Codominance refers to a relationship between two alleles of a gene. It occurs when the contributions of both alleles at a single locus are visible and do not overpower each other in the phenotype.
For instance, in the ABO system, the IA and IB alleles are co-dominant in producing the AB blood group phenotype, in which both A- and B-type antigens are made. 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. (The medical condition produced by the heterozygous genotype is called an incomplete dominant, see above). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed into RNA Co-dominance and incomplete or semi-dominance are not the same thing. For example, in some plant species, pink flowers may be the product of a mixture of red and white pigments (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).