Which statement correctly describes codominance with a classic example?

Which statement correctly describes codominance with a classic example?

• A. The AB blood type demonstrates codominant expression of A and B antigens
• B. In codominance, only the dominant allele is expressed
• C. In codominance, the phenotype is always a blend between two alleles
• D. In codominance, the environment decides which allele is expressed

Answer: (A)

In codominance, both alleles contribute to the phenotype, so you see both forms present at once. The classic example is the ABO blood group system. The IA and IB alleles are codominant to each other, and both are dominant over i. Individuals with IAIB have red blood cells displaying both A and B antigens, producing the AB blood type. This shows that both alleles are fully expressed rather than one masking the other or mixing into a single intermediate trait. The other statements don’t fit codominance: only the dominant allele expressed would treat one allele as if the other isn’t present; a blended phenotype describes incomplete dominance; and environment driving expression is not what defines codominance.

In codominance, both alleles contribute to the phenotype, so you see both forms present at once. The classic example is the ABO blood group system. The IA and IB alleles are codominant to each other, and both are dominant over i. Individuals with IAIB have red blood cells displaying both A and B antigens, producing the AB blood type. This shows that both alleles are fully expressed rather than one masking the other or mixing into a single intermediate trait. The other statements don’t fit codominance: only the dominant allele expressed would treat one allele as if the other isn’t present; a blended phenotype describes incomplete dominance; and environment driving expression is not what defines codominance.