Topic 17.4 · Supplement
Codominance and ABO Blood Groups
Everything on the monohybrid inheritance page assumed one allele completely masks the other in a heterozygote. Codominance is the case where that assumption breaks — and ABO blood groups are the required Cambridge example of exactly how.
What codominance actually means
Codominance is the relationship where both alleles in a heterozygous organism contribute to the phenotype. That’s a genuinely different situation from the simple dominance covered on the monohybrid inheritance page. In simple dominance — Tt, for example — the recessive allele’s effect is completely masked in a heterozygote, and only the dominant phenotype shows. In codominance, neither allele masks the other: both are expressed together, producing a distinct combined phenotype. That combined phenotype isn’t a blend of the two, and it isn’t one allele quietly winning — it’s both showing up at once.
The ABO alleles
Human ABO blood groups have four phenotypes — A, B, AB and O — produced by three alleles: IA, IB and Io. IA and IB are codominant with each other, and each of them is dominant over Io. It’s worth being precise about a detail that trips students up: there are three alleles circulating in the population, but any one diploid individual only ever carries two of them.
| Blood-group phenotype | Possible genotype(s) |
|---|---|
| A | IAIA or IAIo |
| B | IBIB or IBIo |
| AB | IAIB |
| O | IoIo |
The AB phenotype is the clearest demonstration of codominance in this whole example: an IAIB individual shows both the A and B characteristics together, because neither allele is masking the other. Notice, too, that the A and B phenotypes each have two possible genotypes — a person with blood group A could be IAIA or IAIo, and phenotype alone can’t tell you which. That’s the same genotype-versus-phenotype gap the test cross deals with in simple dominance.
A worked ABO cross
Take one parent with blood group A and genotype IAIo, and another with blood group B and genotype IBIo. The first parent’s gametes carry IA or Io; the second parent’s gametes carry IB or Io.
| IA | Io | |
|---|---|---|
| IB | IAIB | IBIo |
| Io | IAIo | IoIo |
The possible offspring genotypes translate to phenotypes AB, B, A and O — one of each — giving an expected phenotypic ratio of 1 A : 1 B : 1 AB : 1 O. This example is worth remembering precisely because it shows, in one cross, both codominance (the AB offspring) and the genotype-ambiguity problem (a phenotype-A or phenotype-B parent whose exact genotype you can’t read off directly from their blood group alone).
Antigen biochemistry can explain why the phenotypes differ at a molecular level, but it isn’t required by the current syllabus objective — the genotype-to-phenotype rules above are the depth an answer needs.
What this sets up next
Codominance shows that a heterozygote doesn’t always default to “one allele wins.” The next page pushes the notation further still, into genes carried specifically on a sex chromosome: sex-linked inheritance, where the same genetic-diagram method has to track the sex chromosome and the allele it carries together.