The ABO blood group gene is known to code for a glycosyltransferase, which acts at the last step of sequential extension of oligosaccharide chains attached to glycoproteins or glycolipids. Since the first delineation of the molecular basis of ABO blood group, genotype-phenotype relationship of various ABO alleles has been extensively studied. Major differences between the coding sequences of them were found to reside in exons 6 and 7. Over 70 alleles have been analyzed for their sequences, more than half of which were found to exhibit hybrid nature in their sequence motifs. These alleles seem to result not from recurrent mutation but most likely from intragenic recombination due to crossing-over or genetic conversion. Occurrence of reciprocal products and de novo recombinant support the idea. The aim of this article is to outline the genetic mechanism underlying the ABO allelic diversity with a speculative model for genesis of an allele.