Studies on Homeostasis and Heterozygosity I. General Considerations. Abdominal Bristle Number in Second Chromosome Homozygotes of Drosophila melanogaster

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Abstract
Homeostasis is defined in this paper as the property of a genotype or collection of genotypes which allows it to respond adaptively to a wide variety of environments. The measure of homeostasis is, then, the average adaptive value over a range of environments. The variance of abdominal bristle number in a series of strains of D. melanogaster homozygous for semi-lethal and sub-vital gene complexes has been examined. It is shown that these homozygotes, which are less homeostatic than the heterozygotes constituted from them, show a lower variance in total bristle number than do the heterozygotes or F2 populations. On the other hand, both the heterozygotes and F2 populations show a higher correlation in bristle number between the two segments examined. It is suggested in conclusion that no one-to-one correspondence exists between phenotypic variance and homeostasis, although a higher phenotypic correlation between two parts of the same organism might more often than not be a concomitant of superior homeostatic adjustment.