In the budding yeast Saccharomyces cerevisiae, mating-type is determined by two non-homologous alleles at the mating-type locus. S. cerevisiae has the capability of undergoing mating-type switching, that is conversion of some haploid cells in a colony from one mating-type to the other. Mating-type switching can occur as frequently as once every generation.
Switching involves homologous recombinational repair of a site specific, programmed double-strand break, a highly organized process. This process replaces one mating type allelic DNA sequence with the sequence encoding the alternative mating-type allele. When two haploid cells of opposite mating type come into contact they can mate to form a diploid cell, a zygote, that may then undergo meiosis.