Evolutionary dynamics of the FMR1 locus.

Rare haplotypes for close flanking markers are associated with increased allele size and frequency of the fragile X mutation. Exceptional founder haplotypes can be identified, but many haplotypes with rare alleles contribute to full mutations. A transition matrix constructed from the data predicts that a population with reduced variability will manifest a slowly increasing frequency of premutations and full mutations, reach a distribution close to the observed one after a few hundred generations, and then slowly be depleted of these alleles. This prediction is opposite to less well supported inference of increasing frequency of progressive amplification, but the data are inadequate to reach any firm conclusions. Factors that may determine the evolution of these systems, but cannot now be evaluated, are discussed.