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Inbred diploid yeast strains heterozygous or homozygous for the rad18-2 allele and carrying markers for detection of mitotic recombination were constructed. The homozygous rad18-2/rad 18-2 strain was highly sensitive to killing by UV light, showed greatly elevated frequencies of spontaneous and induced mitotic recombination and was more sensitive to trimethoprim than the wild-type diploid. The heterozygous strain RAD18/rad 18-2 was intermediate in its response for these same phenotypic characters. These findings are discussed in the light of other studies in which incomplete dominance of genes involved in some aspect of DNA repair has been reported.
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