Medical Genetics The level is usually substantially higher in people who carry one pku gene(even though they show no signs of disease than in individuals who are homozygous for the unmutated gene. Both parents must be heterozygous (i.e. must be carriers" of the trait to produce a child with pkU. the chance of their doing so is 1 in 4 卜 oNna homozygotes Heterozygotes Plasma phenylalanine levelMedical Genetics The level is usually substantially higher in people who carry one PKU gene (even though they show no signs of disease) than in individuals who are homozygous for the unmutated gene. Both parents must be heterozygous (i.e., must be "carriers" of the trait) to produce a child with PKU. The chance of their doing so is 1 in 4