Codominance is a pattern of inheritance in which two versions of a gene are both fully expressed at the same time, producing a phenotype that displays both traits side by side rather than one masking the other. The most familiar example is the AB blood type: if you inherit an A allele from one parent and a B allele from the other, your red blood cells carry both A and B markers on their surface. Neither allele “wins.” Both show up.
This makes codominance different from the classic dominant-recessive model you may remember from biology class, where one allele hides the effect of the other. It’s also distinct from blending inheritance, where two traits mix into something in between. In codominance, both traits appear fully and separately in the same organism.
How Codominance Differs From Incomplete Dominance
These two concepts get confused constantly, and the difference comes down to one question: do the two traits blend together, or do they both appear distinctly?
In incomplete dominance, the result is a blend. Cross a red snapdragon with a white snapdragon, and the offspring have pink flowers. The red and white pigments mix into an intermediate color. Neither parent’s trait shows up fully.
In codominance, there’s no blending at all. Both parental traits appear in full, just in separate areas or forms. A camellia flower showing codominance, for instance, displays distinct patches of pink and white on the same petals. You can see both colors clearly, not a uniform mix of them. The key distinction is that codominant traits coexist rather than compromise.
The AB Blood Type Example
The ABO blood group system is the textbook case of codominance in humans. Your blood type depends on which sugar molecules sit on the surface of your red blood cells. The A allele codes for one type of sugar (N-acetylgalactosamine), the B allele codes for a different sugar (D-galactose), and the O allele doesn’t produce a functional surface marker at all.
If you inherit one A allele and one B allele, both enzymes are active. Your red blood cells end up decorated with both A-type and B-type sugars, giving you type AB blood. The A and B alleles are codominant with each other, while both are dominant over the O allele, which is recessive. This is why someone with type AB blood can’t pass on an O blood type to their children, since they simply don’t carry that allele.
Roan Coats in Horses and Cattle
One of the most visually striking examples of codominance appears in animal coat color. When a homozygous red horse is crossed with a homozygous white horse, the offspring can have what’s called a roan coat. From a distance, a roan animal looks like an even pinkish-gray, but up close you can see the coat is actually a mixture of individual red hairs and individual white hairs. Each hair is entirely one color or the other. Both alleles are fully expressed, just in different cells, creating a salt-and-pepper effect across the body.
This is a useful example because it shows so clearly what “both traits expressed simultaneously” actually looks like in practice. The red pigment and the white pigment don’t blend into a single pink hair. They each do their thing independently, and the result is a patchwork.
Sickle Cell Trait at the Molecular Level
Sickle cell disease offers a fascinating case where the same gene can look codominant or not, depending on what level you’re examining. At the molecular level, people who carry one normal hemoglobin allele and one sickle hemoglobin allele (carriers, sometimes called “sickle cell trait”) produce both types of hemoglobin in their red blood cells. Both alleles are actively making their respective protein. That’s codominance.
This mixture of normal and sickle hemoglobin means carriers have a population of red blood cells that includes both normal-shaped and sickle-shaped cells. Under everyday conditions, carriers typically don’t experience symptoms, which is why sickle cell trait was historically described as recessive. But at the protein level, both gene products are clearly present and functioning, making it a genuine example of codominant expression.
Why Codominance Matters for Your Immune System
Codominance plays a critical role in how your immune system recognizes threats. Your body uses a set of genes called the major histocompatibility complex (MHC) to produce surface proteins that help immune cells distinguish your own tissue from foreign invaders like bacteria and viruses. These MHC genes are codominant: if you inherit different versions from each parent, your cells display both sets of surface proteins.
This doubles the range of foreign molecules your immune system can detect. The more diverse your MHC proteins, the wider your net for catching pathogens. This is why MHC genes are among the most variable in the entire human genome. By the 1980s, researchers had already identified between 8 and 39 codominant alleles for each of the major MHC genes, and the number of possible combinations across those genes reaches into the millions. The selective advantage of being heterozygous at these genes, meaning you carry two different alleles, has been linked to stronger immune responses against infectious diseases including HIV.
Predicting Codominant Inheritance
When two organisms that are both heterozygous for a codominant trait reproduce, the offspring follow a predictable 1:2:1 ratio. One quarter will be homozygous for the first allele, one quarter homozygous for the second, and half will be heterozygous, showing both traits. This is the same genotypic ratio you get in any standard monohybrid cross, but in codominance the phenotypic ratio matches it exactly, since each genotype produces a visibly distinct result.
Compare this to simple dominance, where a cross between two heterozygotes gives a 3:1 phenotypic ratio because the heterozygotes look identical to one of the homozygotes. In codominance, you can tell all three genotypes apart just by looking, which makes these traits especially useful in genetics education and in practical applications like paternity testing and blood typing.
Codominance in Plants
Certain flowering plants make codominance easy to see. Camellias can produce flowers with distinct patches of pink and white on the same bloom, where heterozygous plants express both pigment alleles in separate groups of cells. The result isn’t a uniform pastel. It’s a mosaic of fully pigmented and fully unpigmented patches, each cell following the instructions of one allele or the other.
This patchwork pattern is the visual signature of codominance in many organisms. Whenever you see two distinct trait expressions appearing side by side in the same individual, rather than a smooth intermediate, codominance is the likely explanation.