Fig. 4. Heterothallic haploid cells retain their mating types as they go through rounds of mitosis. In contrast, homothallic cells frequently switch mating type. Thus the progeny of a single homothallic spore colony can mate with each other to form diploids, while those of a single heterothallic spore colony cannot. This figure was adapted from H. Roman. Development of yeast as an experimental organism. In The Molecular Biology of the Yeast Saccharomyces: Life Cycle and Inheritance. Pp. 1-9. Cold Spring Harbor Laboratory, 1981.