Many ways. PKU is an inherited recessive disorder where an individual does not make the enzyme needed to metabolize phenylalanine, an amino acid in protein. Elevated levels in childhood cause developmental delays, but can be prevented with a special diet that should be continued lifetime to maintain mental abilities. During pregnancy, elevated levels of pku can affect fetus. See http://www.mayoclinic.org/diseases-conditions/phenylketonuria/basics/definition/con-20026275.