Which term best describes the genetic system that determines human blood type?

• A. Genome
• B. Mitochondrial DNA (mtDNA)
• C. ABO blood type system
• D. Regulatory genes

Answer: (C)

The ABO blood type system is the genetic system that determines human blood type. It rests on a single gene with three common alleles: I^A, I^B, and i. The I^A and I^B variants encode enzymes that add specific sugars to surface molecules on red blood cells, producing A and B antigens, while the i allele encodes a nonfunctional enzyme, resulting in no A or B antigens and the O type. Because I^A and I^B are codominant, individuals with IAIB express both A and B antigens on their cells. Transfusion compatibility hinges on these antigens and the antibodies in plasma that recognize foreign antigens. The other terms—genome, mitochondrial DNA, and regulatory genes—refer to broader genetic material or gene expression control, not the specific system that determines blood type.