Explain codominance with the AB blood type.

Explain codominance with the AB blood type.

• A. Both alleles are expressed equally, producing AB blood type antigens.
• B. Only the dominant allele is expressed.
• C. The two alleles form a hybrid trait that is intermediate.
• D. One allele dominates while the other is silenced.

Answer: (A)

Codominance means that both alleles contribute to the phenotype and are expressed at the same time. In the ABO system, the IA and IB alleles are codominant, while the i allele is recessive. When someone has IAIB, their red blood cells display both A and B antigens on the cell surface because the enzymes produced by IA and IB add their respective sugars, and neither antigen is masked or suppressed. That’s why the result is AB blood type: both traits appear simultaneously without blending. This differs from complete dominance (only one allele’s trait shows), and from incomplete dominance (the heterozygote shows a blended intermediate). It also isn’t a case of one allele being silenced.

Codominance means that both alleles contribute to the phenotype and are expressed at the same time. In the ABO system, the IA and IB alleles are codominant, while the i allele is recessive. When someone has IAIB, their red blood cells display both A and B antigens on the cell surface because the enzymes produced by IA and IB add their respective sugars, and neither antigen is masked or suppressed. That’s why the result is AB blood type: both traits appear simultaneously without blending. This differs from complete dominance (only one allele’s trait shows), and from incomplete dominance (the heterozygote shows a blended intermediate). It also isn’t a case of one allele being silenced.