Evolutionary fate of an unstable human minisatellite deduced from sperm-mutation spectra of individual alleles.

Although mutation processes at some human minisatellites have been extensively characterized, the evolutionary fate of these unstable loci is unknown. Minisatellite instability is largely germline specific, with mutation rates up to several percent and with expansion events predominating over contractions. Using allele-specific small-pool polymerase chain reaction, we have determined sperm-mutation spectra of individual alleles of the highly unstable human minisatellite CEB1 (i.e., D2S90). We show that, as allele size increases, the proportion of contractions rises from <5% to 50%, with the average size of deletion increasing and eventually exceeding the average size of expansion. The expected net effect of these trends after many generations is an equilibrium distribution of allele sizes, and allele-frequency data suggest that this equilibrium state has been reached in some contemporary human populations.