How would you classify the relationship between alleles in co-dominance?

In co-dominance, the relationship between alleles is characterized by the expression of both alleles in a distinct manner within the phenotype of the organism. This means that instead of blending or one allele completely overshadowing the other, both alleles contribute equally and visibly to the organism's characteristics. A classic example of co-dominance is seen in the ABO blood group system, where individuals with genotype IAIB express both A and B antigens on their red blood cells, leading to blood type AB.

The other choices describe relationships that do not accurately reflect the nature of co-dominance. Complete recessiveness implies that one allele would not manifest at all, while blending suggests that the traits would merge into an intermediate form rather than being expressed distinctly. Additionally, stating that alleles are not expressed contradicts the fundamental aspect of co-dominance where both traits are visibly represented. This understanding of co-dominance helps clarify how certain traits appear in offspring and the role of different alleles in genetic expression.