Heterogeneity of the blood group Ax allele: genetic recombination of common alleles can result in the Ax phenotype.

The Ax phenotype is an important subgroup of the ABO blood group system. Its inheritance does not always follow Mendelian rules and recent studies suggested that different alleles can result in this phenotype. This suggestion has been explored by cloning and sequencing exons 6 and 7 of the ABO gene and the intervening intron from members of six unrelated families expressing the Ax phenotype. Two families showed the previously described T646A 'Ax' mutation as the only deviation from the consensus A1 allele. In two other families the Ax phenotype was inherited as two different recombinational gene products. Combination of exon 6 derived from A or B/O2 alleles with exon 7 from the O1v allele created two novel alleles that have four O1v-characteristic nucleotide substitutions in exon 7, including T646A. Sequencing and analysis of polymorphisms in intron 6 defined the crossing-over zones of these hybrid alleles. Southern blot confirmed the hybrid formation by detecting ABO-related polymorphisms approximately 1.35 kb downstream from the ABO reading frame. The remaining two families expressed the Ax phenotype via an allele having A2-specific mutations. Thus, a heterogeneous molecular background leads to the serologically defined Ax phenotype and may well explain the different modes of inheritance observed.