The genetical activity of heterochromatin

Abstract

Heterochromatin, which occurs as supernumerary chromosomes as well as intercalated with euchromatin of regular chromosomes, is not genetically inert although it is not essential to normal growth and survival. While heterochromatin lacks the critical effects of major genes, it possesses polygenic properties which lead to quantitative variations in characters, subject in part to environmental modifications. The Y chromosome of D. melanogaster is heterochromatic and largely polygenic. 12 special stocks were prepared for analysis of polygenic activity in variations of the number of sternopleural chaetae. Analysis of variance of the chaetae counts on interaction of 6 Y chromosomes indicated that the gross effect of one was small, and signified the presence of + and [long dash]genes in balanced proportions. Recombination between X and Y, also demonstrated, reveals the presence of heterochromatic material in X corresponding to Y. X and Y show genetical variation of the same order of magnitude, but are non-additive. Both major and polygenes appear to be essential to optimum adaptation of the individual to the environment.