Molecular characterization of a spontaneously generated new allele at a VNTR locus: no exchange of flanking DNA sequence

Variable-number tandem-repeat (VNTR) DNA markers are contributing new power to human genetic studies because their hypervariable nature allows individualization at the DNA level. The practical value of VNTR markers has been well established for genetic linkage mapping, forensic biology, paternity testing, and monitoring of bone marrow transplants. A popular hypothesis attributes generation of variability at VNTR loci to unequal exchange between homologous chromosomes at meiosis. Contrary to the prediction of this hypothesis, we report here the finding that a newly generated VNTR allele is parental for closely spaced flanking markers; the new allele was generated by loss of one repeat unit, without exchange of flanking DNA sequences. These results are consistent with sister chromatid exchange and polymerase slippage or deletion, as well as with some models for gene conversion.