Synopsis
Explain inheritance, penetrance, genomic variation and testing limits; calculate family probabilities; and use genomic results only when phenotype, test scope and classification support action.
- Separate genotype, phenotype, penetrance and variable expression: inheriting a pathogenic variant may not predict whether, when or how severely disease appears.
- Autosomal dominant, autosomal recessive, X-linked and mitochondrial patterns generate probabilities per pregnancy, conditional on the parental genotypes and variant mechanism.
- A negative result excludes only variants the chosen assay could detect in genes and regions relevant to the tested indication.
Reasoning priorities
Represent affected status, ancestry, reproductive history and biological relationships across a family.
Record ages and uncertainty, verify diagnoses where possible and avoid inferring inheritance from a small or incomplete family alone.
Worked reasoning
Inputs: both parents are confirmed heterozygous carriers of the same pathogenic recessive variant; their first child is affected; calculate outcomes for the next pregnancy.
- Represent each parent as A/a, where a is the disease-associated allele; each produces A and a gametes with probability one half.
- Combine independent gametes: AA probability one quarter, A/a plus a/A total probability one half, and a/a probability one quarter.
- State the next-pregnancy outcomes: 25% affected, 50% unaffected carrier and 25% unaffected non-carrier, assuming the model and parentage are correct.
- Explain that the affected first child does not change the next conception's probabilities because each meiosis is a new event.
- Discuss reproductive options and test limitations through genetics services without treating probability as a prediction of the individual outcome.
- Verify by enumerating four equally likely gamete pairs—A/A, A/a, a/A and a/a—which reproduces the one-two-one distribution.