Phenylketonuria (PKU) is an autosomal recessive genetic disease. The mutation occurs in the gene for phenylalanine hydroxylase, an enzyme that catabolizes (breaks down) the amino acid phenylalanine. Mutations in the phenylalanine hydroxylase gene lead to a phenylalanine hydroxylase enzyme that does not function normally. In this case the concentration of phenylalanine rises, since it cannot be degraded. For reasons that are not understood the increase in phenylalanine concentration leads to severe mental retardation and autistic features in people with the disease. Newborn infants are normal, but their condition deteriorates if they have a diet with normal protein levels, since the protein contains phenylalanine.
Suppose a man who has PKU and a woman who is homozygous for the normal allele of
the phenylalanine hydroxylase gene have children. What are the expected genotypes
of their children? What are the ratios of the genotypes? What are the expected
phenotypes and their ratios?
Phenylketonuria can be diagnosed in newborn infants by a blood test. When PKU is present the infants are put on a low protein diet, so that they ingest very little phenylalanine. These people have intelligence in the normal range. In California and most other states all newborns are screened for PKU.
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