Co-dominance:

Co-dominance occurs when the contributions of both alleles are visible in the phenotype.

When the dominant character is not able to suppress, even incompletely the recessive character and both the characters appear side by side in F1hybrids, the phenomenon is called co-dominance. In this case the F1generation resembles both parents.
For example, in catties, if a cattle with black coat is crossed to a cattle with white colour, the F1 hybrids possess roan coat. In roan coat, both black and white patches appear separately. So, the alleles which are able to express themselves independently when present together are called co- dominant alleles

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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. The IA and IBalleles produce different modifications. The enzyme coded for by IA adds an N-acetylgalactosamine to the membrane-bound H antigen. The IB enzyme adds a galactose. The iallele produces no modification. Thus IA and IB alleles are each dominant to i (IAIA and IAiindividuals both have type A blood, and IBIB and IBi individuals both have type B blood. But IAIBindividuals have both modifications on their blood cells and thus have type AB blood, so the IA andIB alleles are said to be co-dominant.)