Neurospora

Neurospora crassa is a mold which in its wild state grows on sugar and a variety of inorganic compounds. Bombarding this mold with x-rays causes it to mutate and sometimes lose the ability to form specific organic compounds. Beadle concluded that the characteristic function of the gene was to supervise the formation of a particular enzyme (one gene to each enzyme).
 George Wells Beadle and Edward Lawrie Tatum United States 1941 Neurospora

Neurospora crassa is a mold which in its wild state grows on sugar and a variety of inorganic compounds. Bombarding this mold with x-rays causes it to mutate and sometimes lose the ability to form specific organic compounds. Beadle concluded that the characteristic function of the gene was to supervise the formation of a particular enzyme (one gene to each enzyme).