In 1941, George W. Beadle and Edward L. Tatum used a species of bread mold as a model organism to demonstrate that the "one gene, one enzyme" hypothesis was true. Their experiment was vital in providing a molecular basis for the action of genes.
Beadle and Tatum irradiated the mold to produce genetic mutations. The bread mold that they used for their experiment, Neurospora crassa, only has a single set of chromosomes. That simplified the process of seeing the genetic changes at work. Normally, Neurospora crassa only needs sugar, biotin and inorganic salts to grow, but some of the mutated strains needed arginine. Beadle and Tatum developed four strains that needed arginine, an amino acid that the unmutated strains could produce for themselves. This demonstrated that the radiation damage had affected a specific gene, which made an enzyme necessary for arginine production.