Tracy Sonneborn's breakthrough research on mating types in Paramecium allowed geneticists to conduct cross-breeding analyses of unicellular organisms. Before Sonneborn identified the mating types, researchers knew that the protozoa reproduced both asexually, through budding, and sexually, through conjugation, but could not control the reproductive path taken by the organism. Sonneborn's findings added protozoa to the growing number of model organisms available to genetics researchers, including maize, Drosophila, Chlamydomonas, and Neurospora. Paramecium's complex sexual reproduction cycle, involving both a macronucleus, two macronuclei, and cytoplasmic factors, led Sonneborn to argue for the importance of cytoplasmic and non-nuclear inheritance. This placed him in opposition to many of the genetic leaders of the day, especially those trained by American T. H. Morgan (1866-1945) at Columbia University, who believed that all hereditary information was stored in the chromosomes of the nucleus. Sonneborn's first major piece of evidence for cytoplasmic inheritance was the Kappa...