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It may seem that monogenic traits are never influenced by the environment, so P = G. This is not always the case. An example of a gene that is influenced by the environment is the gene that causes phenylketonuria or PKU in humans. It is a rare recessively inherited metabolic disorder. Only if two carriers have a child, that child can have PKU. In the Netherlands 1 in 18,000 children has it, but all are tested for it as newborn baby in the blood collected with the heel pinch. In people with PKU the enzym Phenylalaninehydroxylases is not present or not functioning, so that it cannot break down the amino acid Phenylalanine. This is thus accumulating in the blood and the spinal fluid, which causes damage to nerve cells, eventually resulting in brain damage. Patients with untreated PKU are usually mentally retarded with behaviour problems, and often suffer from skin diseases. The treatment is very basic: a life-long diet with very little protein and no intake of aspartaam as that contains phenylalanine. Patients do get amino-acid additives to prevent deficiencies. Thus, the expression of this monogenic traits can be influenced by the environment, by the diet in this example.