Mendel recognized the impossibility of a pangenesis-like model to explain his experimental observations as well as those of earlier plant hybridizers. They saw that recessive traits could be carried unchanged through several generations, that the traits could reappear by the F2 generation, and that the recessive homozygotes extracted from such crosses could form pure breeding stocks indistinguishable from the original par ental strains. Mendel’s explanation, with genes present in pairs that segregate during gamete formation, is the basis of the present science of genetics. The coupling of the idea of single- gene inheritance with differences for individual traits was the breakthrough that unified biology in its disciplines ranging from evolution to physiological function.