What is the characteristic of blood type that exemplifies codominance?

Blood type is an excellent example of codominance, which occurs when two different alleles are expressed simultaneously, leading to a phenotype that shows characteristics of both alleles. In the case of blood groups, individuals can inherit an A allele from one parent and a B allele from the other, resulting in type AB blood. In type AB blood, both A and B antigens are present on the surface of red blood cells, clearly demonstrating codominance since neither antigen overshadows the other; instead, both are fully expressed.

The other options don’t represent codominance in the same way. Type O blood, which has no antigens, reflects a different genetic inheritance pattern where both A and B alleles are absent. Dominance occurs when one allele masks the expression of another in a heterozygous condition, which does not apply here as both alleles contribute to the phenotype in codominance. Lastly, describing blood type determination by a single allele pertains to traits controlled by simple dominance rather than the simultaneous expression seen in codominance.