Problem
PKU is an autosomal recessive disease resulting from the inability of the body to process the chemical phenylalanine, which is contained in proteins that we eat. PKU manifests early in infancy and, if left untreated, generally leads to cognitive disabilities. A couple intends to have children but decide to consult a genetic counselor because one of them has a sister with PKU and the other has a brother with PKU. There are no other known cases in their families. You are their genetic counselor, what is the probability that their first child will have PKU?