Abstract
Heterozygous effects of lethal genes on viability in Drosophila melanogaster were examined using 81 second chromosomes carrying newly arisen lethal genes and 49 second chromosomes carrying lethal genes extracted from a Raleigh, N. C. population. The same number of lethal-free control chromosomes were employed. The following findings were obtained: (1) No evidence of superiority of lethal heterozygotes over the control was obtained in either almost completely homozygous or in intra- and interpopulational heterozygous genetic backgrounds. (2) The average degrees of dominance of lethal genes (h) are probably negatively correlated with the viabilities of their genetic backgrounds. The h values were 0.0076-0.038 for newly arisen lethals and 0.0057 for lethals from the natural populations. (3) The newly arisen lethals that showed more deleterious effects in one genetic background seemed to be also more detrimental in all the other genetic backgrounds employed. (4) Significant synergistic interaction could not be observed when single individuals had two lethal genes. For the natural lethals, a clear linear relationship was observed between the average viabilities of heterozygotes and the numbers of lethal genes per individual, while in newly arisen lethals, a tendency of synergistic interaction was observed but not significantly so.
On the basis of the above findings and of the experimental results of Dobzhansky and Spassky (1968) and Oshima (1963), the behavior of lethal genes in natural populations was discussed.
