In codominance, what is observed in a heterozygote?

• A. One allele dominates over the other
• B. Both alleles are expressed
• C. Phenotype intermediate
• D. Neither allele expressed

Answer: (B)

In codominance, a heterozygote expresses both alleles. There isn’t a single dominant allele masking the other, and the phenotype isn’t a blend. A classic example is the AB blood type, where both A and B antigens are present on red blood cells because the A and B alleles are both expressed. This is different from complete dominance, where one allele hides the other, and from incomplete dominance, where the phenotype is a mix. So, the observed outcome in a heterozygote under codominance is that both alleles contribute to the phenotype.