codominant

Codominance is a mode of inheritance in which both alleles of a gene are fully expressed in heterozygotes, resulting in a phenotype that shows both traits simultaneously. This contrasts with complete dominance, where one allele masks the other, and with incomplete dominance, where the phenotype is an intermediate blend of the two alleles.

The ABO blood group system is the classic example. The IA and IB alleles are codominant to each other, while the i allele is recessive to both. Individuals with IAIB express both A and B antigens on red blood cells, producing blood type AB. In the MN blood group, the M and N alleles are codominant, and MN heterozygotes express both antigens. Codominance also appears in some animal coat colors, such as roan cattle and horses, where heterozygotes display a pattern that reflects both alleles rather than a single blended color.

Mechanistically, codominance arises when both alleles encode functional gene products that are produced in the organism and contribute distinct phenotypic effects. There is no single dominant allele masking the other.

Historically, the concept was described in the early 20th century by William Bateson and Reginald Punnett to account for inheritance patterns that did not fit simple dominance. Codominance is a key example of how genetic expression can involve multiple alleles contributing to phenotype.

Understanding codominance aids in interpreting genetic variation, the outcomes of crosses, and practical considerations in transfusion medicine and typing systems in humans and other organisms.