and the other group inhibited. 

 Wettstein later called this the realizator function. Later, when the 

 location of the sex determiners in the sex chromosomes became known, 

 F had to be located in the X, and M in the Y-chromosome, which 

 of course left all cases of XO in the air, as well as Drosophila without 

 a sex determiner in the Y. Then came intersexuality and the balance 

 theory, which required only F (M with female heterogamety) in the 

 X-chromosome and M outside, which in male heterogamety could 

 only be the autosomes, but in female heterogamety, also the Y-chromo- 

 some, as we have seen. Thus AG became a superfluous notion, FF = 

 9 , FM = 6 did not work; and, in order to explain intersexuality, the 

 different valencies of F and M, the balance idea, and the epistatic 

 minimum had to be introduced. All this means that the formulation 

 of Correns which was good in 1907 (though even at that time it 

 neglected the sex chromosomes) ceased to be useful after 1910-1912. 



