Some genes are more prone to mutation than others. An example is Fragile X syndrome, which involves a single gene on the X chromosome. It arises spontaneously fairly infrequently, usually becoming the "pre-mutation" version of the gene (which is symptomless or very faintly symptomatic, i.e. may produce a very subtle impairment in intelligence but not outside the normal range). The premutation version very often spontaneously mutates into the retardation-inducing "full mutation" when being copied from parent to offspring.
Then how come that the sub-saharans are unusually resistant to it? Wouldn't that mean that the mutated [or the "pre-mutated", if you like] version arose between 100K and 40Kyrs before present, for it not to be found there? For if it were there, then the full mutation would be, too.