Classic example of pleiotropy is:

When a single gene can exhibit multiple phenotypic expression. Such a gene is called a pleiotropic gene. An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.