Codominance is an inheritance pattern where two different alleles for a specific trait are both fully expressed in the phenotype of a heterozygous individual. Neither allele dominates or masks the other. Instead, both alleles contribute equally to the observable characteristics, resulting in a phenotype that displays both traits simultaneously.
Understanding How Codominance Works
The mechanism of codominance involves the production of distinct products from each allele. When an organism inherits two different codominant alleles, both are transcribed and translated. This leads to the presence of both corresponding products, which then independently manifest their traits in the organism’s phenotype.
This contrasts with complete dominance, where one allele overshadows the other. For example, a plant with alleles for red and white flowers might only produce red flowers. In incomplete dominance, alleles blend to create an intermediate phenotype, such as a pink flower. Codominance is unique because both original traits remain identifiable and expressed side-by-side, without blending or being hidden.
Human ABO Blood Groups: A Key Example
The human ABO blood group system provides a key example of codominant inheritance. It is determined by a gene with three alleles: I^A, I^B, and i. The I^A allele directs A antigen production on red blood cells, while I^B leads to B antigen production. The ‘i’ allele does not code for any functional antigen.
When an individual inherits both the I^A and I^B alleles, they have the I^A I^B genotype, resulting in AB blood type. Both A and B antigens are fully expressed on their red blood cells. This simultaneous presence directly manifests codominance, as neither allele masks the other. The ‘i’ allele is recessive to both I^A and I^B; individuals with I^A i have A blood type, and those with I^B i have B blood type.
The codominant expression of the I^A and I^B alleles is significant for blood transfusions. Recognizing both A and B antigens in AB blood type is important for safe medical practices. This interaction highlights how codominance influences physiological traits.
More Examples of Codominance in Nature
Codominance is also observed in other organisms. Roan cattle serve as an example, exhibiting a coat color pattern where both red and white hairs are distinctly present. A roan cow, heterozygous for red and white coat color alleles, expresses both traits simultaneously, with individual red and white hairs interspersed.
Andalusian chickens also show codominance. When a black-feathered chicken is crossed with a white-feathered chicken, their offspring display a “blue” phenotype, which is not a solid color. It results from the codominant expression of black and white feather alleles, creating a speckled pattern that appears grey-blue.
The sickle cell trait in humans also demonstrates codominance. Heterozygous individuals (HbAS) produce both normal (HbA) and abnormal (HbS) hemoglobin. Both types are present in their red blood cells, allowing them to avoid severe sickle cell symptoms while gaining malaria resistance. This illustrates the simultaneous expression of two distinct protein products.