The inheritance of traits is fundamentally governed by genes, which exist in different versions called alleles. For any given gene, an organism inherits two alleles, one from each parent, and the relationship between these two copies determines the observable trait, or phenotype. In classical Mendelian genetics, one allele is often dominant, meaning its trait is expressed and masks the recessive allele’s trait. Codominance, however, presents a different pattern of inheritance where two different alleles are expressed equally and distinctly in the phenotype of a heterozygous individual.
The Mechanism of Simultaneous Expression
Codominance is a direct result of the organism’s genetic makeup, specifically the heterozygous state where two non-identical alleles for a single gene are present. In this scenario, both alleles are fully functional and capable of producing their respective gene products, typically proteins or enzymes. The mechanism deviates from simple dominance because neither allele is silenced or masked by the other within the cell.
Instead of one allele dictating the entire phenotype, both alleles direct the cellular machinery to synthesize their unique products independently and simultaneously. For example, if one allele codes for a specific protein structure and the other codes for a slightly different protein structure, the resulting cell will produce both versions. This simultaneous production means the traits associated with both alleles are displayed at the same time.
The cellular environment allows for the activity of both genetic instructions without one interfering with the function or visibility of the other. The two traits are not chemically or structurally combined; they simply exist side-by-side, each fully representing its corresponding allele.
Distinguishing Codominance from Incomplete Dominance
Codominance is frequently confused with another non-Mendelian pattern called incomplete dominance, but the distinction lies in the final appearance of the heterozygous organism. In incomplete dominance, the heterozygous phenotype is an intermediate blend of the two parental phenotypes. If a red flower is crossed with a white flower, for instance, incomplete dominance results in offspring with pink flowers, a completely new color that is a mix of the two parental colors.
This blending occurs because the dominant allele’s product is insufficient to produce the full trait, and the recessive allele’s product is either absent or non-functional. The resulting phenotype is a dilution or combination of the two original traits, like mixing red and white paint to get pink.
Codominance, by contrast, does not involve any blending or intermediate color. Instead, both parental traits appear fully and distinctly in the offspring, just in different regions or parts of the organism. If a red-flowered plant and a white-flowered plant exhibited codominance, the offspring would have flowers with both red petals and white petals, or distinct red and white patches.
The visual difference is that incomplete dominance results in a homogeneous mix, where every part of the trait is the same intermediate color. Codominance results in a heterogeneous display, where the parent traits are physically visible at the same time, such as distinct patches or spots of each original color.
Classic Examples of Codominant Traits
One of the most widely recognized instances of codominance in humans is the inheritance of the AB blood type within the ABO blood group system. The gene responsible for blood type has three main alleles: \(I^A\), \(I^B\), and \(i\). The \(I^A\) and \(I^B\) alleles are codominant with respect to each other, while both are dominant over the recessive \(i\) allele.
An individual with the \(I^A I^B\) genotype has type AB blood because both the \(I^A\) allele and the \(I^B\) allele are fully expressed. The \(I^A\) allele instructs the red blood cells to produce the A antigen on their surface, and the \(I^B\) allele simultaneously instructs the same cells to produce the B antigen. The resulting red blood cell features both A and B antigens displayed on its membrane.
A visible example of this pattern is the roan coat color seen in certain breeds of cattle and horses. Roan animals are the heterozygous offspring of a cross between a parent with a solid red coat and a parent with a solid white coat. The resulting roan coat is not a uniform pink or light red, which would be incomplete dominance, but rather a mixture of individual red hairs and individual white hairs.
Each hair is either fully red or fully white, with no intermediate color. This patchy or speckled appearance across the animal’s body confirms that both the allele for red hair pigment and the allele for white hair are active and fully expressed in the animal’s cells.