The mother-daughter mating type switching asymmetry of budding yeast is not conferred by the segregation of parental HO gene DNA strands.

The Saccharomyces cerevisiae HO gene, which encodes a site-specific endonuclease, is transcribed in the parent (mother) cell but not in the daughter (bud) cell. Consequently mother cells can switch their mating type whereas bud cells cannot. Whether or not the different capabilities of these cells are due to a nonrandom segregation of parental HO gene DNA strands to progeny cells is tested here by assaying the pattern of switching in cells where the HO gene is inverted in the chromosome. In an inverted HO strain, as is the case with the normal HO arrangement strains, only the mother cells were found to switch. Thus, the possibility of asymmetric segregation of parental HO gene DNA strands does not regulate the mother-daughter asymmetry of switching.