SUPER-GENES 



The Sex-Ratio super-gene is again scx-liniited : in females it is 

 submerged, like the super-gene of Lebistes. Males carrying it in their 

 X chromosomes have an abnormal meiosis. Their X and Y chromo- 

 somes suffer from a nucleic acid surfeit and one which is greater 

 at a lower temperature. In consequence of the surfeit they fail to 

 pair. The X chromosome reproduces twice. The Y chromosome, 

 which, as we know, is still more largely heterochromatic than the X, 

 is so heavily flooded with nucleic acid that it fails to divide at all 



Fig. 84. — The X chromosomes in the saHvary glands of a female Drosopliila azteca 

 heterozygous for the sex-ratio super-gene, to show the three inversions associated 

 with sex-ratio in this species (after Dobzhansky, 1939). 



and is lost. This course of events results in almost all sperm 

 containing an X chromosome. The progeny of Sex-Ratio males consist 

 of 94 per cent of females at 25° C. and 99 per cent at 16-5° C. 

 Owing to the lower efficiency of the gene at higher temperatures, 

 it is not surprising to fmd that it exists in a higher proportion of 

 the population at the warmer end of its habitat. The supergene, 

 as it floats in the population, is evidently working to modify the 

 sex-ratio as determined by simple X — Y segregation in such a way 

 as to economize in the reproduction of the species. No doubt its 

 frequency is raised above the optimum for this effect by the general 

 advantage of hybridity in respect of an inverted segment. 



Elements of Genetics 



337 



