THE LIMITS OF Till: SUPER-GENE 



males arc completely fertile and have the male-Hmited polymorphism 

 shown normally only by X Y. The male coloration, which had 

 been sex-linked as well as sex-limited, is now only sex-limited. 



The easy substitution of the Lehistes switch mechanism argues 

 a simple gene structure. The sex determiner is part of the super-gene, 

 but sex-determination is not finally bound up with it. Quite a 

 different situation meets us in a species with an ancient system of 

 sex differentiation such as we find in Drosophila. Three differences 

 are outstanding. In the first place, large differential segments in the 

 two sex-chromosomes fail to pair or of course to cross-over : and 

 one of these contains many major genes. Secondly, sex is determined 

 by a more completely and inelastically adjusted balance between 

 the X chromosome and the autosomes. Thirdly, new switch genes 

 such as have been found several times in the third chromosome 

 of D. melaiiooaster fail to achieve full success, for the new sex 

 genotype, the XX male, is always sterile. 



The advanced sex switch gene is therefore an adaptive complex, 

 a super-gene. And we know very well how this super-gene has 

 been sheltered and protected from recombination: it is by the 

 localization of crossing-over at meiosis in the heterozygous sex. 

 Such localization is indicated elsewhere. A difference in the 

 frequencies of crossing-over of the two sexes is characteristic of the 

 advanced systems of sex-determination. In some, to be sure, as in 

 the grasshoppers, it probably does not occur. But in them it would 

 in fact have no meaning, for their Y has completely disappeared. 

 The X chromosome is then in the final stage of its development. 

 So far as the heterozygous male is concerned, and so far as 

 sex-determination is concerned, it constitutes a single inviolable 

 super-gene. 



The Limits of the Super-Gene 



The next step in the consideration of the super-gene must be 

 Oenothera. Elsewhere super-genes make only one side of each 

 system: some individuals arc homozygous for them, and selection 

 can then cause readjustment of their components. In the complex 

 heterozygote of Oenothera, on the other hand, two complexes 

 develop. Each is a super-gene including large differential segments 

 of each chromosome and representing, in fact, almost the whole 



341 



