LOH events in S. cerevisiae generating half sectors can be further classified as either reciprocal (LOH occurs in both cells) or nonreciprocal (one cell undergoes LOH while the other remains heterozygous) (see Figure 2A and reviewed in [5]). This classification has facilitated a better mechanistic understanding of LOH events. For instance, in a previous examination of mutations that increase LOH rates in young cells, we found that the ratio of reciprocal to non-reciprocal LOH events reflected the type of defect [17]: Mutations that affect specific DNA repair pathways bias LOH events away from the wild type ratio. By contrast, mutations that increase LOH rates but do not alter the ratio were consistent with a general increase in DNA damage that did not alter normal repair pathways. To examine reciprocal and non-reciprocal events in aging cells, we used the diploid MEP strain UCC5185 carrying ADE2 and MET15 in opposition at the MET15 locus. By examining the color of both half sectors, it is possible to distinguish reciprocal from non-reciprocal events (Figure 2A).
