Minisatellite allele diversification: the origin of rare alleles at the HRAS1 locus.

Three genetic markers within the promoter-exon 1 region of the HRAS1 locus have been employed to investigate lineage relationships among alleles of the highly polymorphic variable tandem repeat (VTR) immediately downstream of the HRAS1 gene. These markers were in absolute linkage disequilibrium with the HRAS1 VTR, allowing the assignment of unique upstream haplotypes to each of the four common VTR alleles. Analysis of 17 rare alleles revealed a stratification of allele fragment size and upstream haplotype in which each rare VTR allele possessed the markers characteristic of the common allele nearest in size. Therefore, hyperallelism emanated from the four common alleles in a defined fashion, the size of a rare allele specifying its origin. As discussed below, this result implies that unequal crossing-over between homologues is unlikely to be the predominant mechanism for generating new VTR alleles at this minisatellite locus.