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From SNPedia
Magnitude 2
Summary AB blood group
Criteria Gs130/criteria

This genoset defines one of the ABO blood group types, that of type AB blood.

The criteria is based on SNPs rs8176746 and rs8176747, which for a (single) type A allele, will be rs8176746(C) and rs8176747(G), and for a single type B allele, will be rs8176746(A) and rs8176747(C).

Therefore, an individual will be an ABO type AB if they are not carrying a type O allele (i.e. not carrying a rs8176719(-) allele), and if they are rs8176746(A;C) and rs8176747(C;G).

There is an important caveat to the above. There exist O alleles that do not have the deletion (-) at rs8176719. O303, for example, is a nondeletional O allele that has been estimated to occur in approximately 1-3% of caucasians. Like an A allele, O303 will have rs8176746 (C) and rs8176747 (G), which means that having rs8176719 (GG) or (II), rs8176746 (A;C) and rs8176747 (C;G) might as easily represent a B allele in combination with O303 (or similar nondeletional O allele) as it might a B allele in combination with an A allele. In this case, the person's blood type will be B rather than AB.

See Sequence Variation At the Human ABO Locus. Yip SP. Annals of Human Genetics. 2002 Jan;66(Pt 1):1-27. [PMID 12014997] (http://onlinelibrary.wiley.com/doi/10.1017/S0003480001008995/pdf)