Blood Type Calculator
Under this simplified model, type A and type B parents leave A, B, AB, and O in the possible ABO set because either parent may carry an unobserved O allele. Enter each parent's reported ABO phenotype and positive or negative Rh label to list the union of possible child labels. The output is not a probability table. A phenotype does not reveal whether an A parent is AA or AO, whether a B parent is BB or BO, or whether an Rh-positive parent carries a negative allele. The code combines every genotype consistent with the selected phenotype and reports any outcome that appears at least once. It does not weight genotypes by family history or population frequency. A listed type may therefore be possible without being equally likely.
Quick answer
The ABO gene has three alleles. A and B are codominant, and O is recessive to both.
What this tells you
- •The ABO gene has three alleles. A and B are codominant, and O is recessive to both.
- •A parent with type A or type B blood could carry a hidden O allele, so this tool lists every outcome consistent with each parent's phenotype.
- •Rh+ is dominant over Rh-. Only two Rh- parents guarantee an Rh- child. Any Rh+ parent could carry a hidden Rh- allele.
- •The result lists possible labels and ruled-out labels but does not calculate percentages.
- •ABO and Rh are modeled independently and then combined into the eight familiar labels.
- •Rare phenotypes, variants, laboratory error, transfusion history, and other blood-group systems are excluded.
How to Use
- 1Select the first parent's ABO blood type: A, B, AB, or O.
- 2Select the first parent's Rh factor: positive or negative.
- 3Select the second parent's ABO blood type and Rh factor.
- 4Calculate to see every possible blood type for their child and which types are ruled out.
- 5Read the result as a simplified possibility set, not a prediction for a pregnancy or child.
- 6Use laboratory typing and clinical care for transfusion, pregnancy, newborn, or Rh-compatibility questions.
- 7Use validated DNA testing, not blood-type matching, for biological relationship questions.
How It Works
Formula
ABO: A and B alleles are codominant, O is recessive
Rh: Rh+ is dominant, Rh- is recessive
Example: type A x type B parents can produce A, B, AB, or O offspringEach parent passes on one ABO allele and one Rh allele to a child. Because a type A or type B phenotype can come from two different genotypes (homozygous or carrying a hidden O allele), this calculator considers every genotype consistent with the phenotype you select and lists the union of possible offspring types. The same logic applies to Rh factor, where an Rh+ parent may or may not carry a hidden Rh- allele.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Type A and type B parents
Both parents could carry a hidden O allele, and both are Rh+, so a hidden Rh- allele is possible on either side. That combination allows every ABO and Rh outcome.
AB type and O negative parents
A type AB parent always passes an A or a B allele, never O, so an O child is impossible. A type O parent never passes an A or B allele, so an AB child is also impossible.
Two O negative parents
Type O is a recessive ABO genotype and Rh- is a recessive Rh genotype, so two O- parents can only pass on O and Rh- alleles.
Two A negative parents
Each A phenotype is treated as potentially AA or AO, leaving A and O in the union. Two negative parents produce only the negative Rh label in this simplified model.
Two AB positive parents
Each parent passes A or B, so no child receives two O alleles. Positive parents are treated as potentially carrying a negative Rh allele, so both Rh labels remain possible.
ABO Parent Combinations and Possible Offspring
Possible ABO types a child can inherit from each combination of parent phenotypes.
| Parent 1 | Parent 2 | Possible child ABO types |
|---|---|---|
| O | O | O |
| A | O | A, O |
| B | O | B, O |
| AB | O | A, B |
| A | A | A, O |
| A | B | A, B, AB, O |
| A | AB | A, B, AB |
| B | B | B, O |
| B | AB | A, B, AB |
| AB | AB | A, B, AB |
Rh factor is calculated separately. An Rh- child is only guaranteed when both parents are Rh-.
Possibility is not probability
The ABO mapping gives A parents the allele set A and O, B parents B and O, AB parents A and B, and O parents O only. This deliberately represents all compatible common genotypes. Pairing those sets produces the union of possible phenotypes.
A true probability requires genotype information or assumptions about how likely each hidden genotype is. For example, an AA parent and a BO parent have different child probabilities from an AO parent and a BB parent even though both couples display A and B phenotypes. The calculator does not choose among those cases.
Rh is reduced to one dominant-positive and one recessive-negative model. Real Rh blood-group genetics and serologic testing are more complex. Weak or partial D variants and clinical management cannot be inferred from a positive or negative menu selection.
ABO/Rh labels are not sufficient for transfusion matching, pregnancy management, or relationship testing. Transfusion services perform typing, antibody screening, crossmatching, and other checks. Pregnancy care may include laboratory testing and preventive treatment based on a clinician's assessment.
Common mistakes
- Assuming a type A or type B parent's exact genotype is known. Both phenotypes can hide a recessive O allele.
- Assuming an Rh+ parent cannot have an Rh- child. A heterozygous Rh+ parent can pass on the recessive allele.
- Treating this tool as a paternity or maternity test. Matching blood types does not confirm a biological relationship, and mismatched types do not rule one out on their own.
- Reading every listed outcome as equally likely when hidden parental genotypes are unknown
- Using ABO and Rh labels to choose blood for transfusion without laboratory compatibility testing
- Using the output to make pregnancy or Rh-treatment decisions
- Assuming the simplified rules cover Bombay phenotype, weak D, chimerism, or other blood groups
Limitations
This calculator assumes common Mendelian ABO inheritance and a simplified dominant-positive, recessive-negative Rh model. It lists a union of outcomes from every genotype treated as consistent with each phenotype and does not assign probabilities. It does not use ancestry, family history, genotype testing, population frequencies, or laboratory uncertainty. It excludes Bombay phenotype, cis-AB, weak or partial D variants, chimerism, transfusion effects, stem-cell transplant effects, and other blood-group antigens. It cannot confirm or exclude biological relationships and cannot assess transfusion or pregnancy compatibility.
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