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I'm using "back-mutation" in rather specific sense to distinquish from true reverse-mutation, as the specific history of the mutations hasn't been studied. Basic human genetics holds that each SNP mutation is considered to have occurred only once during human history, but neverheless revrse-mutations have been obsered and studied in many species. I haven't been abe to find any information on how common true reverse mutations are considered to be in humans, nevertheless there are a number of other circumstances from which this could have arised especially considering the sample size. One is, of course, genotyping error. Another is two recombination events likely at different points in history splicing only the section containing that SNP from another haplotype. A more interesting conjencture due to the apparent lack of intermediate versions of the African and European haplotypes would be that it could represent a middle phase between the chain of mutations between the dominant haplotypes. I've only checked HapMap haplotypes so far though, and there could be phasing issues especially with regards to simpleton SNP's so will have to check phasing on 1000 Genomes to verify the conclusions. --Donwulff (talk) 13:05, 18 May 2014 (UTC)