The Beadle & Tatum Experiment:
The nature of heterotroph mutants

    Beadle and Tatum grew spores from each of the Neurospora heterotroph strains on a series of minimal media, each supplemented with a different amino acid. Each of the heterotroph strains had a requirement for a specific amino acid. They inferred that a separate mutation in each strain affected synthesis of that amino acid. In this example, supplementing the minimal media with tyrosine permits growth of the heterotrophic mutant: the inference is that a gene mutation has affected the ability to synthesize tyrosine, and this strain is called a tyr^- mutant.