446 



Genetics of Sex Determination 



1. Pure races 2. Fi and F2 combinationa 

 a"*" a"*" = monoicum a*^ a^ = c?" 



aP a"* = dioicum <f a"*" a° = c?* 



a"* a"* = dioicum 9 a+ a+ = 5^ 



a+ a^ = ^ 

 a"* a° = 9 



It is easy to fit these facts into our former discussion. (Bridges, 

 1939, tried to explain the analogous results of Correns on the basis of 

 his views on sex in Drosophila, assuming that females are XX AA; A = 

 autosomes with a male tendency. The details of Galan's crosses do 

 not agree with this interpretation.) It is obvious that in the formula 

 M(A)M(A)F(X)F(X) = 9 , M(A)M(A)F(X)Y = S , with the Y-chromo- 

 some acting as a modifier, as shown in Melandrium, the a of Galan is 

 a condition of F with the potency F = M (a+ = F) in the dioecious 

 plant. Galan's a*^ is then the F in the dioecious form, which has a 

 higher potency than the one (a+) in the monoecious form, say F^, so 

 that F^F^MM or a'^a'' = 9 ; a^ then becomes the Y-chromosome, absent 

 of course in the monoecious form and always present in males. Since 

 a+a** = F(X)FHX)MM is monoecious, just as a+a+ = F(X)F(X)MM, 

 the F^ of the dioecious form (= a'') has a potency (pull in the 

 female direction) which is not far above that of F = a+. Thus, in 

 Galan's terminology, what looks like a completely different story is in 

 fact identical with the Melandrium case as we expounded it. But we 

 cannot say whether the Y in the male acts as a male enhancer as in 

 Melandrium, or only as the absence of one F^ as in the standard 

 case. Only polyploidy experiments hke those made with Melandrium 

 could decide this point. 



e. Once more the Y-chromosome 



All these facts and interpretations lead us to consider the problem 

 of the modifiability of the sex balance system. Before doing this, one 

 more point should be made in relation to the present discussion. I 

 emphasized above that in male heterogamety no room is left for 

 genuine sex determiners in the Y-chromosome, whereas in female 

 heterogamety the Y(Z) can carry a female determiner, at least in 

 animals where predeterminative action of the Y(Z) in the immature 

 egg permits the production of intersexuality in the XX sex. There are 

 now available some interesting facts concerning sex determination in 

 the silkworm (Tazima, 1943 ff.), in which the female is XY(WZ), 

 which we have already presented (IV 3 B a). A brief repetition at 

 this point will serve as the basis for further discussion. Tazima found 

 that the female determiners are located in the Y-chromosome, as in 



