As both the genes get equally expressed in the human blood group genes, both the proteins or characters expressed by those genes get prominent in the phenotype of the organism.
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Explanation:</u></h3>
In case of human blood type, there are co dominance between the genes coding for A blood group and B blood group. Absence of both of them produce the O blood group and presence of both of them produce the AB blood group.
The gene for A blood group codes for enzyme α-1,3-N-acetylgalactosamine transferase which attaches the N Acetylgalactosamine to surface of RBCs which are actually the A antigens.
The gene for B blood group codes for α-1,3-galactosyl transferase which attaches galactose to surface of RBCs which are actually B antigens.
Presence of both of these genes make both N Acetylgalactosamine and galactose to be present on RBC membrane. This is why co dominance occurs in case of AB blood group persons.
