What is pleiotropy?

Pleiotropy refers to the condition where a single mutation causes more than one observable phenotypic effect or change in characteristic. For example, in the human genetic disease, phenylketonuria (PKU), a single simple recessively inherited mutation inactivates the enzyme para hydroxylase which converts phenylalanine to tyrosine. This results (in homozygous mutant individuals) in excessive amounts of phenylalanine, a deficiency of tyrosine, and an excess of phenyl pyruvic acid which is an alternate degradation product of phenylalanine. These effects can result in mental retardation and also abnormally light hair and skin colour. Such a combination of phenotypic effects of a genetic defect is called a syndrome.