Genetic recombination in the ABO blood group gene region

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Cloning and sequencing of the ABO genes from a family where ABO inheritance had been discrepant between the child (genotype ; A01) and the father (genocype ; 101) showed that the apparent A allele in the child was composed of the B allelic sequence in the 5' portion and the 01 allelic sequence in the 3' portion (B-01 array). The recovery of the 261G deletion in the 01 allele by the B allelic sequence allowed the authentic reading frame to be res : ored in the hybrid allele, resulting in expression of glycosyltransferase with the A1 activity.Screening of a random Japanese population then reveal a relatively frequent occurrence (about 1%) of hybrid alleles wich various arrays. Extensive sequencing of the major four alleles, A1, B, 01, and 01v, showed allele-specific sequences in both exons and introns and nucleotide differences in the same alleles. These polymorphic sites enabled us to define che recombination breakpoints in those hybrid alleles. These breakpoints were found to locace variously within the ABO gene. A hybrid allele was found to have two breakpoints, one in incron V (01v-B) and the other in exon 7 (B-01v), probably resulting not from crossing-over but from gene conversion.We scudied population samples from Japanese, some minor ethnic groups in South America, and German for the occurrence of ABO hybrid alleles. The frequency of the ABO hybrid alleles seems to differ in ethnic groups because we found no hybrid alleles in a German population and only a single allele in the minor ethnic groups.