Codominance is a pattern of inheritance where both versions of a gene (called alleles) are fully expressed at the same time in a heterozygous individual. Instead of one allele masking the other, both contribute visibly to the trait. The most familiar example is AB blood type, where a person inherits one A allele and one B allele, and both proteins appear on the surface of their red blood cells equally.
How Codominance Works at the Cellular Level
Every person carries two copies of most genes, one from each parent. In simple dominant inheritance, only one allele’s instructions get noticed in the final trait. The other allele is there, but it’s effectively invisible. Codominance breaks that pattern: both alleles independently produce their own protein, and both proteins show up in the body without interfering with each other.
Take the ABO blood group as an example. The A allele codes for a specific protein that sits on the surface of red blood cells. The B allele codes for a different surface protein. The O allele codes for no protein at all. If you inherit one A allele and one B allele, your cells manufacture both the A protein and the B protein in equal amounts. The result isn’t some blend of A and B. It’s a distinct phenotype: type AB, where both proteins coexist on every red blood cell. This is what makes codominance different from a simple mixing effect.
Codominance vs. Incomplete Dominance
These two concepts get confused constantly, and the distinction matters. In codominance, both alleles are fully and separately visible in the trait. In incomplete dominance, the two alleles blend together into an intermediate trait that looks like neither parent.
The classic incomplete dominance example is snapdragon flowers. Cross a red-flowered plant with a white-flowered plant, and the offspring are pink. The red and white haven’t both shown up side by side. They’ve blended into something in between. Compare that to camellia flowers, where petal color follows codominance. Cross a red camellia with a white one, and the offspring don’t turn pink. Instead, individual petals display both red and white patches on the same flower. Both colors are distinctly present, not merged.
Here’s a quick way to remember the difference:
- Complete dominance: only one allele’s trait is visible
- Incomplete dominance: the two alleles blend into an intermediate trait
- Codominance: both alleles’ traits are separately and simultaneously visible
Sickle Cell Trait as Codominance
AB blood type is the textbook example, but sickle cell trait is equally important and often overlooked as a case of codominance. Sickle cell disease involves a change in hemoglobin, the protein inside red blood cells that carries oxygen. A single amino acid difference alters the shape of the hemoglobin molecule, which in turn warps the red blood cell into an elongated, curved “sickle” shape.
People who inherit one normal hemoglobin allele and one sickle hemoglobin allele are heterozygous carriers. Their bodies produce both normal hemoglobin and sickle hemoglobin simultaneously. Under a microscope, you can see a mixture of normally shaped red blood cells and sickle-shaped ones circulating together. Both alleles are independently expressed, and both protein products coexist in the same person. That’s codominance in action. These individuals typically don’t experience the severe symptoms of full sickle cell disease, but the presence of both hemoglobin types is measurable and clinically relevant.
What Happens in a Codominant Cross
When two heterozygous codominant individuals have offspring, the expected ratio follows the same basic genetics as any monohybrid cross, but the phenotypic ratio looks different from what you’d see with simple dominance. With complete dominance, crossing two carriers gives a 3:1 phenotypic ratio because heterozygotes look identical to one of the homozygous parents. With codominance, heterozygotes have their own distinct appearance, so the phenotypic ratio matches the genotypic ratio: 1:2:1.
Using blood type as an example, if both parents are type AB, one quarter of their children would be expected to have type A, half would have type AB, and one quarter would have type B. Each genotype produces a visibly different phenotype because there’s no masking going on.
Why Codominance Matters for Blood Transfusions
Understanding codominance isn’t just an academic exercise. It has direct consequences in medicine, particularly for blood transfusions. Because people with type AB blood express both A and B surface proteins on their red blood cells, their immune system recognizes both proteins as “self.” This means AB individuals can receive red blood cells from type A, type B, type AB, or type O donors, making them universal recipients. Conversely, if type AB blood were given to someone with type A, that person’s immune system would attack the B proteins it doesn’t recognize.
The same principle applies in genetic counseling. When parents want to understand the likelihood of their children inheriting certain traits, codominance changes the math. Unlike recessive conditions where carriers show no outward signs, codominant traits are always visible in carriers. A person with sickle cell trait, for instance, can be identified through a blood test showing both hemoglobin types, which helps families understand inheritance risks before symptoms ever appear.
Codominance in Plants and Animals
Codominance shows up across many species, not just humans. In cattle, crossing a red-coated animal with a white-coated one can produce offspring with roan coloring, where individual red and white hairs grow side by side across the coat. From a distance, it might look blended, but up close, each hair is distinctly one color or the other. Both alleles are doing their job independently.
Camellia flowers offer one of the most visually striking plant examples. Red camellias crossed with white camellias produce flowers with clearly defined red and white sections on the same petal. If you cross a homozygous red camellia with a heterozygous red-and-white one, about half the offspring will be solid red and half will display the red-and-white pattern. The white patches aren’t diluted pink. They’re fully white, sitting right next to fully red areas, because both alleles are expressed without compromise.