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Pleiotropy occurs when one gene will code and control the phenotype or expression of several different and unrelated traits. Phenylketonuria is an autosomal recessive disorder controlled by a mutant gene present on the 12th chromosome. This mutant gene fails to code for the enzyme phenylalanine hydroxylase required for normal metabolism of amino acid phenylalanine to tyrosine. Due to this, there is an accumulation of phenylalanine in body fluids such as blood, sweat, CSF. An abnormal breakdown product phenyl ketone is found in urine A higher level of phenylalanine and pheylketone cause severe brain damage leading to mental retardation. Thus phenylketonuria is the best example to explain pleiotropy.