 How do differences in genes bring about differences in phenotypic traits? The modest answer given above to the question ‘What do genes do?’ provides the basis for explaining how differences in genes can bring about differences in phenotypic traits. A difference in the nucleotide sequence of a gene will result in the difference in the nucleotide sequence of RNA molecules, which in turn can result in a difference in the amino acid sequence of a polypeptide. Differences in the linear sequences of amino acids in polypeptides (and in the linear sequences of nucleotides in functional RNA molecules) can affect the roles they play in the cell and organism, sometimes having an effect that is observable as a phenotypic difference. The mutations (differences in genes) identified by the Morgan group (e.g., the purple mutation) have been routinely identified as differences in nucleotide sequences in DNA.