A Family Risk Estimate After One Affected Child
Amara and Dev are healthy and have no relatives with any genetic disorder, yet their first child was diagnosed with an autosomal recessive condition. Testing found two different variants in the same gene, one inherited from each parent, and confirmed that both parents carry one variant each. Amara's brother Marcus and his partner Elena are now expecting a baby. Elena has no family history of the condition, but she and Marcus both grew up and still live in a region where carrier testing of 4,000 adults found the carrier rate for this gene to be about 1 in 22. All four grandparents are alive and unaffected. Marcus and Elena meet with a genetic counselor before deciding whether to be tested.
1. Estimate the probability that Marcus carries the family variant, then estimate the probability that his and Elena's baby is affected. Show each factor in the calculation and state the assumptions behind it.
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Step 1 - Marcus's carrier risk is about 1/2. Amara is an obligate carrier because she has an affected child. Since the condition is rare, the variant almost certainly entered her family through one of her two parents, who is also a carrier. Marcus has a 1 in 2 chance of inheriting that same variant from that parent.
Step 2 - Elena's carrier risk is about 1/22, taken from the regional carrier testing rather than from her family history, which tells us nothing about a recessive allele.
Step 3 - If both are carriers, the chance both pass the variant to a child is 1/4.
Combined: 1/2 × 1/22 × 1/4 = 1/176, or about 0.57%.
Assumptions: that both grandparents are not carriers (which would slightly raise Marcus's risk), that the 1/22 figure applies to Elena's population, and that the regional figure reflects variants in this gene generally rather than a single variant. The third assumption is the fragile one and is worth flagging honestly: carrier screens that test specific variants may miss others.
2. Elena says, "Nobody in my family has ever had this, so the risk must be close to zero." Evaluate that reasoning, and explain what single test would most change the estimate.
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The reasoning is mistaken. A recessive condition appears only when two carriers have an affected child, and carriers are typically healthy with no symptoms. A family can therefore carry a recessive variant through many generations without a single affected relative, so an unremarkable family history is the normal situation, not evidence of low risk. This is why the counselor used a population carrier rate - a measured base rate - instead of family history for Elena.
The claim also treats the risk as near zero when the calculation gives roughly 1 in 176, which many families would consider worth acting on.
The single most useful test: carrier screening for Elena, aimed at the family variant. If Elena does not carry it, the chance of an affected child collapses to near the population baseline for untested variants. Testing Marcus first is also reasonable - if he is not a carrier, the question is closed immediately, and his result is cheaper to interpret because we already know exactly which variant to look for in Amara.
3. The affected toddler will someday ask about their own future children. Explain why that child's risk depends almost entirely on their future partner, and estimate the risk if the partner comes from the same region.
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Why the partner determines everything: the affected child carries a variant on both copies of the gene, so every gamete they produce will carry one variant or the other. They cannot pass on an unaffected copy. An affected child of their own is possible only if a partner also contributes a variant - and a partner carrying one copy is healthy, so there is no way to know without testing.
Estimate: if the partner comes from the same region, carrier risk is about 1/22. If the partner is a carrier, the chance the partner passes the variant is 1/2. So the chance of an affected child is roughly 1/22 × 1/2 = 1/44, or about 2.3%.
Important caveats to state: this is a substantially higher risk than for the general population precisely because the child is a confirmed carrier of two variants, not one. The estimate is approximate because the 1/22 figure may not cover every disease-causing variant in the gene. Partner testing would replace the estimate with something close to a certainty in either direction, which is exactly what a counselor would recommend.