Different Genetic Possibilities within the Balance Theory 441 



pletely inapplicable to the well-established cases of intersexuality 

 through genie balance. I wish to raise the question whether we are not 

 faced here by one peculiar variant of the general balance scheme, 

 which means, in the presence of male heterogamet)', the same basic 

 mechanism, X-chromosomes : autosomes = F/M balance as in Dro- 

 sophila, with the additional unusual feature of specific behavior of the 

 Y-chromosome. In my opinion all the facts (omitting, for the moment, 

 the Y-chromosome) point to the presence of the usual balance mech- 

 anism between autosomal male determiners and X-chromosomal female 

 determiners. When this mechanism gets a chance to work, as in the 

 case of accumulation of X-chromosomal female determiners, it works 

 just as in other cases, producing male intersexuality (subandroecious 

 flowers). So far, no analysis of the opposite condition is available — 

 autosomal male determiners being conditioned to produce female in- 

 tersexuality (subandroecious flowers). But the fact of Strasburger's 

 Ustilago case and the perhaps rare occurrence of natural sub- 

 gynoecists show that this is a possible situation. Thus the only fact in 

 the Melandriwn case which, in my opinion, requires a new interpre- 

 tation is the overwhelming male action of the Y-chromosome. I con- 

 ceive of this as a specific modifying action upon the otherwise normal 

 balance mechanism, superimposed for unknown reasons in this species 

 upon the standard type of sex determination, and working in the same 

 way as known modifiers in Drosophila and Lymantria. (These will be 

 discussed in the next chapter.) If in Drosophila a mutation in the 

 Y-chromosome should appear, acting as a strong modifier for maleness, 

 the same situation would result; and such mutants in autosomes are 

 well known. 



These deliberations lead to further discussion of the Y-chromo- 

 some of Melandrium. Some interesting facts, not mentioned thus far 

 in the interest of clarity, are known about it. As in other X-Y pairs of 

 sex chromosomes, the Y-chromosome contains a segment which is 

 homologous to a segment of the X-chromosome, with which it pairs 

 and produces a chiasma. Actually, X-chromosomal loci are known 

 (Baur, Shull), also Y-chromosomal mutants (Winge), and crossing 

 over between Y and X loci ( Winge ) in the pairing segment. The rest, 

 the differentiating segment alone, contains whatever sex determiners 

 or modifiers there are. Westergaard (also Warmke before him) have 

 found Y-chromosomes in the polyploid strains in which parts of the 

 Y are missing: in Y^ a part of the left half is missing; in Y^, a part of 

 the right half. Y- probably contains only the right half plus a trans- 

 located piece from X. Plants with Y^ or Y- are always genuine her- 



