How is codominance different from complete dominance?

Codominance is characterized by the full expression of both alleles in a heterozygous individual, leading to distinct and separate expressions of both traits without blending. This means that instead of one allele completely masking the effect of another, as seen in complete dominance, both alleles contribute equally to the organism's phenotype. A classic example of codominance is seen in the ABO blood group system, where individuals with one allele for type A and one for type B express both A and B antigens on the surface of their red blood cells.

Complete dominance, on the other hand, results in the phenotype of the dominant allele being expressed exclusively in the presence of a recessive allele, effectively masking the recessive trait. For instance, in a complete dominance scenario, if an allele for tall plants dominates an allele for short plants, the resulting plants will all exhibit the tall phenotype regardless of the presence of the short allele.

This distinction highlights why the assertion that both alleles are expressed fully in codominance accurately captures its essence, while other statements do not appropriately describe this genetic interaction.