I’ve had this question for a while and figure there is someone (more than one probably) who knows the answer.
One of my cousins works in California for a company that apparently makes high-end ancestry testing (among other things) that costs like $10,000. He says that is the only kind of testing worth getting and that the cheaper tests for the proletariat really don’t tell you much of anything. I can’t remember exactly what all he said now, but something in particular kind of threw me off and made me think that the cheaper tests might not be worth much.
He said that if you go far back enough in your family line, you really aren’t related to those people anymore. You wouldn’t be here without them, but you don’t really have any genes from them per se. He also said that if you get a cheaper test and it tells you that you have ancestry from a certain part of the Middle East or Europe or Africa or whatever, that may not actually be true, but I wasn’t sure WHY that wasn’t true - can it pick up on ancestry from too far back in your lineage that you don’t actually have (or something)?
It’s been a while since he said this so I probably am not remembering everything.
Recently a friend of mine got results from one of these cheaper tests and it gave her 4 or 5 countries or regions that she has genetic material from, and it brought up in my mind my questions about what my cousin said and whether or not the results she got could be in any way accurate. It would be fascinating if they are, but if not she paid $100 for a load of bunk.
To the first part of your question, yes, it is entirely possible for certain ancestors to get washed out of your genetic makeup. You only inherit one of each of your parents’ two copies of each gene. That means half their genetic material gets thrown away.
The reality is slightly more complicated in that the two chromosomes sometimes swap material before the sex cells are created, so the chromosome you get can be a hybrid of their two chromosomes. But ancestors can get totally erased over time, yes.
To the second part of your question, yes, any ancestry result that names modern countries or even regions is a touchy-feely, public-facing interpretation of the information that’s actually in your genome. Once you have your genome–from any company–you can run it through other services that give you a more honest breakdown of your ancestry, though the results might disappoint you. They identify groups like “Neolithic farmer” and “Eastern Hunter-Gatherer” rather than “French” or “German.” They will also contain information that needs contextualization–for example, lots of people of German descent will ping 1-2% Native American. This isn’t actual Native American ancestry, but shared ancestry between Native Americans and the Huns/Mongols/etc that invaded Europe.
There is a free website called dna.land that will run your genetic data through half a dozen different models done this way. You can them compare and research to your heart’s content.
This site is a good source to understand your 1st question. In a nutshell, when you look at an ancestor 8 generations ago, you start to get to where there is some small chance that you have no DNA from that ancestor. At 10 generations, it’s 50/50. When you get back to 14 generations, it’s almost certain that you got no DNA from certain ancestors.
There are three types of tests/test interpretations given by 23andMe and similar companies today.
Ethnicity estimates. These are the most hyped, and the least accurate. They are extrapolations from comparing the 4-8 hundred thousands SNPs the companies tests to modern reference populations. They vary a lot between the several companies, and although they are being refined, there is only so much that can be done because the concept is flawed to begin with. Beyond telling you the likely continent each of your great-grandparents came from they should be taken with a huge grain of salt.
These tests might be possible to improve with a full genome test (for 10 grand it ought to be one of those), but I wouldn’t be too sure, the main problem, it seems to me, is picking reference populations.
yDNA and mtDNA tests determining your strict paternal and maternal lines. These can give you data back hundreds of years, if you have someone to compare with. Few companies offer these, and the cheapest offers are fairly inaccurate. 10 grand could buy you perfect knowledge of this DNA, but in the end it is worthless unless you have direct paternal or maternal cousins to compare with.
autosomal family finder tests. This is where the real benefit to the average DNA genealogist lie today, and the tests are pretty good. The companies do give you a lot of results labelled “uncertain”, and not enough tools to understand why a small match of “shared” DNA with a cousin on paper might not actually prove anything at all. But they are pretty good. And serious research on this have found that whole genome analysis does not do a much better job, except in weeding out the least reliable results. Not really worth 10 grand.