Heterozygosity, environmental variation and heterosis

Abstract

WHEN a wild strain of Drosophila melanogaster is inbred under optimum conditions, the phenotypic variance of body-size is reduced by not much more than 50 per cent1. This suggests that nearly half the variance of the original strain was due to the effects of uncontrollable environmental variations within the culture. But highly inbred lines tend to have a much higher variance than crosses between them, so that the environmental variance in such quantitative characters is not constant for all genotypes and tends to be smaller in heterozygotes than homozygotes. This is illustrated in Table 1 for six highly inbred lines and the crosses between them. The lines came from two unrelated wild stocks, Nettlebed and Edinburgh, and include two lines selected for long wings (LN and LE), two selected for short wings (SN and SE) and two unselected lines (UN and UE). Wing-length is closely correlated with body-size, and variance is expressed as squared coefficient of variation, since this measure eliminates most of the effect of differences in absolute size. © 1952 Nature Publishing Group.