Pedigrees & Inheritance
Rule out inheritance patterns and calculate probabilities with incomplete family information.
What to do when the question looks unfamiliar
- Step 1Write the inheritance assumptions before excluding a model.
- Step 2Trace which parent can pass the allele to whom.
- Step 3Condition on information you already have before computing probability.
Use exclusions before making a diagnosis
Two unaffected parents with an affected child support a recessive model under complete penetrance and no new mutation. An affected daughter with an unaffected father is inconsistent with classic X-linked recessive inheritance under those assumptions.
An affected father does not directly pass his X to a son. If he passes a dominant X-linked allele, his daughters receive it. Mitochondrial DNA is usually transmitted through the egg, so maternal inheritance has a different pattern. A small family may fit multiple models by chance.
Condition on what is already known
For Aa × Aa, the four equally likely ordered outcomes are AA, Aa, aA, aa. If a child is already known to be unaffected by a completely penetrant recessive trait, remove aa. Two of the three remaining outcomes are carriers.
The outcomes of previous births do not change the next child’s Mendelian probability when the parental genotypes are known. If the parental genotypes are uncertain, family history can change your inference about those genotypes.