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

Abstract

Abstract 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 /* O heterozygous individuals, but as phenotype A 2 in * R101 /* B heterozygotes, thus giving rise to a high A 2 B phenotype frequency.