Guide to PKU - Mead Johnson Nutrition

of the disorder, which is shownas “Pp” in Figure 1. A carrier hasone working gene (P) for producingthe enzyme phenylalaninehydroxylase and one non-workinggene (p) which does not producethe enzyme, but no symptomsof PKU.In every pregnancy, the eggand the sperm combine to makea fetus. One of the mother’s genesfor a trait (P or p in this case) fromthe egg and one of the father’sgenes (P or p) from the sperm goto make the fetus. The inheritanceof genes is a completely randomprocess, and if the baby inheritstwo p genes, he will be born withPKU. This child (pp, in geneticterms) requires nutritional managementto keep excess phenylalaninefrom being toxic to his body.As shown in Figure 1, whentwo carriers for the non-workinggene have children together, thereis a one in four chance (25%) thatthey will have a child with PKU.Likewise, there is a 75% chancethey will have a baby who doesnot have PKU; this child may bea carrier (Pp) like the parentsor may have two working (PP)genes. These odds don’t meanthat a family of four childrenwill have one child with PKU,because the chances are thesame with every pregnancy.PpNon-AffectedCarrierNon-AffectedCarrierWith every child, the odds in thisexample are one in four of havingPKU.Because PKU is hereditary,everyone in the families of bothparents should know about thepresence of PKU. This will helpmake certain that all babies borninto the family are screened forPKU at birth. Families should talkwith genetic counselors for themost up-to-date information onthe genetic aspects of PKU.How Recessive Inheritance WorksppAffectedChildPpP=Working Genep= PKU GenePpPpNon-AffectedCarrierNon-AffectedCarrierPPNon-AffectedFigure 1: Two PKU (pp) genes areneeded for a child to be affected.Each child has a 25% chance ofbeing affected. If one parent is affectedby PKU, then each child has a 50%chance of being affected.6