Gene Concept

The one gene-one enzyme hypothesis was given by the geneticists George W. Beadle and E. L. Tatum in 1941.
They worked on the pink bread mould Neurospora crassa.
They exposed it to X-rays to produce nutritional mutants (auxotrophs), which could grow only when a particular nutrient was added to the minimal medium.
Each mutant was found to lack one specific enzyme of a biosynthetic pathway, owing to a mutation in a single gene.
They concluded that each gene in an organism controls the production of a specific enzyme, and it is these enzymes that catalyse the reactions leading to the phenotype.
Diplococcus pneumoniae was used by Griffith for transformation, and Escherichia coli for the lac operon and for the Meselson-Stahl experiment.
So the relationship was first established in Neurospora crassa.

George W. Beadle and Edward L. Tatum postulated the one gene-one enzyme hypothesis in 1941.
They worked on the pink bread mould Neurospora crassa and produced nutritional mutants with X-rays.
Each mutant was unable to make one particular enzyme of a biosynthetic pathway, owing to a mutation in a single gene.
They concluded that each gene controls the synthesis of one specific enzyme.
A. Garrod had earlier studied inborn errors of metabolism such as alkaptonuria, but did not postulate this hypothesis.
Rosalind Franklin produced the X-ray diffraction pictures of DNA, and Hershey and Chase proved that DNA is the genetic material of bacteriophages.
So the hypothesis was postulated by Beadle and Tatum.