The rad59-Y92A mutation has a dominant, hyper-rec effect on hetero-allelic recombination in diploid strains. (A) The spontaneous hetero-allelic recombination system: Diploid strains possessing a sam2-∆EcoR V-HOcs allele at the SAM2 locus on one copy of chromosome IV, a sam2-∆SalI allele on the other copy, and a sam1::LEU2 allele at the SAM1 locus on both copies of chromosome XII (not pictured) were grown to saturation in YPD medium supplemented with AdoMet, and plated onto medium lacking AdoMet to select for cells in which a recombination event generates a functional SAM2 gene and an AdoMet prototrophic cell. Either reciprocal or non-reciprocal recombination events between sam2-∆EcoR V-HOcs and sam2-∆Sal I can generate AdoMet+ recombinants. The sam1::LEU2 allele is missing sufficient information to recombine with the sam2 alleles. The white bars indicate the positions of the sam2 mutations. (B) Rates of heteroallelic recombination in wild-type, heterozygous and homozygous mutant strains. Rates were determined from a minimum of 10 independent cultures as described in the Methods. Fold decreases (−) and increases (+) from wild-type indicated in boxes.