Beadle and Tatum studied the mould on bread and came up with the 'One gene One Protien' theory. They used x-rays to create a mutation in bread mould (Neurospora), which affected particular genes, so certain enzymes weren't being produced. They thought that this meant that each gene codes for a particular protien. But they didn't know that the protein they were studying only had one polypeptide.
It was later changed to 'One gene One polypeptide' as it was found more than one polypeptide can make up a protien and that each gene coded for a particular protien.