 Environment plays a major role in effects of the human genetic disease phenylketonuria . The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine , causing a toxic build @-@ up of an intermediate molecule that , in turn , causes severe symptoms of progressive mental retardation and seizures . However , if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid , they remain normal and healthy . 
