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The page below is a sample from the LabCE course Introduction to the ABO Blood Group System. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

Learn more about Introduction to the ABO Blood Group System (online CE course)

Those who type as group O must have two O genes present (since both the A and B genes would have produced recognizable antigens, neither of which is present on group O cells). Therefore, in the case of an AB individual or an O individual, we can tell exactly which genes are present, or a genotype. Group A or group B typing reveals only one gene product and thus only a phenotype can be determined. Persons of phenotype A can be genotype AA or AO , while those of phenotype B can be genotypically BB or BO. Family studies may be done to determine the genotype of an A or B individual. For example, if the mating of one A and one O parent produced a group O child, the second gene present in the A parent must have been O since the child has inherited one O gene from each parent.