What best defines codominance in genetics?

Codominance in genetics occurs when both alleles in a heterozygous individual contribute equally and visibly to the phenotype. This means that neither allele is dominant or recessive; instead, the traits associated with both alleles are expressed simultaneously. A classic example of codominance can be observed in the ABO blood group system, where individuals with genotype IAIB will express both type A and type B antigens on their red blood cells.

In contrast, the other definitions provided fall under different genetic principles. One allele completely masking the effect of another describes complete dominance, where the dominant allele overshadows the recessive one in the phenotype. Only one allele being expressed represents a scenario of complete dominance or sometimes incomplete dominance, where the phenotype may be a blend rather than a display of both traits. Lastly, neither allele being expressed typically does not fit under standard dominance inheritance patterns, as at least one allele usually contributes to the phenotype. Thus, codominance accurately captures the essence of both alleles being expressed and contributing to the organism's phenotype.