Blood Type Inheritance Calculator

How ABO is inherited

The ABO blood group is controlled by one gene with three common alleles: A and B, which are codominant, and O, which is recessive to both. A person with genotype AA or AO has type A, BB or BO type B, AB type AB, and OO type O. Each parent passes one allele to each child at random. Two type A parents who are both AO can have a type O child, a quarter of the time, which surprises many families. A type AB parent cannot normally have a type O child, and two type O parents normally have only type O children.

A worked example

A type A parent with genotype AO and a type B parent with genotype BO can have children of all four ABO types, each with probability 25%: A from A and O, B from B and O, AB from A and B, and O from the two O alleles. If both parents are also Rh positive with genotype Dd, each child has a 75% chance of being Rh positive and 25% of being Rh negative. Combined, the eight possible types are A+, B+, AB+ and O+ at 18.75% each and A−, B−, AB− and O− at 6.25% each. A type O, Rh negative mother and a type AB, Rh positive (Dd) father can have only A or B children, each equally likely to be Rh positive or negative.

Genotype versus blood type

A blood test reports the phenotype, such as A positive, not the genotype. Type A can be AA or AO, and Rh positive can be DD or Dd; the calculator asks for genotypes so that the probabilities are exact. If you know only the types, try each possibility, or use family information: a type A person with a type O parent or child must be AO. In populations of European ancestry, roughly 15% of people are Rh negative, so many Rh positive people are Dd carriers.

Why blood types cannot settle parentage

A common misconception is that an “impossible” blood type proves non-paternity. Rare variants exist. In the Bombay phenotype, a separate gene prevents A and B antigens from being expressed, so a person tests as type O while carrying A or B alleles they can pass on. The rare cis-AB allele carries both A and B on one chromosome, so an AB parent can have an O-like pattern in a child’s family tree. Laboratory errors and weak subgroups such as A₂ and A_x also occur. Modern parentage testing uses DNA markers, not blood groups. Rh type matters medically in pregnancy, where an Rh negative mother carrying an Rh positive baby receives preventive anti-D treatment, which is a matter for the prenatal care team.

How to use it

1. Choose each parent’s ABO genotype; if you know only the type, try each option.
2. Choose each parent’s Rh genotype.
3. Read the possible blood types for a child with their probabilities.
4. Remember that rare variants can produce results these rules do not predict.