The one gene-one enzyme hypothesis
(one gene one polypeptide synthesis is a better name since genes do not always code for enzymes but also structural proteins such as keratin of hair and collagen in our skin)
It is the idea that genes act through the production of enzymes, with each gene responsible for producing a single enzyme that in turn affects a single step in a metabolic pathway.

BEEDLE AND TATUM chose the fungus NEUROSPORA for their experiments.
Neurospora crassa had several advantages:

1.  it required a simple growth medium,
2.  it grew quickly,
3.  because of the production of ascospores during reproduction it was easy to isolate genetic mutants for analysis. 

EXPOSURE AND IDENTIFICATION OF MUTANT STRAINS:
They produced mutations by exposing the fungus to X-rays and then identified strains that had metabolic defects by growing them in minimal medium.

They had created single gene mutations that incapacitated specific enzymes and the mutant strains could be grown by adding the necessary and (formerly non-essential) nutrient to the minimal medium.

ARGININE DEFICIENCY:

In some cases it was the amino acid, Arginine which had to be added in order for the spores to grow.
Arginine is produced by Neurospora in a metabolic pathway controlled by enzymes where the product of one reaction serves as the substrate or precursor for the next.

ACETYLCHOLINE----ORNITHINE----CITRULINE----ARGINOSUCCININE-----ARGININE
                           
The following are the three mutant strains that are blocked at some stage in the above metabolic pathway.

• MUTANT 1:would grow if Ornithine,Citruline or Arginine were added to it showing that the enzyme that produced Ornithine was missing.

• MUTANT 2:would grow if C or A were added showing that the pathway was blocked due to enzyme B that produced Citruline.

• MUTANT 3:would grow if A was added to the minimal medium thus in this case enzyme D was absent and could not contribute to the formation of A.

 CONCLUSION:
They concluded that the irradiation had altered the genes in such a way that one precursor could not convert into another.
Since the addition of one enzyme seemed to be required for the production of Arginine in each mutant Beedle and Tatum argued that the each gene was producing an enzyme.