Residual Carrier Risk When Penetrance Is Incomplete
A 40-year-old man is referred for genetic counseling. His father carries a pathogenic variant in a gene causing an autosomal dominant cancer syndrome with 80% penetrance by age 40. The man is healthy at age 40 and has not been tested. He has a 16-year-old daughter. He asks two questions: 'What is my chance of carrying the gene?' and 'If I am negative, is my daughter safe?' Assume no other family history, no early signs, and that penetrance is complete by age 40 in carriers.
1. Calculate the man's probability of carrying the pathogenic variant given that he is unaffected at age 40. Show a Bayesian calculation and state the assumptions.
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Use Bayes. Prior probability of carrying the variant is 1/2 because his father is affected by an autosomal dominant condition and the man is the son of an affected parent. If he carries the variant, the probability he remains unaffected at age 40 is 1 − 0.80 = 0.20. If he does not carry it, the probability he is unaffected is essentially 1.0. Posterior = (0.5 × 0.20) / (0.5 × 0.20 + 0.5 × 1.0) = 0.10 / 0.60 = 1/6 ≈ 17%. Assumptions include 80% penetrance by age 40, no other causes of the phenotype, and no additional family information. This residual risk is not zero; he should be offered testing for the known familial variant.
2. If he tests negative for the known familial variant, how should you counsel his daughter? What if the test is a panel that did not confirm the father's specific variant?
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Confirm first that the laboratory tested specifically for the father's pathogenic variant and that the result is a true negative. If so, the man did not inherit the variant; his risk falls to the population level, and his daughter's risk is also population-level, not 50%. The family variant should be documented. If the test was a panel that did not include the familial variant, or if the result is a variant of uncertain significance, the negative is uninformative. Then the man's residual carrier risk remains about 17%, and his daughter's risk is half of that, about 8–9%, because she has a 50% chance of inheriting his at-risk allele. In that situation, do not tell the family she is 'safe'; arrange testing for the correct variant and genetic counseling.
3. If the daughter is tested at 16 and is positive, what does the result mean for her immediate management? Does a positive result mean she will certainly develop cancer?
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A positive result in the daughter confirms she carries the pathogenic variant, but it does not mean she will certainly develop cancer: penetrance is 80%, so about 20% of carriers may not. Management depends on the specific syndrome and age-specific guidelines, but it generally includes counseling about surveillance, risk-reducing options, and reproductive implications. At 16, immediate prophylactic surgery is not indicated; psychosocial support and a plan for transition to adult surveillance are important. Also address confidentiality and insurance concerns. The key judgment is that a positive predictive test changes surveillance, not certainty of disease.