The defective gene responsible for the disease PKU is found on chromosome __

A specific gene on the autosomal chromosome 12 is responsible for the synthesis of an enzyme phenylalanine hydroxylase (PAH) that converts amino acid phenylalanine to other important compounds in the body. Mutations on this gene, lowers the activity of PAH resulting in improper metabolism of phenylalanine. This in turn, leads to a situation wherein phenylalanine obtained from the diet is not processed properly, causing the accumulation of phenylalanine in blood. This affects nerve cells and causes brain damage, although severity may vary. If not treated early enough during childhood, this can cause severe mental retardation and other serious health issues.