If two individuals mate who are both heterozygous for the allele associated with Phenylketonuria (PKU), their offspring have a 25% chance of expressing the PKU phenotype. In a Punnett square where N represents the normal allele and p represents the recessive PKU allele, a cross between two heterozygous parents (Np) results in a 1 in 4 chance for the offspring to inherit the homozygous recessive (pp) combination.