Both alleles express equally. Iᴬ and Iᴮ both expressed in AB blood.

What Is Codominance? ABO Blood Group Example

Question

In genetics, what is codominance? Explain with the example of ABO blood group system. Why is the AB blood group considered a classic case of codominance and not dominance or recessiveness?

(CBSE 2024 Board Exam — 3 marks)

Solution — Step by Step

Codominance occurs when both alleles in a heterozygous individual are fully expressed simultaneously — neither allele is dominant or recessive over the other. The phenotype shows both traits at the same time, not a blend.

This is different from incomplete dominance, where you get a mixed/intermediate phenotype. In codominance, both original phenotypes appear together.

The ABO blood group is controlled by a single gene with three alleles: $I^A$ , $I^B$ , and $i$ .

$I^A$ produces antigen A on RBC surface
$I^B$ produces antigen B on RBC surface
$i$ produces no antigen (recessive to both)

Both $I^A$ and $I^B$ are codominant to each other — neither suppresses the other.

A person with genotype $I^A I^B$ has AB blood group.

Both $I^A$ and $I^B$ are expressed — antigen A and antigen B both appear on the RBC surface. The cell doesn’t show only A, only B, or some halfway version. It shows both, fully and simultaneously.

This is the textbook definition of codominance.

Cross two parents: one $I^A I^B$ (AB) × one $ii$ (O):

\begin{array}{c|cc} & I^A & I^B \\ \hline i & I^A i & I^B i \\ i & I^A i & I^B i \\ \end{array}

Offspring: 50% blood group A, 50% blood group B. No AB offspring — confirms that $I^A$ and $I^B$ are separate, independently expressed alleles.

AB blood group is codominance because:

Both $I^A$ and $I^B$ alleles are expressed in the heterozygote
Both antigens (A and B) are present on the RBC surface
Neither allele masks the other

The AB blood group (genotype $I^A I^B$ ) is the classic example of codominance in humans.

Why This Works

The key to codominance is what’s happening at the molecular level. Each allele codes for a specific enzyme that attaches a specific sugar to the RBC surface protein. $I^A$ codes for an enzyme adding N-acetylgalactosamine (antigen A), and $I^B$ codes for a different enzyme adding galactose (antigen B).

When both alleles are present, both enzymes are produced. There’s no competition — they do different jobs on the same surface protein. So the cell ends up displaying both antigens.

This is why you can’t have a “dominant” allele here — dominance would require one gene product to suppress the other. But these two enzymes work independently, so both phenotypes show up in full.

Alternative Method — Spotting Codominance in MCQs

For NEET and CBSE MCQs, use this quick check:

Codominance vs Incomplete dominance — the 3-second test:

Ask: Does the heterozygote show BOTH original phenotypes, or a NEW intermediate phenotype?

Both original → Codominance (AB blood = A antigen + B antigen)
New intermediate → Incomplete dominance (red × white → pink flower)

The AB blood group is codominance because the AB person genuinely has both A and B antigens — not some new “AB antigen”.

This shortcut has saved marks in NEET 2022 and CBSE 2023 board papers where both terms appeared in the same question.

Common Mistake

Confusing codominance with incomplete dominance.

Students write: “In AB blood group, $I^A$ and $I^B$ blend to form a new blood group.”

This is wrong. Blending would be incomplete dominance. In codominance, there is no blending — antigen A and antigen B both appear completely and independently on the same cell. The AB blood group is not a “new” blood group phenotype; it’s the simultaneous expression of two existing ones.

CBSE examiners specifically check this distinction for full marks in 3-mark questions.

Scoring note: For 3-mark CBSE questions on codominance, examiners want: (1) definition with “simultaneous expression”, (2) genotype $I^A I^B$ with both antigens mentioned, (3) contrast with dominance/recessiveness. Hit all three and you get full marks.

Question

Solution — Step by Step

Why This Works

Alternative Method — Spotting Codominance in MCQs

Common Mistake

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