Figure 2.

Inter-homolog allelic mitotic recombination leading to loss-of-heterozygosity (LOH) of a counter-selectable marker. The sequence of panels (from top to bottom) illustrates the steps of a DSB repair event by HR. Other possible scenarios such as gene conversion and break induced replication are not shown. A pair of duplicated homologous chromosomes represent the S288c/YJM789 hybrid background; S288c in red, YJM789 in blue. Black circles represent centromeres, a “U” labeled yellow circle represents the counter-selectable URA3 marker which has been inserted at a distal region of the chromosome. One of the chromatids in the S288c homolog sustains a DSB. Repair of the DSB occurs through HR using as template the allelic position from a chromatid in the YJM789 homolog. The recombination intermediate is resolved as a crossover resulting in a reciprocal exchange of genetic information. In the subsequent mitosis, two possible outcomes of segregation of the parental and recombinant chromatids produce genetically distinct sets of daughter cells. Left: one daughter cell receives two parental chromatids while the other receives two recombinant chromatids. Both daughter cells remain heterozygous for the URA3 marker insertion and are thus 5-FOA sensitive, although HetSNP phasing is altered in one of them. Right: each daughter cell receives one parental and one recombinant chromatid. One cell becomes homozygous for the S288c sequences at positions distal to the crossover site, including the URA3 marker insertion and remains sensitive to 5-FOA. The other daughter cell (bold) becomes homozygous for the YJM789 sequences, thus losing the URA3 marker and becoming resistant to 5-FOA.