Quick answer
Each person carries two ABO alleles, one from each parent, chosen from A, B and O. A and B are codominant and O is recessive, so AO shows as type A, BO as type B, AB as type AB, and only OO shows as type O. A child inherits one allele from each parent at random, which is why children often have a different blood type from both parents.
The three alleles
The ABO gene on chromosome 9 comes in three common versions (alleles). The A allele produces an enzyme that adds the A sugar to red cells, the B allele adds the B sugar, and the O allele produces an inactive enzyme that adds nothing.
| Genotype (two alleles) | Blood type (phenotype) |
|---|---|
| AA | A |
| AO | A |
| BB | B |
| BO | B |
| AB | AB |
| OO | O |
Because type A and type B people can carry a hidden O allele, knowing someone's blood type does not always tell you their genotype. That uncertainty is why calculators give probability ranges for some parent combinations.
Worked example: two type A parents
Suppose both parents are type A. If each is AO, a Punnett square shows four equally likely outcomes for the child: AA, AO, OA and OO. Three of the four are type A and one is type O, so there is a 25 percent chance of a type O child. If either parent is AA instead, every child will be type A.
| Parent 2: A | Parent 2: O | |
|---|---|---|
| Parent 1: A | AA (type A) | AO (type A) |
| Parent 1: O | AO (type A) | OO (type O) |
Every parent combination
Possible and not-expected child ABO types for each pair of parent blood types under the standard model.
| Parents | Possible children | Not expected |
|---|---|---|
| A × A | A, O | B, AB |
| A × B | A, B, AB, O | — |
| 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 |
The baby blood type calculator adds Rh factor and shows probability ranges for each outcome.
Exceptions to the standard model
A small number of genetic variants break the simple rules. In the Bombay phenotype, a person lacks the H antigen that A and B are built on, so they test as type O even though they may carry A or B alleles and pass them to children. In cis-AB, one chromosome carries both A and B activity, so an AB parent can have an O child. Weak A or B subgroups can also be misread in routine tests. These are rare, but they mean an unexpected result should be investigated by a laboratory, never taken as proof about parentage.