ABO blood groupsSpec 3.17B
In short
The ABO blood group is controlled by one gene with multiple alleles: IA, IB and IO. A person inherits two, one from each parent. IA and IB are codominant, so a person with both is blood group AB. IO is recessive to both, so a person is only blood group O with the genotype IO IO.
The ABO blood group is controlled by a single gene with multiple alleles: more than two alleles of the same gene. There are three alleles, written here as IA, IB and IO. A person inherits two, one from each parent.
In exam papers the second letter is printed small and raised above the I, but it means the same as IA, IB and IO here. Some books write IO as i.
IA and IB are codominant: if both are present, both are expressed in the phenotype. IO is recessive to both.
| Phenotype (blood group) | Possible genotypes |
|---|---|
| A | IA IA or IA IO |
| B | IB IB or IB IO |
| AB | IA IB |
| O | IO IO |
Parents with group A and group B
A mother is IA IO and a father is IB IO. What blood groups could their children have?
- Mother's gametes: IA or IO. Father's gametes: IB or IO.
- Possible children: IA IB, IA IO, IB IO and IO IO.
Answer: Group AB, A, B and O, each with a 1 in 4 (25%) probability.
Define codominance as both alleles being expressed in the phenotype of a heterozygous person. Do not call it blending.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026