phenylketonuria
Some foods contain a building block called phenylalanine, an amino acid the body normally breaks down with a specific enzyme. In phenylketonuria that enzyme is missing or weak, so phenylalanine piles up like an unsorted package that never leaves the warehouse. Very high levels are toxic to the developing brain, which is why untreated PKU can cause intellectual disability.
PKU is autosomal recessive, usually caused by mutations in the PAH gene that encodes the enzyme phenylalanine hydroxylase. A child must inherit a faulty copy from each parent to be affected. The crucial point is that the harm is largely preventable: with a special diet low in phenylalanine started in infancy, affected children can develop normally.
This is why PKU was one of the first conditions added to routine newborn screening, where a heel-prick blood spot is tested days after birth. PKU is a landmark example of how knowing the genotype lets us change the environment (here, the diet) to prevent the harmful phenotype. This entry is educational, not medical advice.
A newborn screening blood spot shows a high phenylalanine level; the diagnosis is confirmed, a low-phenylalanine formula is started at once, and the child grows up with typical development.
Early screening turns a once-disabling disorder into a manageable one.
PKU is the classic case where the phenotype depends on both genes and environment: the same genotype leads to severe disability on an ordinary diet but to normal development on a controlled one.