## The environment allows the signs to manifest

One example that demonstrates how the environment can influence the resulting phenotype is the disease phenylketonuria (PKU). In affected individuals (recessive homozygotes for the mutated allele), toxic compounds accumulate in the brain and consequently damage its development, leading to mental retardation. PKU is a recessive disorder of amino acid metabolism, resulting in the inability to convert phenylalanine to tyrosine. Phenylalanine, which is commonly found in our diet, is not toxic in itself, but it is metabolized to other compounds that are toxic. Infants with this disease who are fed a normal diet thus ingest phenylalanine, and this causes the development of symptoms of the disease. Infants diagnosed with PKU who are given a diet low in phenylalanine usually develop without severe mental impairment. Screening for PKU, the frequency of which ranges from 1:4,500 in Ireland to 1:100,000 in Finland, is therefore done immediately after birth, and early implementation of a modified diet makes it possible to alleviate the clinical manifestations of this disease. This example illustrates how the influence of an external factor, in this case diet, can affect a change in phenotype.