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I can tell you this much, many such test are concerned with the Y chromosome or mitochondrial DNA.

The Y, if present, comes exclusively from your paternal lineage just as mitochondrial DNA comes exclusive from the maternal. If they are basing their numbers off of these then that just represents your mom’s mom and her mothers in turn and your dad’s dad etc the same.

Thus if you’re a guy for your great grandparents that means just 2 out of 8 are represented. This level of representation halves every generation farther back you go.

With ladies this gives just 1 or 8 grandparents represented.

It somewhat improves if you toss in the X chromosome since these can come by way of paternal influence rather than just maternal.

The problem here is that the X chromosome, like all others besides the Y or mitochondrial DNA mentioned above, mixes things up every generation. Traits are less well defined as belonging to this or that group and people in two populations well dispersed from each other in time and place can still have similar genes even though not especially related in terms of conventional thinking about ancestry.

The uncertainty associated is only enhanced if you’re trying to assert membership in a group underrepresented in DNA material as, for instance, many indigenous Americans are. You may recall this last in relation to Fauxcohontas but here’s the rub ... geneticists don’t have a lot of North American Indian genetics for various reasons and they make up for it with DNA samples from Mexico ... Warren may be barely Native American but the only thing her genes prove is she might be barely Mexican. *bada*boom*kisk*

On a more serious note: I’m very suspicious of these DNA testing folks. With the odds of learning much of real use about our ancestry so easily undermined by our own genes it seems a scam to me ... and worse than a scam I deeply suspect the real underlying motives as this may have more to do with identifying us for the benefit of big government so that everything else we think it’s about is just pretty window dressing for a bait and switch.