Look up possible child blood types from parents' ABO types, or infer the other parent's possible type.
Reverse: from one parent + child
Pick both parents' ABO types and click Calculate to see which child blood types are possible and which are impossible. The lower section reverses the question: given one parent and the child, it lists the types the other parent could be. ABO only — Rh factor is not modeled here. Based on standard Mendelian genotypes (A=AA/AO, B=BB/BO, AB=AB, O=OO).