In 1941, American scientists George Beadle and Edward Lawrie Tatum proposed the one gene-one enzyme theory. The four main tenets of this theory (as modified by Tatum in 1959) were:

• All biochemical processes in all living organisms are under genetic control.
• All biochemical reactions in an organism are resolvable into separate steps.
• Each step or reaction is under the control of a single gene.
• Mutation of a single gene results in the loss of function of the appropriate enzyme.

In other words, each gene controls the reproduction, function, and specificity of a particular enzyme.

The theory was based on results originally obtained from Neurospora crassa, a fungus that was grown in a medium containing only the bare minimum of nutrients necessary (the fungus being capable of manufacturing the rest). After inducing mutations in the mold using radiation, some of the progeny were unable to grow on...