Abstract
Female flies heterozygous for four kinds of recessive lethal genes on the second chromosome in homozygous genetic background showed high fecundity and the following results were ascertained by their persistence in the artificial populations.
1) Some lethals showed heterotic effect in homozygous genetic background.
2) Even these lethals which did not show high female fecundity in heterozygous background have been maintained at higher level than the theoretical one.
3) Decreasing patterns of each lethal heterozygote were similar in heterozygous and its irradiated genetic backgrounds.
4) The effect of irradiation gave rise to decrease the frequencies of lethal heterozygotes, but it was not clear in the case of heterozygous population.
However, the intimate relationship between female fecundity of lethal heterozygotes and equilibrium frequency of these lethal genes was presumed in the homozygous populations. So the fecundity was assumed to be an important factor in fitness and persisting mechanisms of lethals.
