Facts S. cerevisiae haploid cells can be of one of two mating types, Roman a or Greek a .
Roman haploids secrete a pheromone that in Greek haploids stimulates processes that lead to mating.
Greek haploids secrete a pheromone that stimulates Roman haploids to mate with Greeks. Genes determining Greek or Roman status are alleles of the same locus , the MAT locus. Diploid cells resulting from the mating of Romans and Greeks do not make any pheromones or pheromone receptors. The two alleles of MAT are ideomorphs , meaning that they are unrelated sequences at identical loci. Cells of either mating type can with low frequency switch to the opposite mating type.