The ABO blood group gene: a locus of considerable genetic diversity

The blood group ABO gene shows considerable polymorphism in most of the 7 exons. Introns examined so far have also shown blood group-related polymorphisms, as has an upstream enhancer region. Several polymorphisms affect the specificity of the gene product (glycosyltransferase) and explain the occurrence of blood group A and B. Various lethal mutations result in blood group O. Other mutations are presumed to alter the activity rather than the specificity of the enzyme and result in weaker A and B blood group phenotypes. In total, 27 A alleles, 15 B alleles, 26 O alleles, and 4 AB hybrid alleles are described and surely more will surface in the near future. Variation in geographic/ethnic distribution of allele frequencies is discussed, along with the confusing nomenclatures currently in use.