The theory that each gene is responsible for the synthesis of a single polypeptide. It was originally stated as the one gene–one enzyme hypothesis by the US geneticist George Beadle in 1945 but later modified when it was realized that genes also encoded nonenzyme proteins and individual polypeptide chains. It is now known that different polypeptides can be made from the same gene by alternative splicing of the RNA transcript, and that some genes code for various types of RNA involved in control of gene expression and protein synthesis.