Write a short note on pleiotropy.

(N/A) There are instances where a single gene can exhibit multiple phenotypic expressions.
$\rightarrow$ Such a gene is called a pleiotropic gene.
- The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes.
$\rightarrow$ An example of this is the disease phenylketonuria,which occurs in humans.
- The disease is caused by a mutation in the gene that codes for the enzyme phenylalanine hydroxylase (single gene mutation).
$\Rightarrow$ This manifests itself through phenotypic expressions characterized by mental retardation and a reduction in hair and skin pigmentation.