Abstract: Loss of heterozygosity (LOH) can be a driving force in the evolution of mitotic/somatic diploid cells, and cellular changes that increase the rate of LOH have been proposed to facilitate this process. In the yeast Saccharomyces cerevisiae, spontaneous LOH occurs by a number of mechanisms including chromosome loss, and reciprocal and non-reciprocal recombination. We performed a screen in diploid yeast to identify mutants with increased rates of LOH using the collection of homozygous deletion alleles of non-essential genes. Increased LOH was quantified at three loci (MET15, SAM2, MAT) on three different chromosomes, and the LOH events were analyzed as to whether they were reciprocal or non-reciprocal in nature. Non-reciprocal LOH was further characterized as chromosome loss or truncation, a local mutational event (gene conversion or point mutation) or break-induced replication (BIR). The 61 mutants identified could be divided into several groups, including ones that had locus specific effects. Mutations in genes involved in DNA replication and chromatin assembly led to LOH predominantly via reciprocal recombination. In contrast, non-reciprocal LOH events with increased chromosome loss largely resulted from mutations in genes implicated in kinetochore function, sister chromatid cohesion or relatively late steps of DNA recombination. Mutants of genes normally involved in early steps of DNA damage repair and signaling produced non-reciprocal LOH without an increased proportion of chromosome loss. Altogether, this study defines a genetic landscape for the basis of increased LOH and the processes by which it occurs.