George Beadle and Edward Tatum used mutant strains of Neurospora to formulate the one-gene-one-enzyme hypothesis. Researchers later updated this hypothesis to propose that one gene encodes one polypeptide because proteins can have multiple subunits. Consider a heterodimeric protein. How many genes are required to form this protein?

(A) two
(B) four
(C) six
(D) three
(E) one