Cambridge O Level Biology · Syllabus 5090 · Inheritance
ABO Blood Group System
What is ABO Blood Group System?
A human gene with three alleles that determines blood group. The alleles I to the power A and I to the power B are codominant with each other, and both are dominant to the allele I to the power O. A person with genotype I-A I-A or I-A I-O is group A, one with I-B I-B or I-B I-O is group B, one with I-A I-B is group AB because both codominant alleles are expressed, and only the homozygous recessive genotype I-O I-O gives group O. The system is the standard Cambridge O Level Biology example of codominance and of a gene with more than two alleles.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Last reviewed Syllabus 2026-2028