A long-standing question in biology concerns the genetic mechanisms by which two sexes can evolve (botanists call this the dioecious condition and zoologists call it gonochory) from a functionally ancestral hermaphroditic state (without separate sexes). In 1932, H. J. Muller, one of the great 20th century geneticists but also a fine evolutionary biologist, pointed out that two mutations were necessary. It was therefore puzzling that sex determination often involves a single genetic locus. Muller believed that the evolution of a single-gene system was possible, because maize geneticists had synthesized a single-gene system with separate sexes. However, this system is highly artificial, requiring geneticists to actively eliminate the wild-type allele at one of the two genes involved. This genetic system cannot therefore explain the natural evolution of dioecy. In 1958, Westergaard reviewed studies from a diversity of flowering plants, and showed that the genetics of natural sex determination in plants does not support the maize system. Instead, the genetic results pointed to a model involving two separate factors, with close linkage creating a single genetic locus. Moreover, Westergaard also pointed out that a two-gene model offers a natural explanation for the evolution of suppressed recombination between sex chromosome pairs. Studying plants allowed genetic analyses of the early steps in the evolution of dioecy, using dioecious species that evolved recently from species without separate sexes, whereas Muller failed to fully understand such evolutionary changes because he focused on animals, where later changes have often happened and obscured the early stages.