Final answer:
Codominance is understood by the equal and simultaneous expression of both alleles in a heterozygote, as seen in human ABO blood types where both A and B antigens are equally expressed in type AB individuals.
Step-by-step explanation:
The key to the recognition of codominance is D) Equal expression of traits. This is a genetic scenario where both alleles for a gene are expressed simultaneously in a heterozygote. Unlike incomplete dominance where the phenotype is intermediate between the two alleles, codominance results in a phenotype that exhibits both alleles distinctly and equally. A classic example of codominance is in the human ABO blood type system, where alleles for both A and B antigens are expressed equally, resulting in type AB blood.
In the case of the ABO blood type, besides codominance, we also encounter multiple alleles, as there are three alleles contributing to the blood type (Iª, I³, and i). These multiple alleles can exist in the population, but any individual can only possess two of them simultaneously.