Codominance: When Both Alleles Are Fully Expressed

What Codominance Is and Where It Occurs

Codominance is a pattern of allelic interaction at a single genetic locus where a heterozygote carrying two different alleles expresses both alleles fully and independently. It is one of the classic departures from simple Mendelian dominance, alongside incomplete dominance and multiple allelism.

The hallmark is that the two allele products do not compete, blend, or mask one another — they are both made, and both are detectable in the phenotype. This happens most cleanly when:

• Each allele encodes a functional, distinguishable gene product (an enzyme, a cell-surface antigen, a structural protein).
• The products act on separate substrates or occupy separate molecular targets, so neither interferes with the other.

Codominance appears at the level of molecular phenotypes: blood-group antigens (ABO, MN, Ss), serum proteins, and HLA/MHC molecules, where a heterozygote co-expresses both maternal and paternal variants on every cell. It also shows at the tissue level, as in roan and calico coats, where two cell populations each display a different allele's output side by side.

The Mechanism, Step by Step

Codominance arises because both alleles are transcribed and translated, and each protein carries out its own biochemical job. Trace it through the ABO system:

• Step 1 — Shared substrate. Nearly everyone builds the H antigen: a fucose-terminated oligosaccharide on red-cell glycoproteins and glycolipids, made by the FUT1 (H) fucosyltransferase.
• Step 2 — Allele-specific enzymes. The ABO gene (chromosome 9q34) encodes a glycosyltransferase. The I^A allele makes α-1,3-N-acetylgalactosaminyltransferase, which adds GalNAc to H to form A antigen. The I^B allele makes α-1,3-galactosyltransferase, which adds galactose to H to form B antigen. The two enzymes differ by only ~4 amino acids (notably at residues 266 and 268) that reset sugar-donor specificity.
• Step 3 — Co-expression. In an I^A I^B heterozygote, both enzymes are made. Each independently modifies H-antigen chains, so the red-cell surface displays both A and B antigens simultaneously.

Because both proteins are functional and act on abundant substrate, neither dominates — the phenotype is genuinely additive, not blended.

Key Molecules and Concrete Numbers

ABO blood group (I^A I^B → AB): A mature red cell carries roughly 1–2 million A/B antigen sites in a type-A or type-B individual; in an AB person these are split between the two antigen types. The immunodominant sugar for A is N-acetylgalactosamine; for B it is galactose — a single-sugar difference on the terminal H structure. The O allele carries a single-base deletion (G at position 261, c.261delG) causing a frameshift and a truncated, catalytically dead ~117-residue protein — which is why O is recessive while A and B are codominant to each other.

• MN blood group: alleles L^M and L^N of glycophorin A (GYPA) differ at amino acids 1 and 5; genotype L^M L^N produces both M and N antigens — a second clean human codominant example.
• Roan coat (KIT locus): heterozygotes intermix fully red and fully white individual hairs; each hair follicle expresses one allele's pigment output, so the animal shows both colors at once rather than a diluted pink.

How Codominance Is Detected and Studied

Codominance is unusually easy to confirm because both allele products can be measured directly, rather than inferred from a masked phenotype.

• Serological typing: The 1900 discovery of ABO by Karl Landsteiner (Nobel Prize, 1930) relied on agglutination — anti-A and anti-B antibodies clump red cells. AB blood agglutinates with both reagents, visibly demonstrating co-expression. This is still the routine crossmatch test in every blood bank.
• Electrophoresis: Codominant protein variants (e.g., hemoglobin, serum esterases, HLA) resolve as two separate bands in a heterozygote — one for each allele — the workhorse of classical population genetics.
• DNA and molecular markers: SNPs, RFLPs, and microsatellites are inherently codominant: a heterozygote shows both alleles on a gel or chromatogram, which is why they are prized for mapping, paternity, and forensic profiling.

At the tissue level, codominant coat patterns are scored simply by counting hairs of each color under magnification.

Codominance vs. Its Close Cousins

Students most often confuse codominance with incomplete dominance, but the distinction is sharp:

• Incomplete dominance gives a blended intermediate. A red × white snapdragon yields pink because one functional allele makes roughly half the pigment — a quantitative, dose-dependent average. Nothing new coexists; one product is simply reduced.
• Codominance gives both phenotypes at once, unblended. AB blood is not a fusion antigen; it is A and B, both present. Roan is not pink; it is red hairs and white hairs.

Codominance also differs from:

• Complete dominance, where the recessive allele's product is missing or masked (e.g., O is recessive to A and B).
• Multiple alleles, a population-level concept (ABO has ≥3 alleles) that combines with codominance rather than opposing it.
• Pleiotropy and epistasis, which involve one gene affecting many traits, or one gene masking another gene — not two alleles of the same gene.

Why It Matters: Medicine, Forensics, and Open Questions

Codominance is not a curiosity — it underpins transfusion medicine. Because A and B antigens are co-expressed, an AB individual is the universal plasma donor and universal red-cell recipient; mismatches trigger antibody-mediated hemolysis that can be fatal. ABO typing before every transfusion and organ transplant is a direct clinical application of this allelic relationship.

• Forensics and parentage: Codominant markers (STRs, SNPs) let both alleles be read directly, enabling exclusion-based paternity testing and DNA profiling with high discriminating power.
• Population genetics: Codominant loci reveal true genotype frequencies, so they feed cleanly into Hardy–Weinberg analysis without the ambiguity of hidden recessives.
• Disease associations: ABO type modulates risk for certain infections, thrombosis, and, notably, was linked to differential COVID-19 susceptibility.

Open questions include why some KIT-associated roan alleles are embryonic-lethal when homozygous in certain species, and how epistatic modifiers (e.g., MC1R) fine-tune the density of the two hair populations — reminders that even a clean codominant locus sits inside a complex regulatory network.

Codominance vs. incomplete dominance vs. complete dominance in a heterozygote (A₁A₂)

Feature	Complete dominance	Incomplete dominance	Codominance
Heterozygote phenotype	Same as dominant homozygote	Intermediate blend of the two	Both parental phenotypes shown together
Example	Round vs. wrinkled peas (Rr = round)	Red × white snapdragon = pink	I^A I^B = blood type AB
Molecular basis	One allele's product is nonfunctional or masked	One functional allele gives ~50% product; dose-dependent	Both alleles make distinct functional products
Is anything blended?	No — one masks the other	Yes — quantitative averaging	No — two products coexist
Detectable at molecular level?	Recessive allele often silent	Reduced amount of one product	Both products directly detectable (e.g., A and B antigens)