Four rec genes, responsible for a deficient mitotic conversion in yeast, have been studied for their effects on mitotic crossing over and in meiotic intragenic and intergenic exchanges. Mutations in rec1 do not affect either mitotic segregation of ade2 or meiotic intragenic or intergenic recombination. Homozygous rec2/rec2 diploids yield 1-2% sporulation and ten percent spore viability. They also exhibit normal frequencies of x-ray induced mitotic crossing over of ade2. Mutations in rec3 suppress sporulation, but do not affect mitotic segregation of cyh2. Diploids homozygous for rec4 show normal frequencies for x-ray induced mitotic crossing over in ade8. Such diploids exhibit 60% sporulation and 90% spore viability. Intragenic meiotic recombination between alleles 2 and 17 in arg4 is reduced about 500 times. Meiotic crossing over appears normal for the region between mating type and ade8 and their respective centromers and also in the intervals between thr1 and CUP1 and the centromere of chromosome VIII. Common processes for mitosis and meiosis are suggested by the existence of mutants in rec2, rec3, and rec4. Mutations in rec4 suggest common steps for single site conversions in mitosis and meiosis.

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