Genetic analysis of spontaneous half-sectored colonies of a diploid strain homozygous for ade2 and heterozygous for linked markers aro1A, hom2, ade8 and trp4 was undertaken. Some of these were due to an unexpectedly high rate of ade2 reversion and others to a low incidence of haploid sectors. About 50% resulted from mitotic recombination. Only a few were due to mitotic crossing over alone, the majority resulted from non-reciprocal ‘conversion’, including recombination extending over two loosely-linked loci, and some colonies featured both mitotic crossing over and mitotic conversion. The latter indicate that negative interference operates for mitotic recombination. The rate of mitotic recombination for ade8, calculated from the frequency of half-sectored colonies was 3·1 × 10−4 per cell division.
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