The ABO gene encodes the glycosyltransferase protein which is best known for ultimately determining blood type, otherwise known as the ABO blood group. SNPs in this gene determine whether an individual is of blood group type A, B, AB, or O. Determination of an individual's blood type based on SNPs can be done via genosets, but this remains a work in progress.
http://abobloodgroup.googlepages.com/home is a useful resource and illustrates how many variants are known.
According to this source the most important snps are
- rs8176719 the O allele frameshift mutation
- rs8176743 codon 235
- rs8176746 codon 266
- rs8176747 codon 268
With these other SNPs commonly involved as well: