For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (IA, IB) and dominant over the recessive i at the ABO locus.
While it is often convenient to talk about a recessive allele or a dominant trait, dominance is not inherent to either an allele or its phenotype.