Codominance and ABO blood groupsSpec 17.4.14, 17.4.15
In codominance, both alleles in heterozygous organisms contribute to the phenotype. Neither allele is hidden and the two do not blend.
The ABO blood group is controlled by one gene with three alleles: IA, IB and Io. Exam papers print the A, B and o as small raised letters after the I. A person inherits two alleles, one from each parent. IA and IB are codominant. 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 |
Group A mother and group B father
A mother is IA Io and a father is IB Io. Which blood groups could their children have?
- Mother's gametes: IA or Io. Father's gametes: IB or Io.
- Punnett square boxes: IA IB, IA Io, IB Io, Io Io.
Answer: Group AB, A, B and O, each with probability 1 in 4 (25%), a ratio of 1 : 1 : 1 : 1.
Exam tip:
IA IB gives group AB because both alleles are expressed. Group O needs two Io alleles.
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