Blood Type Calculator: Child Inheritance from Parents
Enter the mother's and father's ABO group and Rh factor to see the probability of each of the eight possible blood types for their child. The calculator uses Mendelian genetics to compute a full Punnett square, shows the chance of Rh-positive and Rh-negative outcomes, and lists which blood types the child could donate to or receive from. Results update instantly as you change any input.
How blood type inheritance works
Blood type is inherited through two overlapping genetic systems: the ABO system and the Rh system. In the ABO system, everyone carries two alleles chosen from three possibilities: A, B, and O. The A and B alleles are codominant, meaning both are expressed when present together (producing type AB), while O is recessive. Type A results from the genotypes AA or AO, type B from BB or BO, type AB only from AB, and type O only from OO. Each parent passes one allele to the child at random, so a parent of type A (genotype AO) has a 50% chance of passing A and a 50% chance of passing O to each child. The Rh factor is controlled by a separate gene: the D allele (Rh positive) is dominant over the d allele (Rh negative). An Rh-positive person can be DD or Dd; an Rh-negative person is always dd. Because we cannot know a parent's exact Rh genotype without additional testing, this calculator assumes an Rh-positive parent is equally likely to be DD or Dd, giving a 75% chance of passing the D allele.
Reading your results: probability and the Punnett square
The calculator crosses all possible parental genotypes and weights each combination equally, producing the classic Punnett-square result expressed as percentages. For example, if the mother is type A (AO or AA) and the father is type O (OO), the child has a 50% chance of being type A and a 50% chance of type O - the same result whether you use a grid or algebra. The Rh probabilities are computed independently and then multiplied by the ABO probabilities to get the joint chance of each of the eight full phenotypes (O-, O+, A-, A+, B-, B+, AB-, AB+). These are statistical likelihoods, not guarantees. Only a blood test can confirm a child's actual blood type.
Rh incompatibility in pregnancy
Rh incompatibility becomes medically important when an Rh-negative mother carries an Rh-positive baby. During delivery (or miscarriage or certain procedures), fetal red cells can enter the mother's bloodstream, triggering her immune system to produce anti-D antibodies. In a subsequent Rh-positive pregnancy, those antibodies can cross the placenta and attack the baby's red cells, potentially causing hemolytic disease of the fetus and newborn (HDFN), which ranges from mild jaundice to severe anemia. Routine prenatal care screens for this: Rh-negative mothers receive an injection of anti-D immunoglobulin (Rh immune globulin) around week 28 and again after delivery, effectively preventing sensitisation. If you are Rh negative and your partner is Rh positive, discuss prophylaxis with your obstetrician.
Blood type and transfusion compatibility
Transfusion compatibility is governed by the antigens on red blood cells and the antibodies in plasma. Type O-negative blood carries no A, B, or Rh antigens, so it can be given safely to anyone in an emergency (universal donor for red cells). Type AB-positive plasma and platelets can go to any recipient regardless of ABO or Rh type. In practice, hospitals always cross-match donor and recipient blood to catch incompatibilities beyond ABO and Rh, including dozens of minor blood group antigens. The compatibility table here covers the main ABO and Rh rules for red cell concentrates only.
Blood type compatibility for red cell transfusions
| Blood type | Can donate to | Can receive from | Population frequency (approx.) |
|---|---|---|---|
| O- | All types (universal donor) | O- | ~6% |
| O+ | O+, A+, B+, AB+ | O-, O+ | ~38% |
| A- | A-, A+, AB-, AB+ | O-, A- | ~6% |
| A+ | A+, AB+ | O-, O+, A-, A+ | ~28% |
| B- | B-, B+, AB-, AB+ | O-, B- | ~2% |
| B+ | B+, AB+ | O-, O+, B-, B+ | ~9% |
| AB- | AB-, AB+ | O-, A-, B-, AB- | ~1% |
| AB+ | AB+ only | All types (universal recipient) | ~3% |
Simplified red-cell compatibility. Plasma and platelet transfusion rules differ. Always confirm with a hospital blood bank.
Frequently asked questions
Can a child have a blood type that neither parent has?
Yes, in some cases. Two parents who are both type A (each carrying an AO genotype) can have a type O child. Two parents who are both type AB cannot, however, produce a type O child, because neither carries the O allele. Whether a child can inherit a type that neither parent expresses depends on the hidden recessive O allele both parents may carry.
Can blood type reveal paternity?
Blood typing can exclude certain men from paternity - for example, two O-type parents cannot have a type AB child, so a type AB child would rule out a type O man. However, blood typing cannot prove paternity, because many people share blood types. DNA testing is the reliable method for establishing biological parentage.
Why does the calculator show probabilities rather than a single answer?
Because most blood types correspond to two possible genotypes. A type A person could be AA (homozygous) or AO (heterozygous), and we cannot tell which without genetic testing. The calculator models both possibilities with equal weight, which produces a range of probabilities rather than a definite outcome. If both parents have been genotyped, the probabilities will be more precise.
What is an Rh-negative blood type and why does it matter?
Rh factor refers to the presence (Rh+) or absence (Rh-) of the D antigen on red blood cells. About 85% of people are Rh positive. Rh factor matters most in two situations: blood transfusions (an Rh-negative patient given Rh-positive blood can develop antibodies) and pregnancy (an Rh-negative mother carrying an Rh-positive fetus can become sensitised, which may harm future pregnancies). Routine antenatal injections of anti-D immunoglobulin prevent sensitisation.
What is the rarest blood type?
AB-negative is the rarest of the eight main ABO/Rh types, occurring in roughly 1% of the population. Within broader blood group systems there are even rarer phenotypes, such as Bombay (Oh), found in about 1 in 10,000 people in some South Asian populations. These rare types require specially matched donors and are managed by specialist blood banks.
What is the universal donor and universal recipient?
O-negative blood is called the universal donor for red cells because it lacks A, B, and Rh antigens, so it can be safely transfused to patients of any blood type in emergencies. AB-positive is the universal recipient for red cells because it carries A, B, and Rh antigens and has no antibodies against them in plasma. For plasma donations the rule reverses: AB-positive plasma is the universal plasma donor.