 To address the immediate chemical function of genes in the cell, i.e., their heterocatalytic properties, it was necessary to conceive of the phenotype itself in chemical terms. In 1908 Archibald Garrod (1857–1936; 1909) had found that some medical conditions that could be associated with “errors in metabolism” were inherited in a Mendelian fashion. Subsequently, physiological geneticists studied the relation of genes and catalytic activities. Conclusive evidence was achieved when George Beadle (1903–1989) and Edward Tatum (1909–1975) used the bread mold Neurospora crassa to search for mutants that affected known nutritional pathways by growing the fungi on selective media that provided or lacked certain substances. This approach established the field of biochemical genetics and stabilized the one gene—one enzyme hypothesis, i.e., the view that every gene was involved in the production of a specific enzyme, which had a specific effect on cellular metabolism (Kay 1993: Ch. 7; Kohler 1994: Ch. 7).