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 A(2)B phenotype is significantly higher than expected based on the A(2) phenotype frequency. To understand the genetic basis of this 'excess' of A(2)B, we examined ABO alleles in individuals with A(2)-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 A(2)-related alleles (*A105, *.A106, *A107 *A111 and *R101) were clearly different between the A(2) and A(2)B phenotypes. In particular, a putative recombinant allele, *R101, was uncommon in the A(2) but common in the A(2)B phenotype individuals. This allele was also detected in 4 of 401 (1%) unrelated A(1) phenotype (AO genotype) individuals. Conclusion: *R101 is presumably expressed as phenotype A(1) in *R101/*B heterozygous individuals, but as phenotype A(2) in *R101/*B heterozygotes, thus giving rise to a high A(2)B phenotype frequency.