None.Because if you said that it is caused by a recessive allele and parent1 has Phenylketonuria then his Genotype must be "ll"(Both are recessive alleles),and if you say that parent2 does not have the Phenylketonuria alleles then her Genotype must be "LL"(Both are dominant alleles because she does not have the PKU alleles which are recessive alleles). Therefore if you use the Punnet square, the Filial generation one will have "Ll" genotypes and the dominant allele ("L") will mask the recessive allele or the PKU allele which will prevent the child from having Phenylketonuria.
Sorry for the long explanation.Hope this helps.