ABO blood group
The ABO blood group system is the best known blood typing system, determining blood group type as A, B, AB, or O, and thereby of importance for blood donors and recipients. Wikipedia Three common alleles (A, B and O) of the ABO gene yield the six common combinations that a person can have, each of them resulting in a particular ABO blood type (as determined serologically):
SNPs in the ABO gene determine the alleles present and thereby predict ABO blood group type.
23andMe checks these 11 snps
The following are also SNPs in the ABO gene, used only to distinguish rare variants or to predict SNPs from the above list (based on linkage):