🧬 Child Blood Type Calculator

A child's blood type comes from the parents' genes: each parent contributes one ABO allele (A, B, or O) and one Rh allele (+ or -), and the combination decides the type. Because A and B dominate O, the parents' visible types do not always reveal which hidden alleles they carry, which is why two type-A parents can have a type-O child.

This calculator enumerates every genotype combination consistent with the parents' types and reports the possible children with their probabilities. It uses a simplified equal-weighting model, good for education and curiosity, not for medical or legal conclusions.

How to use this calculator

Enter each parent's ABO group and Rh factor.

How inheritance works

Each person carries two ABO alleles. A and B are dominant over O, so genotype AO looks like type A, and only OO looks like type O. AB shows both. For Rh, positive dominates negative: only dd (two negative alleles) is Rh-negative.

The calculator tries every genotype pair consistent with the parents' types, builds all four child allele combinations per pair, and tallies the phenotypes. Equal weighting of possible genotypes is a simplification; real population frequencies would shift the numbers slightly.

Worked example

Parent 1 is type A, parent 2 is type B. Genotypes: (AA or AO) x (BB or BO), four pairings, four children each, 16 outcomes. Tally: AB 9, A 3, B 3, O 1. As percentages: AB 56%, A 19%, B 19%, O 6%. Every type is possible, which surprises people who expect "A and B make AB."

Two type-O parents: OO x OO gives only O children, 100%. That is the one pairing with a single certain outcome, and the basis of the classic rule that O parents cannot have AB children.

The Rh factor

Rh-negative children need a d allele from each parent. Two Rh-negative parents: always Rh-negative children. One negative parent: the child gets d from that side for sure, and needs d from the Rh-positive parent, roughly a coin flip in this simplified model. Two Rh-positive parents: usually Rh-positive children, with a small chance of Rh-negative if both carry a hidden d.

Rh matters most in pregnancy: an Rh-negative mother carrying an Rh-positive baby can develop antibodies affecting later pregnancies, which is why Rh immune globulin (RhoGAM) is given preventively. Blood typing in early pregnancy exists largely for this reason.

What blood type cannot do

Blood type cannot prove or disprove parentage by itself. It can only exclude: if the alleged parents could not possibly produce the child's type, that is informative, but a "possible" result proves nothing, since millions of people share each type. Legal parentage needs DNA testing.

Rare variants (cis-AB, Bombay phenotype, weak D) break the standard rules in ways this calculator does not model. Surprising lab results deserve repeat testing and, if needed, a genetics consult, not family drama.

Why O parents cannot have AB children

The rule follows directly from the alleles. Type O means genotype OO: two O alleles and nothing else. Each parent contributes one allele, so two O parents contribute O and O, making every child OO, type O, with 100% certainty. There is no A or B allele anywhere in the system to pass on.

The reverse surprises people: two type A parents can have a type O child, because "type A" hides two possible genotypes, AA and AO. If both parents are AO, each child has a 25% chance of OO. The visible type conceals the alleles; the calculator reveals them by enumerating every consistent genotype.

Exceptions exist but are rare: the Bombay phenotype (hh genotype) suppresses ABO expression entirely, so someone testing as "O" can carry A or B alleles silently. Cis-AB, where A and B travel on one chromosome, breaks the rules differently. Genuine rule-breaking results deserve repeat testing before any conclusions.

Blood type and transfusion compatibility

Inheritance decides your type; compatibility decides whose blood you can receive. Type O-negative red cells go to anyone (universal donor); AB-positive plasma goes to anyone. AB-positive people can receive any red cells (universal recipients). The logic is antibodies: type A blood carries anti-B antibodies, so it attacks B cells, and the reverse.

Rh adds a second layer: Rh-negative recipients should get Rh-negative blood to avoid sensitization, while Rh-positive recipients take either. In emergencies, O-negative is the universal standby, which is why blood drives especially prize O-negative donors.

Plasma compatibility runs in reverse (AB plasma is universal), a detail that confuses everyone exactly once. The short version: red cell rules are about the recipient's antibodies; plasma rules are about the donor's.

For informational purposes only, not medical advice. Consult a healthcare professional.