A genetic counselor is reviewing a case where a couple has three children. The probability of a child having a certain autosomal dominant disorder (with 100% expressivity) from an affected parent is 50%. What is the probability that at least one child shows the disorder?

Sat, 18 Apr 2026 05:55:29 +0000

**Why More People Are Exploring Risk Patterns in Family Genetics – A Genetic Counselor’s Insight**
From rising interest in hereditary health risks to growing conversations around family planning, a detailed review of a family’s genetic profile reveals more than just biology—it uncovers patterns that matter. A couple with three children presents a compelling moment to reflect on inheritance: if one parent carries a dominant disorder with 100% expressivity—where every child has a 50% chance of inheriting it—what’s the likelihood at least one child shows symptoms? This question is gaining attention as more individuals seek clarity on hereditary conditions, especially when planning families or managing long-term health. Understanding genetic risk isn’t just academic—it shapes how families prepare, make informed decisions, and connect with care. While science provides the data, appointments with a genetic counselor offer context, interpretation, and peace of mind. The conversation around inherited disorders is shifting from niche genetics journals to mainstream health discussions, with many curious to understand their personal risk in clear, step-by-step terms. **How Does the 50% Chance Translate Across Three Children?**
For a disorder with 100% expressivity, each child independently has a 50% (or 1/2) chance of being affected. To determine the likelihood at least one of three children develops the condition, it helps to first consider the opposite: the probability that *none* of them are affected. For each child, the chance of not having the disorder is 50% or 0.5. Since risks are independent, multiply:
0.5 × 0.5 × 0.5 = 0.125, or 12.5%.
Thus, the probability that *at least one* child shows signs of the disorder is:
1 – 0.125 = 0.875, or 87.5%.
This counterintuitive result shows that even with only a 50% chance per child, three pregnancies dramatically increase cumulative risk—an insight many sequencing through family health histories. **Key Questions People Often Ask**
When reviewing such cases, several questions naturally arise:
- How sure is it at 87.5%?
- Do risks compound with each child?
- What if the first child is unaffected?
Under 100% expressivity, each birth carries independent risk—so early children don’t “lower” later risks. However, family history often informs broader screening and preparation, especially when planning further pregnancies. Some ask about genetic testing