If an organism carries a harmful genetic mutation, it can still survive and sometimes even function normally if something in the organism’s environment can compensate for the genetic defect. In this situation, the organism may not appear mutant at all (in other words, though its genotype is mutant, its phenotype is normal – click here for more definitions). Sometimes missing substances normally made by the product of the mutant gene can be provided, or substances normally removed by the product of the mutant gene can be limited in the diet.

The genetic disease phenylketonuria, or PKU, is an example of a disease whose negative effects can be lessened by changing the diet of the individual with the mutation. In a normal person, the PKU gene product changes the amino acid phenylalanine into another chemical. People with PKUmutations do not make this product and are therefore unable to remove phenylalanine from the body. As a result, phenylalanine builds up in the body. If the condition is untreated, severe mental retardation results.

If the indivdual is identified as mutant for PKU at birth, simple dietary changes can help to overcome the effects of the mutation. Because the cause of this genetic defect is now understood, foods containing phenylalanine are labelled to help people with PKU avoid eating them and endangering their health.

To identify people mutant for PKU early enough to help them avoid the serious effects of the mutaion, all newborn babies are tested for PKU deficiency using a simple blood test.