When an organism inherits two different versions of the same gene, known as alleles, their interaction determines the resulting trait, or phenotype. Inheritance is not always a simple case of one allele overpowering the other. Codominance is a distinct pattern of inheritance where a heterozygous organism expresses both of its different genetic instructions simultaneously.
The Mechanism of Codominance
Codominance is an inheritance pattern where a heterozygous individual fully expresses the distinct traits of both alleles they possess for a particular gene. Both alleles are completely functional and produce their respective gene products simultaneously. Neither allele masks the effect of the other.
The resulting phenotype is not a mixture or an intermediate blend of the two parental traits. Both traits are visible side-by-side, each retaining its original appearance. This occurs because the cell translates the genetic information from both alleles into functional products, often proteins, in roughly equal amounts.
This full expression of both alleles differentiates codominance from classic Mendelian dominance, where one allele would be completely hidden. If the two alleles code for different surface proteins on a cell, a heterozygous individual will have both types of proteins present. This results in a phenotype distinctly different from either homozygous parent.
Key Biological Examples
The human ABO blood group system provides a classic example of codominance. This system involves three alleles: \(I^A\), \(I^B\), and \(i\), which determine the presence of specific sugar molecules, called antigens, on the surface of red blood cells. The \(I^A\) allele codes for the A antigen, and the \(I^B\) allele codes for the B antigen.
The \(I^A\) and \(I^B\) alleles are codominant. If an individual inherits both (\(I^A I^B\) genotype), their red blood cells express both the A and B antigens simultaneously. This results in blood type AB, which is distinct from type A, B, or O blood.
Another visible example of codominance is seen in the coat color of livestock, such as roan cattle or horses. A cross between a homozygous red-coated animal and a homozygous white-coated animal produces offspring with a roan coat. This roan color is not a uniform pink or light red, which would suggest blending.
Instead, the animal’s coat is made up of individual hairs that are either fully red or fully white, interspersed across the body. The alleles for red and white hair are both expressed in the skin cells, but they express in different, separate cells. This results in a patchwork appearance, demonstrating that the genetic instructions for both pigments are expressed without blending.
Codominance vs. Other Inheritance Patterns
Codominance is often compared to two other major inheritance patterns: complete dominance and incomplete dominance. In complete dominance, the heterozygous individual expresses only the trait of the dominant allele, completely masking the recessive allele. The visible outcome in the heterozygote is identical to the homozygous dominant individual.
In contrast, incomplete dominance results in a blended or intermediate phenotype in the heterozygote. Neither allele is fully dominant, and the two traits appear to mix, much like combining two colors of paint. For example, a cross between a red flower and a white flower might yield offspring with pink flowers.
Codominance is distinguished because the heterozygous phenotype shows both parental traits distinctly and simultaneously, rather than a blend. While incomplete dominance creates an intermediate, third trait, codominance results in a third trait where both original traits are fully recognizable. This difference (side-by-side expression versus a blended expression) is the primary factor used to differentiate codominance.