Codominance Explained- Definition and Examples

What Is Codominance?

Codominance is a genetic inheritance pattern where both alleles are fully expressed in the offspring. Neither allele hides or blends into the other. The result? A phenotype that shows both parental traits simultaneously.

Think about it this way: if you cross a red-flowered plant with a white-flowered plant and both colors appear in the offspring—not pink, not a blend, but actual red AND white patches—that's codominance.

Simple enough? Good. Let's dig deeper.

Codominance vs. Incomplete Dominance

Students mix these up constantly. Here's the difference:

Incomplete dominance: The alleles blend. Red + White = Pink. You get something in between.
Codominance: Both alleles show fully. Red + White = Red AND White side by side. No blending.

This distinction matters. Incomplete dominance creates intermediate phenotypes. Codominance creates dual expression—both traits visible at once.

Real-World Examples of Codominance

MN Blood Group

Your blood type isn't just about A, B, and O. The MN blood group system shows codominance in action. People with the MM genotype have M antigens on their red blood cells. People with NN have N antigens. People with MN have both M and N antigens on every single cell.

Neither allele dominates. Both get expressed. That's codominance.

Roan Cattle

When you cross a red cow with a white cow, you don't get pink calves. You get roan—cattle with a coat that's a roughly 50/50 mix of red and white hairs. The red hairs and white hairs exist side by side throughout the animal's body.

This is the textbook example instructors love, and for good reason. It's visual, it's obvious, and it demonstrates the principle cleanly.

ABO Blood Types

The ABO system has a codominant element. The A and B alleles are both expressed in individuals with the AB genotype. Type AB blood carries both A and B antigens simultaneously.

However, O remains recessive to both A and B. So ABO is partially codominant—A and B show codominance, but neither is dominant over O.

Sickle Cell Trait

People with sickle cell trait have one normal hemoglobin allele and one sickle cell allele. In most conditions, both forms of hemoglobin are produced. This is another example where codominance plays a role in human genetics.

How to Work With Codominance in Genetics Problems

You'll encounter Punnett squares for codominant traits. The process is straightforward, but the notation trips people up.

Use superscript letters to denote the different alleles. For example:

R^W = White allele
R^R = Red allele

A cross between a homozygous white (R^WR^W) and homozygous red (R^RR^R) produces all heterozygous offspring (R^WR^R)—all roan.

Punnett Square Example

Cross: R^WR^W × R^RR^R

	R^W	R^W
R^R	R^WR^R	R^WR^R
R^R	R^WR^R	R^WR^R

Result: 100% roan offspring. All four squares show the same heterozygous genotype.

Comparison: Dominance Patterns

Pattern	Alleles	Phenotype Result	Example
Complete dominance	1 dominant, 1 recessive	Only dominant shows	Tall vs. short pea plants
Incomplete dominance	2 different alleles	Blended intermediate	Red + White = Pink flowers
Codominance	2 different alleles	Both traits fully visible	MN blood, Roan cattle
Multiple alleles	3+ alleles at one locus	Varies by combination	ABO blood types

Getting Started: Identifying Codominance

When you see a genetics problem, ask yourself:

Are there two distinct, non-blended traits appearing together?
Does the heterozygote look like both homozygotes simultaneously?
Is there no intermediate shade or blend?

If yes to all three, you're looking at codominance. The offspring will express the phenotype of both parents without mixing.

Why This Matters

Codominance isn't just a test question topic. It appears in real genetic testing, blood transfusion compatibility, and understanding inherited conditions. The ABO system alone determines medical decisions daily.

Once you see the pattern—full expression of both alleles, no blending—you'll spot it everywhere in genetics problems and real biology.