Blood Type Inheritance Calculator
Which ABO blood types two parents can and cannot have — where an AB parent and an O parent have children of type A or B, matching neither of them.
This is the ABO gene only — the Rh positive/negative part is a separate gene and is not modelled here. The list of impossible types is exact. The percentages beside the possible ones are not: a type A parent may be AA or AO and their observed type cannot tell you which, so the figures weight both equally, where a real population weights them by how common each allele is. Rare exceptions such as the Bombay phenotype and cis-AB break even the impossibility rules, which is one reason blood type is not used as evidence about parentage. Nothing is uploaded.
An AB parent and an O parent have children of neither type
AB contributes an A or a B; O contributes an O. So every child is AO or BO — type A or type B, an even split, and never AB and never O. It is the one common pairing where both parents' own types are off the table, and the reason the folk rule that a child must match a parent is simply false.
| Parents | Children | |
|---|---|---|
| AO × AO | A 75%, O 25% | two type-A parents, both carrying O |
| AO × BO | AB 25%, A 25%, B 25%, O 25% | type A and type B, both carrying O |
| AB × AB | A 25%, AB 50%, B 25% | two AB parents |
| AB × OO | A 50%, B 50% | AB and O |
| OO × OO | O 100% | two O parents |
Six genotypes produce four types, and only AB and O name their genotype outright. A and B each hide two possibilities, which is why an A parent can carry an O nobody knew about.
What is impossible is the only part this settles
Two type A parents can have a type O child — at 25% when both carry an O — though never a type B one. An O parent never has an AB child and an AB parent never has an O child. And an A parent with a B parent rules out nothing at all: all four types are on the table, which makes it the least informative pairing there is.
| Parents | Possible | Impossible |
|---|---|---|
| A × A | A, O | B, AB |
| A × B | A, B, AB, O | nothing ruled out |
| A × AB | A, B, AB | O |
| A × O | A, O | B, AB |
| B × B | B, O | A, AB |
| B × AB | A, B, AB | O |
| B × O | B, O | A, AB |
| AB × AB | A, B, AB | O |
| AB × O | A, B | AB, O |
| O × O | O | A, B, AB |
Two O parents are the only pairing with a single possible outcome. Every other combination leaves at least two types open and A × B leaves all four, which is why the useful reading of this table is the right-hand column rather than the middle one.
How to use
- Pick the first parent blood type.
- Pick the second parent blood type.
- Read which child types are possible.
- Check the impossible list, which is the exact half.
Frequently asked questions
Can a child have a blood type neither parent has?
Yes, and AB with O is the clearest case: AB gives an A or a B, O gives an O, so every child is type A or type B. Neither parent type is possible in the children at all.
Can two type A parents have a type O child?
Yes. Type A covers two genotypes, AA and AO, and their blood type cannot tell you which they are. If both carry an O then a quarter of their children are type O — though none can be type B.
Which blood types are actually impossible?
An O parent can never have an AB child, and an AB parent can never have an O child. Two O parents can only have O children. Those exclusions hold for the ordinary ABO gene regardless of what the other parent is.
Which pairing tells you the least?
Type A with type B. All four types are possible from it, so the result rules out nothing whatsoever — the pairing carries no information about what a child will be.
Why are the percentages approximate?
Because a type A parent may be AA or AO and their observed type cannot distinguish the two. The figures weight both equally, where a real population weights them by allele frequency. The impossible list does not depend on that assumption.
Is this proof of parentage?
No, and it should never be used that way. Rare variants such as the Bombay phenotype and cis-AB break even the impossibility rules, and the ABO gene carries far too little information for the question. DNA testing is the only sound answer.
What about positive and negative?
That is the Rh gene, which is separate from ABO and not modelled here. Rh-negative is recessive, so two Rh-positive parents can have an Rh-negative child if both carry the negative allele.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.