|This article does not cite any references or sources. (January 2012)|
In genetics, codominance is a phenomenon in which a single gene has more than one dominant allele. An individual who is heterozygous for two codominant alleles will express the phenotypes associated with both alleles.
Codominance and incomplete or semidominance are not the same thing. For example, in some plant species, white and red spotted flowers may be the product of codominance between the red allele for the gene and the white allele for the gene (co-dominance on the pigment level, no dominance on the color level), or the result of one allele that produces the usual amount of red pigment and another non-functional allele that produces no pigment, so as to produce a dilute, intermediate pink color (no dominance at either level).
In the ABO blood group system, the IA and IB alleles are codominant to each other. Individuals who are heterozygous for the IA and IB alleles express the AB blood group phenotype, in which both A- and B-type antigens are present on the surface of red blood cells.
Another example occurs at the locus for the beta globin component of hemoglobin, where the three molecular phenotypes of HbA/HbA, HbA/HbS, and HbS/HbS are all equally detectable by protein electrophoresis. For most gene loci at the level, both alleles are expressed co-dominantly, because both are transcribed into RNA.