Problems in paternity testing. Subtypes of AB.

Paternity test results demonstrated that a child of type A1 has a mother who types as an A2B; the putative father is type O. According to the recent AMA-ABA joint report, these results prove nonpaternity. All other erythrocytic and HLA typing indicated a high probability that the putative father was the biologic father. Since the expression of the A1 gene may be weakened on the erythrocytes of a type AB person, the possibility that the mother carried an A1 gene was considered. Review of published data reveals that in black persons there appears to be an excess of type A2B and a deficiency of type A1B compared with the numbers expected from gene frequencies. It is possible that as many as one of five black persons with A1 and B genes has an A2B phenotype.