codominans

Codominance is a genetic phenomenon in which two different alleles at a locus are both expressed in the phenotype of the heterozygote. In codominance, neither allele is recessive, and the effects of both alleles are visible together. This differs from complete dominance, where one allele masks the other, and from incomplete dominance, where the phenotype is a blend of the two alleles.

In codominant systems, the gene products from each allele contribute separately to the phenotype. The classic example is the ABO blood group system. Individuals with IAIB have red blood cells that express both A and B antigens on their surfaces, producing type AB.

Other examples include the MN blood group, where M and N alleles are codominant, yielding red blood cells with both antigens in heterozygotes; and the roan coat color seen in some cattle and horses, where red and white pigments are both produced in different hairs, giving a mottled coat.

Genetic interpretation of codominance emphasizes that in heterozygotes, both alleles produce functional products that contribute to the phenotype, so neither allele is completely dominant over the other. Codominance is distinct from incomplete dominance and from epistasis. It helps explain non-Mendelian patterns of inheritance observed in certain blood group systems and animal coat colors, and it is considered in both breeding and medical genetics.