On which human chromosome is the ABO blood-group gene located?

The human ABO blood-group gene is located on chromosome 9.

The gene encodes an enzyme that modifies a carbohydrate structure on the surface of red blood cells. Different ABO alleles produce A, B or little functional transferase activity, giving rise to the A, B, AB and O blood-group phenotypes. A and B alleles are codominant, while the common O allele is usually recessive to them.

ABO type is inherited separately from the Rh blood-group system, which includes the familiar positive and negative labels. Thus, someone can be A-positive or A-negative, for example. The ABO gene itself does not determine the entire blood type notation.

Blood-group compatibility matters because antibodies can attack incompatible red-cell antigens. Hospitals test blood rather than relying only on family predictions, since rare variants and additional blood-group systems can complicate simple inheritance charts.