ABO Blood Types: Genotypes, Phenotypes, and Inheritance Patterns

The ABO system is determined by alleles at the ABO locus on chromosome 9. Three main alleles: I^A, I^B, i.

Genotype-Phenotype Table

Blood Type	Possible Genotypes	Antigens on RBCs	Antibodies in Plasma
A	IAIA, IAi	A	Anti-B
B	IBIB, IBi	B	Anti-A
AB	IAIB	A and B	None
O	ii	Neither A nor B (H antigen only)	Anti-A and Anti-B

Rh Factor

Independent locus (RHD). Rh+ = D antigen present (RHD functional); Rh− = D absent (RHD deletion/inactivating mutation). Inherited as dominant/recessive.

Combined ABO/Rh

e.g., A+ = genotype I^AI^A or I^Ai + RHD/RHD or RHD/rhd.

Clinical Relevance

• Transfusion compatibility: Donor RBCs must lack recipient's target antigens.
• Hemolytic disease of fetus/newborn: Anti-D (Rh) or anti-A/B (ABO) cross placenta.
• Organ transplant: ABO compatibility required (except some liver/kidney protocols).