A letter from Bennett Greenspan, FamilyTreeDNA Founder

[defacto7]

11. Understood.

You say, "where a person's DNA segments come from" and that's a point. Not all DNA segments are passed to us or pass from us. That changes the outcome. If it were not so we would be clones. A simplistic example is that one parent can have genes that say 40% German, the other that they have no German but are 50% Italian, then their child's DNA says 60% German, 30% French and 5% Italian. Genes can skip several generations or pile up in unsuspecting places and this is a mild example. Look up chimerism in humans where the DNA says the father is not the father but the uncle. Genealogical records are the only real tell.

I would add that the probabilities are better if you sample multiple generations and as many siblings as possible the more the better; the gene pool would show a broader perspective. If you don't have several siblings or close family members you're out of luck. Individual tests are almost useless because the probability of being a correct reading of your past is low even though the test itself is extremely accurate.