Example of Codominance: Blood Type
Human blood types are encoded by a single locus with three alleles: IA, IB, and I0.
IA and IB code for two different proteins, cell surface antigen A, or antigen B. I0 codes for the lack of that particular protein.
Since we are diploid, we have a blood type, a phenotype, that depends upon the proteins on the surface of our blood cells.
IA IA and IA I0 are A, IBIB and IB I0 are B, I0I0 is O.
Ia and IB are therefore CODOMINANT with respect to each other, and both are DOMINANT with respect to I0.