Mutation in this type of gene will simultaneously affect more than one trait. An example is phenylketonuria, a human disease caused by mutation(s) in a single gene that codes for the enzyme, phenylalanine hydroxylase. The disease is characterized by mental retardation and reduced hair and skin pigmentation.
Word origin: Greek pleio, meaning "many", and trepein, meaning "influencing".
Related forms: pleiotropic (adjective).