What is codominance — explain with ABO blood group system

Question

What is codominance? Explain with the ABO blood group system as an example.

Solution — Step by Step

Codominance is a pattern of inheritance where both alleles in a heterozygous individual are fully expressed simultaneously. Neither allele is dominant or recessive — both show up together in the phenotype.

This is different from complete dominance (one allele masks the other) and incomplete dominance (a blended intermediate phenotype appears).

The ABO blood group is controlled by a single gene with three alleles (a situation called multiple allelism):

$I^A$ — codes for antigen A on red blood cells
$I^B$ — codes for antigen B on red blood cells
$i$ — codes for no antigen (recessive to both $I^A$ and $I^B$ )

Every person inherits two of these three alleles, one from each parent.

Genotype	Blood Group	Antigens Present
$I^A I^A$ or $I^A i$	A	Antigen A
$I^B I^B$ or $I^B i$	B	Antigen B
$I^A I^B$	AB	Both A and B
$ii$	O	Neither

The codominance is seen in genotype $I^A I^B$ . A person with this genotype has blood group AB — both antigen A and antigen B are present on their red blood cells. Neither allele dominates; both are fully expressed.

When $I^A$ and $I^B$ are together in one individual:

$I^A$ directs production of enzyme that adds A-antigen to the cell surface
$I^B$ directs production of a different enzyme that adds B-antigen to the cell surface
Both enzymes work independently — so both antigens appear

The phenotype is not a blend (not some “AB intermediate”) — both full antigens are present. This is the hallmark of codominance.

If one parent is $I^A i$ (blood group A) and the other is $I^B i$ (blood group B):

	$I^B$	$i$
$I^A$	$I^A I^B$ (AB)	$I^A i$ (A)
$i$	$I^B i$ (B)	$ii$ (O)

Offspring ratio: 1 AB : 1 A : 1 B : 1 O

The AB child (25% probability) demonstrates codominance — inheriting one allele from each parent, expressing both.

Why This Works

In complete dominance, one allele’s protein product suppresses or outcompetes the other. In codominance, both alleles produce functional, non-competing products. In the ABO system, the A and B transferases work independently on the same precursor substance (H antigen on the red cell surface) without interfering with each other. So both products — antigen A and antigen B — accumulate on the cell surface.

This independence at the molecular level is why the phenotype is the sum of both alleles’ effects, not a compromise.

Alternative Method — Contrasting with Incomplete Dominance

To cement the concept, compare:

Incomplete dominance (snapdragons): Red (RR) × White (rr) → Pink (Rr). The intermediate pink colour shows neither allele is fully expressed.

Codominance (ABO blood): Blood group A ( $I^A I^A$ ) × Blood group B ( $I^B I^B$ ) → AB ( $I^A I^B$ ). The child shows BOTH antigens — full expression of both alleles, no blending.

NEET frequently tests: “Which of the following is an example of codominance?” The answer is always ABO blood group (AB genotype). Sickle cell anaemia in heterozygous carriers (showing some normal and some sickled cells) is also sometimes cited as codominance — mention this if the question asks for additional examples.

Common Mistake

Students confuse codominance with incomplete dominance. In incomplete dominance, the phenotype is a blend (intermediate). In codominance, both phenotypes appear fully and simultaneously — no blending. For AB blood group: both antigen A AND antigen B are present — not a blended “AB antigen.” Keep this distinction clear for NEET MCQs.

Question

Solution — Step by Step

Why This Works

Alternative Method — Contrasting with Incomplete Dominance

Common Mistake

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