A single gene can exhibit multiple phenotypic expression in an organism. Such a gene is called a pleiotropic gene. The underlying mechanism of the pleiotropy is the effect of a gene (and its alleles) on the metabolic pathways which contribute towards different phenotypes.

Some common example of pleiotropy is phenylketonuria, a disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This results into the phenotypic expression characterised by mental retardation and a reduction in hair and skin  pigmentation.