Reverse also known as back mutations are seemingly a reality; we generally do not know when an observed mutational difference is a back mutation...and a back mutation can appear as either an up or down repeat value. In some large lineages we may suspect where a back mutation is evident. Some observations on the molecular level have suggested that the alleles have a preferred repeat sweet spot; hence the repeats may extend up or down but are inclined to return to the sweet spot. The statisticians and model builders have made allowances for back mutations in their analytical models. Most calculating TMRCA will use analytical models which have built into the model a means to correct for back mutations. Susan On 11/22/2013 5:30 PM, Malcolm McClure wrote: > However, I am bothered about the influence of reverse mutations on > ancestral projections and comparisons. We need widespread sampling of > several related generations to establish the importance of this > possible effect, but those data are unlikely to become available for > many years, if not generations. >