Phenylketonuria is an example for 

A single gene exhibiting multiple phenotypic expression is called as pleiotropy. A disease known as phenylketonuria is caused due to mutation in the gene that codes for the enzyme phenyl alanine hydroxylase. If the gene is a recessive allele, and in homozygous condition, due to absence of this enzyme there is no formation of tyrosine from phenyl alanine and phenyl alanine is accumulated. It results in mental retardation due to affect on central nervous system, reduction in hair and skin pigmentation due to non-synthesis of melanin. The adverse affect is based on amount of phenylpyruvic acid synthesis from phenylalanine.