kodominan

Codominance, or kodominan, is a pattern of genetic inheritance in which both alleles at a single gene locus are fully expressed in the heterozygous phenotype. In codominance, neither allele masks the other, and the phenotype displays traits from both alleles.

By contrast, complete dominance occurs when one allele completely masks the other, and incomplete dominance yields an intermediate phenotype that blends the two alleles.

Among the best-known examples is the ABO blood group system. For the IA and IB alleles, heterozygotes IAIB express both A and B antigens on red blood cells, producing the AB blood type. The O allele is recessive to both IA and IB. Other codominant systems include the MN blood group, where the M and N alleles are expressed equally on the surface of red blood cells, and roan coat color in some cattle and horses, where heterozygotes show a mixture of red and white pigments.

Mechanistically, codominant alleles encode gene products that operate independently and contribute to the phenotype. Expression levels can vary, and in some cases one allele may be more productively expressed in certain tissues. The pattern can affect clinical interpretation and breeding outcomes.

Codominance is an important concept in genetics and population biology because it preserves genetic diversity in heterozygotes and provides distinct phenotypes in some cases. The term codominance was popularized in the early 20th century with the ABO system serving as a canonical example of the phenomenon.