Heterozygote detection in phenylketonuria

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Abstract
Phenylalanine loading was carried out on 105 patients of children with phenylalanine hydroxylase deficiency and 33 apparently normal individuals with no family history of phenylketonuria. The best discriminant was the logarithmic transformation of the slope of the rise in serum tyrosine multiplied by the maximum serum tyrosine concentration over the maximum serum phenylalanine concentration obtained after an oral load with a pure solution of L-phenylalanine. The overlap between heterozygotes for phenylketonuria and normal homozygotes was 2.4%. The distribution of the discriminant values suggested 3 heterozygous phenotypes for phenylalanine hydroxylase deficiency, and the phenotypic combination of parents could be correlated to the phenotype of their affected offspring, i.e., classical phenylketonuria, mild phenylketonuria or hyperphenylalaninemia. The probability of heterozygosity for phenylketonuria was determined by means of the distribution of the discriminant values of the heterozygotes and that of normal homozygotes. The likelihood of being a heterozygote was corrected for the genetic background of the person requiring genetic counseling, and was finally expressed as the percentage probability of being a heterozygote for phenylketonuria.

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