The segregation of mitochondrial genes in yeast

A large number of spontaneous, cytoplasmic petite mutants from six grande strains of Saccharomyces cerevisiae were crossed to a pair of isogenic tester strains. Suppressivity values were obtained by randomly sampling the diploid progeny from these crosses, and on this basis, crosses were broadly categorized as having high, intermediate, or low suppressivity. For each cross, individual zygotes were obtained also. All successive first-generation buds were isolated from the zygotes, and analyzed for the presence of petite genotypes. We found that, though early buds may be mixed, all zygotes eventually produce a succession of buds which have the same genotype—either all petite or all grande. Many more zygotes from crosses in all categories of suppressivity purified to petite than expected from the population values for suppressivity. Reconstruction experiments indicate that most petite mutants may actually generate over 90% petite progeny in a petite x grande cross.