POPULATION VARIATION ANALYSIS AT 9 LOCI CONTAINING EXPRESSED TRINUCLEOTIDE REPEATS

The polymorphisms of nine loci containing reiterated CAG repeats were examined in four populations from three continents. Their normal varia tion was analysed across populations or in subsets of loci grouped acc ording to either the presence/absence of disease-associated expansions or CAG interruptions. A unifying feature of the allele distributions of all loci in all populations was the marked non-normality. Significa ntly larger numbers of alleles, average lengths, length ranges and var iances in repeat number were observed in loci with vs. without known e xpansions. Significantly longer alleles were found at loci with vs. wi thout interruption of the (CAG)n motif. The nine loci detected levels of inter-population variability comparable to other loci. Altogether t he data are at odds with a model assuming that autosomal expressed tri nucleotides accumulate variation exclusively by insertion/deletion of a single unit.