<P> (Note: a and α yeast share the same mating response pathway, with the only difference being the type of receptor each mating type possesses . Thus the above description, given for a-type yeast stimulated with α - factor, works equally well for α - type yeast stimulated with a-factor) </P> <P> Wild type haploid yeast are capable of switching mating type between a and α . Consequently, even if a single haploid cell of a given mating type founds a colony of yeast, mating type switching will cause cells of both a and α mating types to be present in the population . Combined with the strong drive for haploid cells to mate with cells of the opposite mating type and form diploids, mating type switching and consequent mating will cause the majority of cells in a colony to be diploid, regardless of whether a haploid or diploid cell founded the colony . The vast majority of yeast strains studied in laboratories have been altered such that they cannot perform mating type switching (by deletion of the HO gene; see below); this allows the stable propagation of haploid yeast, as haploid cells of the a mating type will remain a cells (and α cells will remain α cells), and will not form diploids . </P> <P> Haploid yeast switch mating type by replacing the information present at the MAT locus . For example, an a cell will switch to an α cell by replacing the MATa allele with the MATα allele . This replacement of one allele of MAT for the other is possible because yeast cells carry an additional silenced copy of both the MATa and MATα alleles: the HML (Hidden MAT Left) locus typically carries a silenced copy of the MATα allele, and the HMR (Hidden MAT Right) locus typically carries a silenced copy of the MATa allele . The silent HML and HMR loci are often referred to as the silent mating cassettes, as the information present there is' read into' the active MAT locus . </P> <P> These additional copies of the mating type information do not interfere with the function of whatever allele is present at the MAT locus because they are not expressed, so a haploid cell with the MATa allele present at the active MAT locus is still an a cell, despite also having a (silenced) copy of the MATα allele present at HML . Only the allele present at the active MAT locus is transcribed, and thus only the allele present at MAT will influence cell behaviour . Hidden mating type loci are epigenetically silenced by SIR proteins, which form a heterochromatin scaffold that prevents transcription from the silent mating cassettes . </P>

Baker's yeast is an organism with 32 chromosomes