Which genetic system is defined by a single gene with three alleles A, B, and O on the surface of red blood cells?

• A. ABO blood type system
• B. Genome
• C. Recessive
• D. Ancient DNA

Answer: (A)

This question probes how a single gene with multiple alleles shapes what sits on the surface of red blood cells. In the ABO system, one gene has three common alleles: A, B, and O. The A and B alleles encode enzymes that add specific sugars to the H antigen on the red blood cell surface, producing A or B antigens. If you have the O allele, no sugar is added, so the H antigen remains unmodified and the blood type is O. The A and B antigens are codominant, so someone with both A and B alleles (A and B) expresses both antigens, resulting in type AB blood. The O allele is recessive to both A and B. This single-gene, three-allele system therefore explains the four common blood types based on surface antigens.

Other options describe different concepts (the genome as the whole genetic material, a recessive pattern, or ancient DNA), which don’t specify how a single gene with multiple alleles determines RBC surface antigens.