codominance
Imagine mixing red and white paint and getting neither pink nor one color hiding the other, but a cloth with distinct red spots and white spots side by side. In codominance, a heterozygote with two different alleles expresses both fully and at the same time — each makes its own visible contribution.
This differs from simple dominance, where one allele's product hides the other, and from incomplete dominance, where the two products blend into an intermediate. In codominance there is no masking and no blending: both gene products are made, and both leave a detectable mark on the phenotype.
Codominance is common at the molecular level, where a heterozygote often produces both protein variants. The blood groups are the textbook case, but the principle extends to any pair of alleles whose products can both be observed — for instance both forms of a protein appearing on a laboratory gel.
A person with the AB blood group has both A and B antigens on their red cells — the A and B alleles are codominant, each producing its own antigen.
Both alleles expressed, both antigens present — the signature of codominance.
The simple test: in a heterozygote, do you see both alleles' effects (codominance) or a single blended one (incomplete dominance)? Both, distinctly, means codominance.