Blood type inheritance calculator.

How ABO and Rh inheritance works

The ABO type is controlled by one gene with three alleles, I^A, I^B and i. I^A and I^B are both dominant over i, and they are codominant with each other, so only ii gives type O and I^AI^B gives type AB (Wikipedia, ABO blood group system). The Rh type depends on the D antigen, which is a dominant trait; the lower-case d used here is shorthand for no D antigen. If both parents are Rh negative, the child is definitely Rh negative (Wikipedia, Rh blood group system). This tool treats ABO and Rh as separate, independent systems and ignores rarer variants.

Worked example. Type A × type B parents, both Rh positive: the tool lists A+, A-, B+, B-, AB+, AB-, O+, O- as possible child types, because an A parent may be AA or AO and a B parent BB or BO. If both turn out to be heterozygous (AO × BO), each of A, B, AB and O has a 25% chance, matching Wikipedia's note that an A and a B parent can have a type O child. Two O-negative parents can only have O- children. Grandparents narrow things down: if a type A parent's own parents are type O and type AB, the A parent must be AO, so the tool offers only AO for that parent.