The A1B genotype expressed as A2B on the red cells of individuals with strong B gene-specific transferases. Results from two paternity cases.

Two paternity cases involving black persons are reported in which possible paternal exclusions existed if the A2B red cell phenotype represented the genotype A2B. ABH transferase assays were performed to determine if the A2B phenotypes arose from the genotype A1B or A2B. These assays confirmed that all group A members carried the A1 gene and not the A2 gene. Therefore, the alleged father was not excluded in either case. The B transferase showed increased activity in both AB individuals, possibly accounting for the observed A2B red cell phenotypes. Therefore, in the presence of a B gene, accurate assignment of A subtypes using standard serological methods may not be possible.