1941
Beadle and Tatum thought of the 'one gene, one enzyme' concept. They thought that there was a connection beween genes and enzymes and they were determined to provide experimental proof of it. They hypothesized that if there really was a one-to-one relationship between genes and specific enzymes, it should be possible to create genetic mutants that are unable to carry out specific enzymatic reactions. To test this theory, they exposed spores bread mold to X-rays or UV radiation and studied the resulting mutations. The mutant molds had a variety of special nutritional needs and could not without the addition of particular vitamins or amino acids to their food. While normal bread mold required only one vitamin (biotin), the mutants that were created also required thiamine or choline.
As Beadle and Tatum had predicted, they were able to create single gene mutations that blocked or repressed specific enzymes, so that the molds with these mutations required an external supply of the substance that the enzyme normally produced, and the substance that the enzyme normally used, piled up in the cell. These results led them to the one gene/one enzyme hypothesis, which states that each gene is responsible for directing the building of a single, specific enzyme.