What is the definition of codominance?

Codominance is a genetic scenario where both alleles contribute equally to the phenotype of an organism. In this situation, neither allele is dominant over the other, which means that traits from both alleles can be observed simultaneously in the organism. A classic example of codominance is seen in the ABO blood group system, where an individual with one allele for type A and one for type B blood expresses both type A and type B antigens on the surface of red blood cells, resulting in the AB blood type.

This concept is distinct from other forms of dominance, such as complete dominance, where one allele masks the expression of another, or incomplete dominance, where the phenotype is a blend of both alleles. Understanding codominance is essential for grasping the complexities of inheritance patterns and how specific traits can manifest distinctly in a population.