Different Alleles Cause an Imbalance in A2 and A2B Phenotypes of the ABO Blood Group

Background and Objectives: In several populations, including the Japanese, the frequency of the A2B phenotype is significantly higher than expected based on the A2 phenotype frequency. To understand the genetic basis of this ‘excess’ of A2B, we examined ABO alleles in individuals with A2‐related phenotypes. Materials and Methods: ABO alleles were identified by means of polymerase chain reaction single‐strand conformation polymorphism (SSCP) and nucleotide sequence analyses. Results: The frequencies of A2‐related alleles (*A105, *A106, *A107, *A111 and *R101) were clearly different between the A2 and A2B phenotypes. In particular, a putative recombinant allele, *R101, was uncommon in the A2 but common in the A2B phenotype individuals. This allele was also detected in 4 of 401 (1%) unrelated A1 phenotype (AO genotype) individuals. Conclusion: *R101 is presumably expressed as phenotype A1 in *R101/*O heterozygous individuals, but as phenotype A2 in *R101/*B heterozygotes, thus giving rise to a high A2B phenotype frequency.