IN the late 1930s when Tracy Sonneborn was in graduate school and just starting his work on the genetics of protozoa, classical studies on genetics were almost finished. The second edition of Principles of Genetics by E. W. Sinnott and L. C. Dunn (1932) that appeared in that period was quite sophisticated, containing a good account of chromosome theory, segregation ratios produced by complex gene interactions, chromosome and genetic maps, polyploidy, aneuploidy, multiple factor inheritance, sex determination, and evolution. The authors argued strongly that virtually all inheritance was ascribable to nuclear genes. Only Correns's work on plastid inheritance was cited as an example of cytoplasmic inheritance, and it was pointed out that this should be considered only a minor exception to the general rule that chromosomal genes determine the hereditary characters of the organism. Other biologists, particularly cell physiologists such as L. V. Heilbrunn at the University of Pennsylvania, were not so sure. They thought that classical genetics dealt only with superficial characters and that the fundamental characters of organisms such as membrane permeability, metabolism, etc., were controlled by the cytoplasm. It is not clear what Sonneborn thought of this controversy, but from what I know about Sonneborn, he must, at least, have had an open mind about the matter. All classical genetics was based on multicellular organisms, in which characters were seen after a complex developmental process. Once Sonneborn had discovered mating types in the protozoa it became possible to carry out classical genetic studies with the protozoa and study their inheritance without an intervening period of somatic development. Perhaps the genetics of single-cell organisms, the protozoa, would prove to be a bit different from the classical picture.