What is expressed when both alleles for a gene are equally expressed as in human blood type

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.

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.

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b is dominant over the allele for the white fur.