A DE-NOVO RECOMBINATION IN THE ABO BLOOD-GROUP GENE AND EVIDENCE FOR THE OCCURRENCE OF RECOMBINATION PRODUCTS

We have encountered a paternity case where exclusion of the putative f ather was only observed in the ABO blood group (mother, B; child, A1; putative father, O), among the many polymorphic markers tested, includ ing DNA fingerprints and microsatellite markers. Cloning a part of the ABO gene, PCR-amplified from the trio's genomes, followed by sequenci ng the cloned fragments, showed that one allele of the child had a hyb rid nature, comprising exon 6 of the B allele and exon 7 of the O1 all ele. Based on the evidence that exon 7 is crucial for the sugar-nucleo tide specificity of A1 and B transferases and that the O1 allele is on ly specified by the 261G deletion in exon 6 of the consensus sequence of the A1 allele, we concluded that the hybrid allele encodes a transf erase with A1 specificity, resulting, presumably, from de novo recombi nation between the B and O1 alleles of the mother during meiosis. Scre ening of random populations demonstrated the occurrence of four other hybrid alleles. Sequencing of intron VI from the five hybrid alleles s howed that the junctions of the hybrid alleles were located within int ron VI, the intron VI-exon 7 boundaries, or exon 7. Recombinational ev ents seem to be partly involved in the genesis of sequence diversities of the ABO gene.