Extremely complex repeat shuffling during germline mutation at human minisatellite B6.7

Human minisatellite B6.7 is a highly variable locus showing extensive heterozygosity with alleles ranging from six to>500 repeat units. Paternal and maternal mutation rates to new length alleles were estimated from pedigrees at 7.0 and 3.9 % per gamete, respective-ly, indicating that B6.7 is one of the most unstable mini-satellites isolated to date. Mutation at this locus was also analysed by small pool PCR of sperm and blood DNA. Male germline instability varied from <0.8 to 14% per allele and increased with tandem array size. In con-trast, the frequency of mutants in somatic (blood) DNA was far lower (<0.5%), consistent with a meiotic origin of germline mutants. Sperm mutants were further char-acterized by minisatellite variant repeat mapping using four major polymorphic sites within the B6.7 repeats. This highly informative system revealed a wide variety of changes in allele structure, including simple intra-allelic duplications and deletions and more complicated inter- and intra-allelic transfers of repeat blocks, as seen at other human minisatellites. The main mode of sperm mutation, however, resulted in ex-tremely complex allele reorganization with evidence of inter-allelic transfer plus the generation of novel re-peats by rearrangement at the sub-repeat level, suggesting that recombinational instability at B6.7 is a complex multistep process