FIGURE 6-5 Segregation of alleles in yeast. Haploid Saccharomyces cells of opposite mating type (i.e., one of mating type a and one of mating type α) can mate to produce an a/α diploid. If one haploid carries a dominant wild-type allele and the other carries a recessive mutant allele of the same gene, the resulting heterozygous diploid will express the dominant trait. Under certain conditions, a diploid cell will form a tetrad of four haploid spores. Two of the spores in the tetrad will express the recessive trait and two will express the dominant trait.