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.
