Blood type calculator: which blood types can the child have?
This blood type calculator shows which blood types a child can inherit from its parents. ABO is decided by three alleles – A and B are codominant, both dominant over O – and Rh positive is dominant over Rh negative. Example: parents A and B can have a child with any of the four blood types.
Example: Blood type of the mother: A · Rh factor of the mother: Rh+ (positive) · Genotype of the mother (if known): unknown · Blood type of the father: B · Rh factor of the father: Rh+ (positive) · Genotype of the father (if known): unknown → Possible blood types of the child: Possible: A, B, AB, O · Rh+ or Rh−. Source: OpenStax Concepts of Biology, 8.3 Extensions of the Laws of Inheritance: three ABO alleles I^A, I^B and i; I^A and I^B are codominant with each other and both dominant over i; I^A I^B = blood type AB (read 10.10.2026). Updated: .
How it is calculated
Every child receives one ABO allele from each parent. There are three alleles: A, B and O. OpenStax explains that A and B are codominant with each other and both dominant over O. So blood type A can be AA or AO, blood type B can be BB or BO, AB is always AB and O is always OO. The Punnett square combines one allele from each parent; each of the four combinations is equally likely, i.e. 25 %.
Which blood types can the child have?
| Parents | Possible blood types of the child |
|---|---|
| O × O | O |
| A × O | A, O |
| B × O | B, O |
| AB × O | A, B |
| A × A | A, O |
| B × B | B, O |
| A × B | A, B, AB, O |
| A × AB | A, B, AB |
| B × AB | A, B, AB |
| AB × AB | A, B, AB |
The table matches the chart of the American Red Cross. Two O parents can only have O children, and an AB parent never has an O child.
Percentages from the Punnett square
The probabilities depend on the genotypes. AO × BO gives 25 % each for A, B, AB and O. AA × BB gives 100 % AB. AO × AO gives 75 % A and 25 % O (the classic 3 : 1). AB × AB gives 25 % A, 50 % AB and 25 % B – the 1 : 2 : 1 ratio OpenStax shows for codominance. If you only know the blood type, the calculator lists every possible genotype combination row by row. The general rules for chances are explained in the probability calculator.
Rh factor: positive is dominant
The Rh factor is the D antigen and is inherited separately from ABO. Rh+ (D) is dominant, Rh− (d) recessive, so an Rh+ person is DD or Dd. Two Rh− parents only have Rh− children. Rh+ × Rh− gives at most 50 % Rh− (if the Rh+ parent is Dd); Rh+ × Rh+ gives at most 25 % Rh− (if both are Dd). Blood types of grandparents help to narrow down hidden alleles – the family tree maker keeps them in one place.
Frequently asked questions
Can two O parents have an A child?
Not in the ABO model: O is always OO, so both parents can only pass on O. The child is O.
Which blood types are possible for parents A and B?
All four: A, B, AB and O. If both parents carry O (AO × BO), each blood type has a 25 % chance.
Can an AB parent have an O child?
No. An AB parent always passes on A or B, so the child has at least one A or B allele. AB × O gives A or B.
Can two Rh positive parents have an Rh negative baby?
Yes, if both are Dd. Then the chance is 25 % for each child.
Can the blood type rule out a father?
In some cases: if a combination is impossible in this model, it is excluded. A possible blood type does not prove parentage, because many people share it.
Why does the calculator show several rows?
Blood type A can be AA or AO, and B can be BB or BO. Without a genetic test the genotype is unknown, so each possible combination gets its own Punnett row.
Sources and legal basis
- OpenStax Concepts of Biology, 8.3 Extensions of the Laws of Inheritance: three ABO alleles I^A, I^B and i; I^A and I^B are codominant with each other and both dominant over i; I^A I^B = blood type AB (read 10.10.2026)
- OpenStax Biology 2e, 12.2 Characteristics and Traits: Punnett square – every combination of the parental alleles is equally likely (read 10.10.2026)
- American Red Cross, “Know your blood type” flyer: chart of the possible blood types of a child for each pair of parents, e.g. AB × O → A or B, A × B → O, A, B or AB (read 10.10.2026)
- Zintegrowana Platforma Edukacyjna (Polish Ministry of Education), “Inheritance of blood groups, Rh inheritance”: the Rh+ allele is dominant, the Rh− allele recessive; Rh factor = D antigen (read 10.10.2026)
- OpenStax Anatomy and Physiology 2e, 18.6 Blood Typing: the Rh group is distinct from the ABO group (read 10.10.2026)
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