Viability of heterozygotes for induced mutations in Drosophila melanogaster. II. Mean effects in irradiated autosomes.

purpose of these studies was to obtain an estimate of the effect of an unTye?ected sample of irradiated chromosomes on viability. The viability of the heterozygotes for irradiated chromosomes was examined in relation to the viability of the homozygotes and the effects on the population were estimated. In the first paper of this series ( FALK, RAHAT and BEN-ZEEV 1965), we studied the effects of irradiation on the X chromosome. It was concluded that a dose of about 3000r induced, besides lethal mutations, many semilethals and subvitals. These effects had a dominance of 5 to 15%. In addition to mutations that could be detected directly, irradiation induced less severe mutations which reduced the mean viability in the normal range by about e%, both in hemizygotes and in heterozygotes for irradiated chromosomes. Although this effect was not statistically significant, it indicated that the majority of the induced mutations (even those with slight detrimental effects) were deleterious to the heterozygotes. There were also indications that the less deleterious mutations penetrate in heterozygotes to a greater extent than the more deleterious ones. If there were “heterotic” mutations, or even “heterotic” combinations, that might have been deleterious in other combinations, they were in the minority in the sample of unselected new mutations. The X-ray dose of 3000r used by us induced many aberrations. Experiments with the X chromosome may not be representative, owing to the possible specific reactions of this chromosome. There were also some technical difficulties in those experiments, which might have biased the results. It was decided to extend these studies to the second chromosome of Drosophila and to study the effects of irradiated chromosomes on viability under diff went environmental conditions. The present paper presents the first results of these experiments and gives the effects on the mean viability of a random sample of irradiated autosomes in a variety of environments.