Genetic inheritance dictates how traits are passed from parents to offspring through basic units of heredity called alleles. For many traits, the dominant allele completely masks the effect of the recessive allele. However, not all genetic interactions follow these simple rules first described by Gregor Mendel. Some inheritance patterns involve a more nuanced relationship between two different alleles, leading to results unexpected under the classic dominant-recessive model.
Defining Codominant Inheritance
Codominance is an inheritance pattern where a single gene has two different alleles that are both fully expressed in a heterozygous individual. Neither allele is masked by the other, and neither is considered dominant or recessive. When an organism inherits both versions of the gene (the genotype), both alleles exert their full influence simultaneously.
Geneticists often represent codominant alleles using a base capital letter for the gene and distinct superscript letters for each specific allele. For example, in the human ABO blood group system, the gene for blood type is denoted by I, with the codominant alleles being IA and IB. This notation emphasizes that both alleles are equally capable of being expressed, resulting in the production of two distinct, fully functional protein products.
Visualizing Codominance: The Phenotype
The observable characteristics (phenotype) resulting from a codominant genotype demonstrate the full presence of both parental traits simultaneously. This result is not a blending or mixing of traits, but rather the distinct display of each one.
For example, if a trait involves color, an organism inheriting alleles for both red and white coloration will not appear pink. Instead, the appearance will show patches, spots, or distinct areas of both pure red and pure white. The resulting pattern is a clear, side-by-side expression of the two separate traits, making codominance easy to identify.
Codominance vs. Incomplete Dominance
Codominance is often confused with incomplete dominance, though they represent fundamentally different genetic outcomes. In incomplete dominance, the heterozygous individual exhibits a phenotype that is an intermediate blend of the two parental traits. For example, crossing a pure red flower and a pure white flower produces offspring with uniformly pink petals.
The pink color is a new phenotype, representing a partial expression where neither allele is fully expressed. By contrast, codominance results in the full and complete expression of both traits, with no blending occurring. In codominant coloration, the red and white traits appear as individual spots or patches on the same organism. The distinction is that incomplete dominance creates a new, blended trait, while codominance displays the two original traits simultaneously.
Common Examples of Codominance
The most widely recognized example of codominance in humans is the ABO blood group system, involving the IA and IB alleles. A person inheriting both alleles will have type AB blood. This blood type is characterized by the presence of both A antigens and B antigens fully expressed on the surface of the red blood cells.
Another example is the coat color of livestock, such as roan cattle. Roan coloring results from crossing an animal with a red coat and one with a white coat. The offspring has a coat composed of individual red hairs and individual white hairs side-by-side. This simultaneous expression creates a mixed, speckled appearance characteristic of codominance.