Codominance is a pattern of inheritance in which two different versions of a gene are both expressed in an individual who carries them. Neither allele completely masks the other. Instead, the observable traits associated with both alleles appear together.
A classic example is the AB blood type. A person with one A allele and one B allele has type AB blood. Their red blood cells display both A and B antigens. The result is not a blend of A and B; both traits are expressed distinctly.
Codominance is easiest to understand by comparing it with other inheritance patterns, especially complete dominance and incomplete dominance. Once those distinctions are clear, several real genetic examples make the pattern much easier to recognize.
What does codominance mean?
Humans typically have two copies of each autosomal gene, one inherited from each biological parent. Different forms of the same gene are called alleles.
In a codominant relationship, an individual who inherits two different alleles—called a heterozygote—expresses information from both alleles.
For example, suppose a gene has two alleles, A and B. If A and B are codominant, an individual with genotype AB expresses the characteristics associated with A as well as those associated with B.
The important point is that the alleles do not merge into an intermediate version of the trait. Both remain identifiable in the phenotype, the observable characteristics of the organism.
Codominance therefore describes a relationship between alleles, not a situation in which one allele is simply “stronger” than another.
Codominance versus complete dominance
In complete dominance, one allele masks the effect of another in a heterozygote.
For a simple example, imagine a gene with a dominant allele A and a recessive allele a. An individual with genotype Aa shows the dominant phenotype. The recessive allele is still present in the person’s DNA, but its associated phenotype is not ordinarily visible in that heterozygous individual.
Codominance works differently:
| Inheritance pattern | Heterozygous genotype | What is expressed? |
|---|---|---|
| Complete dominance | Aa | Primarily the dominant allele’s phenotype |
| Incomplete dominance | Aa | An intermediate phenotype |
| Codominance | AB | Both alleles’ phenotypes |
This distinction is especially important because codominance is sometimes incorrectly described as one trait “blending” with another.
Codominance versus incomplete dominance
Incomplete dominance occurs when neither allele completely determines the phenotype and the heterozygote has an intermediate appearance.
A familiar textbook example involves certain flower colors: if one allele contributes red pigmentation and another contributes white pigmentation, a heterozygote may produce pink flowers. The phenotype falls between the two parental phenotypes.
Codominance is different. If two alleles are codominant, the heterozygote does not produce an intermediate version. Instead, both characteristics can be observed simultaneously.
A useful way to distinguish them is:
- Codominance: A + B → A and B are both expressed.
- Incomplete dominance: A + B → an intermediate phenotype.
The distinction concerns how the alleles affect the phenotype, not whether the alleles themselves physically mix. DNA sequences do not blend together simply because two different alleles are inherited.
The ABO blood group is a classic human example
The ABO blood group system provides one of the clearest examples of codominance in humans.
The ABO gene has three common alleles, traditionally represented as Iᴬ, Iᴮ, and i. The Iᴬ and Iᴮ alleles are codominant with each other, while i is recessive to both.
The Iᴬ allele leads to production of the A antigen on red blood cells, while Iᴮ leads to production of the B antigen. A person who inherits both has genotype IᴬIᴮ.
Because the two alleles are codominant, that person’s red blood cells carry both A and B antigens. The person’s blood type is therefore AB.
The relevant genotypes can be summarized as follows:
| Genotype | Blood type |
|---|---|
| IᴬIᴬ or Iᴬi | A |
| IᴮIᴮ or Iᴮi | B |
| IᴬIᴮ | AB |
| ii | O |
This example also shows why a gene can have more than two alleles in a population while an individual still carries only two copies of an autosomal gene.
The AB phenotype is codominant because the A and B alleles are both expressed in the heterozygote. It is not an intermediate blood type between A and B.
Sickle cell hemoglobin provides another important example
The genetics of sickle cell disease illustrate codominance at the molecular level, although the medical phenotype is more complicated than a simple dominant-versus-recessive label suggests.
The relevant gene is HBB, which encodes a component of hemoglobin, the protein that carries oxygen in red blood cells. Two commonly discussed alleles are the normal hemoglobin allele, often designated HbA, and the sickle hemoglobin allele, HbS.
A person with genotype HbA/HbS produces both types of hemoglobin: normal hemoglobin and sickle hemoglobin. In that molecular sense, the two alleles are codominantly expressed.
However, this does not mean that a person with HbA/HbS simply has “half normal blood and half sickle disease.” The biological consequences depend on the amount and behavior of different hemoglobin forms, as well as other genetic and environmental factors.
This example demonstrates why inheritance terminology should be applied carefully. Codominance describes the expression of alleles; it does not by itself predict the severity of a disease.
People with two HbS alleles can develop sickle cell disease, whereas people with one HbS allele and one HbA allele generally have sickle cell trait rather than sickle cell disease.
Roan coat color illustrates codominance in animals
Codominance is also observed in animals. Roan coloration in cattle is a commonly used example.
Consider cattle with alleles associated with red and white hairs. A heterozygous roan animal can have a coat containing both red hairs and white hairs.
The hairs themselves do not necessarily become a uniform intermediate color. Instead, the two types of pigmentation are present together and can be distinguished.
That makes roan coloration a useful visual illustration of codominance:
- One homozygous genotype produces predominantly red hair.
- Another homozygous genotype produces predominantly white hair.
- The heterozygote produces a mixture of red and white hairs.
The heterozygous phenotype contains evidence of both alleles rather than a single blended color.
Codominance can involve molecular products, not just visible traits
A common misconception is that codominance must produce two visibly different physical features. In reality, the underlying evidence for codominance may be found at the molecular or cellular level.
For example, two codominant alleles can cause a heterozygous person to produce two distinguishable versions of a protein. The products of both alleles can be detected even if the person does not have two obviously different physical characteristics.
This is one reason modern genetics often examines gene expression, proteins, enzymes, and cellular markers rather than relying only on visible traits.
The phenotype is the observable result of genetic and environmental influences. Codominance concerns the fact that both allelic contributions are expressed rather than one completely concealing the other.
How codominance appears in a genetic cross
A simple genetic cross can show why codominance produces a distinctive heterozygous phenotype.
Suppose a hypothetical gene has two codominant alleles, R and W. An RR individual expresses the R-associated phenotype, while WW expresses the W-associated phenotype. An RW individual expresses both.
If two heterozygous individuals reproduce:
RW × RW
Each parent can contribute either R or W. The possible offspring genotypes are:
- RR
- RW
- RW
- WW
So the expected genotype ratio is 1 RR : 2 RW : 1 WW.
Because the heterozygote has its own phenotype—the simultaneous expression of both alleles—the phenotype ratio in this simplified example is also 1 : 2 : 1.
This differs from a simple dominant-recessive cross, where two heterozygotes commonly produce a 3:1 phenotype ratio because the heterozygous and dominant-homozygous individuals share the same phenotype.
The 1:2:1 pattern is therefore often a useful clue when studying a straightforward codominant trait, although real genetic systems can be more complicated.
What codominance does not mean
Codominance does not mean that two alleles are equally beneficial, equally powerful, or equally common.
It also does not mean that the DNA from two alleles physically combines into a new allele. Each allele remains a distinct DNA sequence.
Likewise, codominance does not mean that every characteristic associated with both alleles must be equally visible. Gene expression can be influenced by regulatory mechanisms, cell type, developmental stage, and other genes.
Most importantly, dominant does not mean stronger and recessive does not mean weaker. In genetics, dominance describes the relationship between alleles and their effects on a phenotype in a particular context.
Why codominance matters in genetics
Recognizing codominance helps explain why a person’s phenotype cannot always be predicted by treating every gene as simply dominant or recessive.
The ABO blood group system is a practical example. A person with type AB blood inherited two different alleles, and the presence of both A and B antigens reflects their codominant expression.
Codominance also illustrates a broader principle of genetics: genes can influence traits through relationships more complex than one allele simply overriding another. Understanding those relationships makes it easier to interpret pedigrees, genetic crosses, blood groups, molecular tests, and inherited conditions.
At its core, codominance is straightforward: when two different alleles are codominant, a heterozygous individual can express the characteristic effects of both alleles at the same time. The ABO blood group system provides a particularly clear human example, while hemoglobin variants and roan coat coloration show how the same principle can appear at molecular and organismal levels.
