Two Affected Sons, an Uncertain Variant, and a Family Wanting Numbers
A couple with no symptoms has four children: two sons with prolonged bleeding after minor injuries (factor VIII activity below 5%), an unaffected son and an unaffected daughter. The mother's brother has the same disorder; the father's clotting is normal and he has no family history. Molecular testing of the younger affected son finds a single-base change in the F8 gene on the X chromosome, reported as a 'variant of uncertain significance'. The parents ask three things: what is the chance the next child will be affected, whether their daughter should be tested now, and whether the report proves the diagnosis. A colleague tells the parents the pedigree already proves X-linked recessive inheritance.
1. Does the pedigree prove X-linked recessive inheritance? Explain what it can and cannot establish.
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It is consistent with, but does not prove, X-linked recessive inheritance. The positive features are two affected males born to unaffected parents, an affected maternal uncle indicating that the mother's mother was likely a carrier, and no father-to-son transmission, which is what X-linkage predicts.
But autosomal recessive inheritance also fits if both parents are carriers, and their having two affected sons is not improbable by chance. The observations that would discriminate - an affected daughter born with an affected father, or all sons of a definite carrier being affected - are absent. Only one family is shown, so linkage is not demonstrated, and the single-base change could be a benign polymorphism or a de novo change unrelated to the phenotype. The pedigree is compatible with several models; the molecular result narrows the possibilities but a 'variant of uncertain significance' is not a diagnosis and should be resolved by segregation testing in the family or functional evidence.
2. Assuming the F8 variant is pathogenic and the mother carries it, calculate the probability that the next child is affected. Why might a genetics professional tell the parents a different figure?
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The mother would be X^H X^h and the father X^H Y.
- Sons receive Y from the father: each son has a 1/2 chance of receiving X^h and being affected.
- Daughters receive X^H from the father: each daughter has a 1/2 chance of being a carrier and a 0 chance of being affected.
- Probability of an affected child regardless of sex = 1/2 (probability of a son) × 1/2 = 1/4, i.e. 25%.
A different figure is defensible because the variant may not be pathogenic at all: if it is benign, the risk falls back to the population level and the family history would need another explanation. The mother could also be a gonadal mosaic, so the recurrence risk might be lower than a carrier's, and if her carrier status has not been confirmed the whole calculation is conditional. Skewed X-inactivation affects how severely a carrier daughter might bleed but does not change the sons' 50% risk.
3. Should the eight-year-old daughter be tested for carrier status now? Give a reasoned recommendation that addresses the parents' wishes and the technical limits of the test.
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Testing the mother first is the priority: if she does not carry the variant, the daughter cannot have inherited it from her, and the question is resolved without testing a child. If the mother does carry it, the daughter has a 1/2 chance of being a carrier.
Carrier testing of a young child for a condition that has no health implications in childhood is usually deferred until she can understand and consent to it; the usual argument is that the child's future autonomy and confidentiality should not be pre-empted, while the benefit of knowing now is limited. The exception is when early knowledge would change medical management, which carrier status for haemophilia does not. Practical limits also matter: the test can detect the variant, but a negative result in a VUS context can be falsely reassuring if the variant is later reclassified.
A defensible answer therefore supports the parents' need for information through testing the mother and counselling, offers a plan to revisit testing in adolescence, and records the discussion.