Codominance describes a genetic inheritance pattern where two different versions of a gene contribute to an observable characteristic. Both are fully and equally expressed in an individual. This results in the simultaneous display of distinct traits from each genetic contribution, without any blending or dilution of either characteristic.
Genetic Building Blocks
Organisms inherit characteristics through genes, which are segments of DNA located on chromosomes. Each gene carries the instructions for a specific trait. Different versions of the same gene are known as alleles. An individual typically inherits two alleles for each gene, one from each parent. These allele combinations determine the observable traits, or phenotypes, an organism displays.
The Core Concept of Codominance
Codominance occurs when two different alleles for a single gene are both fully and equally expressed in a heterozygous individual. In this inheritance pattern, neither allele is dominant nor recessive to the other, meaning both contribute to the observable outcome. The traits associated with each allele are simultaneously visible, rather than one trait being hidden or a blended intermediate appearing. The resulting phenotype clearly shows the presence of both parental characteristics side-by-side, without any mixing or modification of either trait.
Real-World Examples
The human ABO blood group system illustrates codominance. This system involves three alleles: Iᴬ, Iᴮ, and i. The Iᴬ allele produces A antigens on red blood cells, while the Iᴮ allele produces B antigens. The i allele does not produce any antigens.
When an individual inherits both the Iᴬ and Iᴮ alleles, their red blood cells express both A and B antigens simultaneously. This results in blood type AB. Neither the A nor the B antigen expression is suppressed or blended; both are present on the cell surface.
Another clear example of codominance is seen in the coat color of roan cattle or horses. This trait involves alleles for red coat color and white coat color. A roan animal inherits one allele for red hair and one allele for white hair.
Instead of producing a blended color like pink, the roan animal exhibits both red hairs and white hairs distinctly interspersed across its coat. Each hair follicle expresses the color determined by its specific allele, resulting in a speckled appearance where both parental colors are plainly visible. This simultaneous presence of individual red and white hairs exemplifies the full and equal expression characteristic of codominance.
Codominance Versus Incomplete Dominance
Understanding codominance often involves distinguishing it from incomplete dominance, another form of non-Mendelian inheritance. In incomplete dominance, the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For instance, if a red-flowered plant is crossed with a white-flowered plant, and the inheritance is incompletely dominant, the offspring will have pink flowers.
This pink color represents a mixing or dilution of the parental traits, where neither the red nor the white allele is fully expressed independently. The resulting phenotype is entirely new and does not distinctly show either original parental color. The amount of pigment produced by a single allele is insufficient for full color, leading to the intermediate appearance.
Codominance, in contrast, results in both alleles being fully and separately expressed in the heterozygote. There is no blending of traits; instead, both parental characteristics are observable side-by-side. For example, in roan cattle, the red and white hairs are individually distinct, not a uniform pink. Similarly, human AB blood type displays both A and B antigens, not a combined or intermediate antigen. This fundamental difference—the distinct, simultaneous expression in codominance versus the blended, intermediate expression in incomplete dominance—is key to differentiating these two genetic patterns.