The distributions of viabilities manifested by individual homozygous second chromosomes isolated from the Suyama-Juriki populations of
D. melanogaster was examined. The viability of heterozygous flies for single lethal and double lethal chromosomes was estimated and compared with that of normal heterozygotes. Complementary effect was recognized in viability of double lethal trans- or cis-heterozygotes either with coadapted genetic background or without it. This effect is assumed to be due to interaction between two loci carrying lethal genes. The mean viability of semilethal heterozygotes was estimated to be similar to that of the normal heterozygotes. The detrimental effect of two semi-lethal trans-heterozygotes is emphasized. The difference between lethal and semi-lethal genes is discussed.
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