A simplified model, with explicit uncertainty
The inheritance tools explore the common ABO groups and a simplified RhD-positive/negative model. A blood-type label is a phenotype: the expressed type. It may be consistent with more than one genotype, the inherited allele combination. We use A, B and O as shorthand for the common ABO alleles, and DD, Dd and dd for the simplified RhD model.
| Selected type | Possible genotype labels |
|---|---|
| A | AA or AO |
| B | BB or BO |
| AB | AB |
| O | OO |
| Rh positive | DD or Dd |
| Rh negative | dd |
For a fixed pair of genotypes, each parent contributes one of two alleles. The model enumerates four equally likely allele pairings, each with a probability of 25%, and combines pairings that produce the same expressed type. It treats ABO and RhD as independent for combined outcomes.
Why a range is not a single predicted probability
The Baby Calculator examines every parental-genotype pair consistent with the selected types. For each child type, it reports the smallest and largest probability found across those scenarios. It does not average the scenarios or assume that AA and AO, for example, are equally common in a population.
A range is a set of bounds under this model, not a confidence interval or an ancestry-specific estimate. Different child types can reach their maximum in different scenarios, so adding all the upper bounds need not give 100%. The probabilities within any one fixed-genotype scenario do sum to 100%.
For a combined ABO/Rh result, the tool multiplies the ABO and Rh probabilities within the model. When a parent's type is unknown, it includes every supported genotype for that input rather than assigning a guessed type or population frequency.
Worked example: an A-negative parent and an O-negative parent
The A parent could be AA or AO; the O parent is OO in the common model. Both Rh-negative parents are represented as dd. Compare the two possible ABO scenarios before reading the final range.
| Parent genotypes | Child type A | Child type O |
|---|---|---|
| AA × OO | 100% | 0% |
| AO × OO | 50% | 50% |
| A parent's allele | O parent's first O allele | O parent's second O allele |
|---|---|---|
| A | AO → type A | AO → type A |
| O | OO → type O | OO → type O |
Each cell in that square has a 25% probability: two cells produce A and two produce O. With dd × dd, the simplified Rh result is negative throughout. The combined calculator therefore displays these ranges:
| Child type | Probability across scenarios |
|---|---|
| O− | 0%–50% |
| A− | 50%–100% |
Those bounds do not say where within the range a particular family belongs. The tool has not determined the A parent's genotype. You can inspect individual scenarios in the Punnett Square Calculator.
The other tools answer different questions
- Parent Calculator: checks which other-parent types can produce the selected child type with the known parent under the same model. It is a compatibility-of-inheritance lookup, not a probability ranking or a parentage test.
- Punnett Square Calculator: shows the allele pairings and lets you examine compatible genotypes. Exact probabilities describe a selected scenario; choosing a genotype does not establish that anyone actually has it.
- Donation tools: apply standard ABO/RhD red-cell and ABO plasma compatibility rules. They do not evaluate a person's antibodies, cross-match results or clinical suitability.
- Rarity tool: looks up the cited general U.S. blood-type frequency estimates. It does not infer a user's ancestry, location-specific frequency or genotype, and those frequencies do not weight the inheritance calculations.
What software checks establish
The calculation library has automated checks for known inheritance examples, genotype enumeration, probability ranges, unknown inputs, reverse lookups and donation rules. Punnett-square checks verify allele pairings and selected-genotype outcomes. Running those checks can identify software regressions against the model; it does not clinically validate the model or every statement on the site.
This methods description was checked against the implemented calculation logic on October 12, 2026. Independent clinical review is not currently claimed. See our editorial policy and review status and publisher information.
Where the model stops
These shorthand genotypes leave out the wider biological complexity of blood groups. Rare variants, including Bombay phenotype, cis-AB and variant D expression, can fall outside the simplified rules. The tools cannot establish a person's actual blood type or resolve an unexpected family result.
Blood typing and clinical interpretation require laboratory testing and an appropriate healthcare professional. Pregnancy and transfusion decisions should not be based on calculator output. If a calculation seems wrong, send a correction with the page, selected types and expected result; omit names or identifying medical records.