Human Genetic Problems Showing Recessive Inheritance 1. PKU: PKU is short for phenylketonuria, a recessive disorder that results from a mutation in the gene for an enzyme that breaks down phenylalanine, an amino acid. Newborns are tested for this disorder at birth, and if they have PKU they will start a diet that restricts their phenylalanine intake, and allowing them to develop normally. If one parent has PKU and the other parent is a carrier (heterozygous, Nn) for the disorder, predict the risk of their children inheriting the disorder. Genotypes of parents: Genotypes and ratios in offspring: Punnett Square: Phenotypes and ratios in offspring: