Define the disorder of phenyketonuria.

Explanation

(i) It is caused by a recessive mutant allele on chromosome 12.
(jj) The affected individuals lack an enzyme that catalyses the conversion of the amino acid phenylalanine into tyrosine.
(iii) Consequently phenylalanie is metabolised into phenly pyruvate and other derivatives.
(iv) Accumulation of these chemicals in the brain results in mental retardation.
(v) These are also excreted in the urine as they are not absorbed by the kidney.