It is not a matter of "how far back does the test go" but rather "what random combination of DNA did I inherit from my ancestors?"

And it can be quite random. And it can be only a few generations until you have absolutely NO shared DNA with an ancestor. I have direct proof of that now.

By using tools not available at AncestryDNA (i.e. I downloaded my results and uploaded them into other sites) I have done some chromosome analysis between me, my mother, my paternal uncle, and my paternal grandmother. On one chromosome (22 in fact) I have absolutely no shared DNA with my grandmother, and yet I share an incredibly long length with my uncle. This tells me that on that particular chromosome both my father and my uncle got almost all (if not completely all) from their father, my paternal grandfather. If all I had was this particular chromosome to go on than I would never get a DNA match or information from my grandmother.

Now that is just one chromosome, but that is why it is always stated that ON AVERAGE you will share 25% of DNA with a grandparent, 12.5% with a great-grandparent, etc etc etc. In my case, if I had my great-grandparents to test I might have less than the 12.5% with them due to not getting that particular chromosome. Notice I said might -- there are other chromosomes to look at and in some of those I can see I got quite a healthy share from my grandmother. We actually do fall in the "average range" with a shared total of 1682 cMs (as opposed to my uncle at 1666 cMs and my mother at 3384 cMs).

Also, as others have said here and in other threads, the ethnicity percentages are guesses and in the infancy stages of science anyway.

--Elizabeth