A couple is planning to have a child. Both prospective parents are healthy and show no symptoms of any genetic disorders. However, they both have a sibling who was diagnosed at birth with a condition caused by a recessive gene. This condition prevents the body from properly processing a specific amino acid found in many foods, and if left untreated by a strict diet, it leads to a toxic buildup that can cause severe intellectual disability. Based on this information, what is the probability that their child will inherit the condition, and what is the underlying biological reason for the symptoms?

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Phenylketonuria (PKU) is an inherited metabolic disorder caused by a recessive gene, meaning a child must inherit a copy of the mutated allele from each parent to develop the condition. Individuals with PKU lack the enzyme needed to metabolize the amino acid phenylalanine. If the condition is not treated through a regulated diet, the resulting buildup of phenylalanine in the brain becomes toxic, leading to severe consequences such as significant deficits in cognitive function, intellectual disability, seizures, and an increased risk of various psychiatric disorders.

Phenylketonuria (PKU)

Genetic Counseling Scenario

Phenylketonuria (PKU) is a genetic disorder that leads to a reduction in myelin and causes abnormalities in the white matter of cortical and subcortical brain structures. Because the myelin sheath acts as an essential insulator for axons, its loss severely impairs normal neural operations. Consequently, this myelin reduction is associated with significant neurological issues, including severe cognitive deficits, exaggerated reflexes, and seizures.

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