SEX DETERMINATION 215 



ism of the A and B factors.^ In Ustilago the A factor is strictly a sterility 

 factor or lethal factor, the homozygote being inviable; mycelia (gameto- 

 phytes) with the same A factor but different Bs can perform a limited 

 copulation giving inviable sporophytes. The B factor directly controls 

 conjugation, which does not take place, even in rudimentary form, 

 unless the B factors in the two mycelia are different. 



C. ZYGOPHASE SEXUALITY 



We know considerably more about the genetic control of zygophase 

 sexuality than that of gamophase. The existence of sex chromosomes 

 leads one to postulate a simple pair of factors homozygous in the 

 homogametic sex and heterozygous in the other; thus in most animals 

 and plants, where the heterogametic sex is the male, the factor in the 

 X chromosome would be a femaleness factor. But this simple scheme 

 is unable to deal with sex reversals and intersexes and hermaphrodites, 

 which show that both males and females contain the potentialities for 

 both types of sexual differentiation. It is necessary then to postulate 

 some underlying bisexual potencies which are set in action or restrained 

 by the controlling genetic mechanism. 



There are two main variants of this hypothesis. ^ According to Correns^ 

 all cells of both sexes contain a set of factors called A A and GG which 

 respectively give the potentialities for male and female differentiation. 

 These underlying complexes are controlled by the "realizers" aa and 

 yy which are contained in the sex chromosomes and act specifically on 

 AA and GG. By making various assumptions as to the strengths of the 

 factors a, y, A and G, this hypothesis can be used to explain the inter- 

 mediate types of sexuality found in intersexes. The other variant of the 

 hypothesis is mainly due to Goldschmidt,* who contrives to formulate 

 the difference between the sex factors in the heterogametic sex as a 

 quantitative difference instead of a difference between the qualitatively 

 different factors a and y. This can be done by invoking two pairs of 

 sex realizer factors FF for femaleness and MM for maleness. In any 

 given species, one of these pairs is always homozygous in all members 

 of the species, while the other is heterozygous in the heterogametic sex, 

 the two allelomorphs being related as hypo-hyper-morphs, so that the 

 difference in effect can be expressed quantitatively. Thus in male 



1 Bauch 1930, 1931. 2 /^g^^ Mainx 1933. 



^ Cf. Correns 1928a. * Goldschmidt 1923, 19316. 



