THE EFFECT OF HETEROZYGOSITY FOR NEW MUTATIONS ON VIABILITY IN Drosophila : A PRELIMINARY REPORT

Abstract

The relative viability of 2 groups of flies (D. melanogaster) were studied by a genetic technique. The control group was homozygous for a 2d chromosome (4 such chromosomes were tested) obtained from a large experimental population. The 2d group was comparable in all respects, except that 1 of the pair of 2d chromosomes had been exposed to 500 r X-radiation; that is, this group was heterozygous for one or another of a wide variety of newly induced mutations. The results presented in this preliminary report indicate that this new, virtually un-selected heterozygosity had an average heterotic effect. These data imply that the alleles found on typical chromosomes taken from a large interbreeding population are of such a nature that, when they are made homozygous, heterozygosity for a "random" mutation is more likely to improve the viability of the otherwise homozygous individual. If one thinks in terms of proportions of loci rather than of the total probability of heterotic or nonheterotlc changes at all loci, the results suggest that a majority of loci on these chromosomes are occupied by "deleterious" alleles. The only reasonable explanation for such a preponderance of "deleterious" alleles is that they owe their existence to selection within the population on the basis of their action in heterozygous individuals.