In November 1941 American geneticists at Stanford University George W. Beadle and Edward L. Tatum published the results of their experiments with the bread mold Neurospora crassa. They concluded that ultraviolet light treatment somehow caused a mutation in a gene that controls the synthesis of an enzyme involved in the synthesis of the essential nutrient. They also showed that the defect is inherited in typical Mendelian fashion.
"...Beadle and Tatum first irradiated a large number of Neurospora, and thereby produced some organisms with mutant genes. They then crossed these potential mutants with non-irradiated Neurospora.
"Normal products of this sexual recombination could multiply in a simple growth medium. However, Beadle and Tatum showed that some of the mutant spores would not replicate without addition of a specific amino acid—arginine. They developed four strains of arginine-dependent Neurospora—each of which, they showed, had lost use of a specific gene that ordinarily facilitates one particular enzyme necessary to the production of arginine" (http://www.genomenewsnetwork.org/resources/timeline/1941_Beadle_Tatum.php, accessed 12-22-2013).
Beadle and Tatum's experiments are often considered the first significant result in what came to be called molecular biology. In 1948 their collaborator at Caltech, Norman Horowitz, characterized their results as the "one gene- one enzyme hypothesis." Although the concept was extremely influential, the hypothesis was recognized as an oversimplification soon after its proposal. More accurately, it was later understood that each gene specifies the production of a single polypeptide— a protein or protein component. Two or more genes may contribute to the synthesis of a particular enzyme, and some products of genes are not enzymes per se, but structural proteins.
Beadle & Tatum, "Genetic Control of Biochemical Reactions in Neurospora," Proceedings National Academy of Sciences 27 (1941) 499-506.
J. Norman, Morton's Medical Bibliography 5th ed (1991) no. 254.3.