The ABO blood group system is controlled by a single gene on chromosome 9 (9q34.2) with three alleles — IA, IB, and i.

IA produces the A antigen.

IB produces the B antigen.

i produces no antigen (O type).

These alleles combine to form four blood groups:

A (IAIA or IAi)

B (IBIB or IBi)

AB (IAIB)

O (ii)

• This reply was modified 1 week, 3 days ago by Hamdy Ahmad.
• This reply was modified 1 week, 3 days ago by Hamdy Ahmad.

The ABO gene encodes a glycosyltransferase enzyme that modifies the H antigen on the surface of red blood cells.

Depending on the allele:

IA adds N-acetylgalactosamine → A antigen.

IB adds galactose → B antigen.

i produces a nonfunctional enzyme → no modification, leaving the H antigen unaltered (blood group O).