In this post, I will go through some common claims I have seen bandied about in Facebook threads and online forums about people of mainly European descent getting unexpected traces of African admixture.
Note: When referring to admixture estimates in this blog post, I am talking about original testing-company results; not GEDmatch calculators or other third-party tools. If used correctly, such tools can be extremely helpful in verifying and debunking African admixture, but they can also be noisy and must be interpreted with care.
"Your trace of African is just statistical noise."
This is probably the most common argument, and also the most sensible one. It is very possible that a small percentage might be statistical noise. However, to find out for sure whether something is real or noise, you need to be prepared to dig very, very deep, using techniques like Roberta Estes' Minority Admixture Mapping. If you do not have the time or motivation to do research at this level, that is OK, but if so, please refrain from making bold claims. It is perfectly fine to say "I don't know". DO NOT dismiss anything out of hand just because it doesn't fit your expectations.
A lot of white Americans go out of their way to prove Native American ancestry on the basis of extremely small admixture traces. This makes the widespread dismissiveness towards African admixture traces all the more peculiar, especially in the American context. It often seems to me to be a kind of special pleading with subtle undertones of racism.
"Your trace of African shows that you can never take admixture estimates seriously. If you come from Sweden, you can't have African DNA. It's impossible."
This shows a lack of historical knowledge, if not an outright racist agenda aimed at maintaining an image of Scandinavian people as "pure whites". Several well-documented African-descendants lived in Sweden as early as the 1700s, some of whom have large numbers of descendants and even dedicated lineage societies. Again, to find the truth, you have to dig deep. Even if you can't find a paper trail, perhaps you have an unknown NPE (non-paternity event) in your tree. Work with your cousin matches. Build trees. Create hypotheses and test them. This takes time and effort, but if you really want to know the truth, that is what you must do.
"All humans come from Africa. That's what your trace of African DNA means."
Statements like this show a total lack of knowledge about how autosomal DNA mutates and recombines over time. The fact that all humans ultimately come from Africa means that even your European DNA is ultimately African in origin if we go back 70,000 years. Those 70,000 years of separation from African populations means that the admixture is no longer read as African, and if by chance an African-like ancestral segment has been passed on to you from that far back, it will be infinitesimally small. If your admixture estimate shows African percentages today, it almost certainly comes from much more recent times, quite possibly modern times (23andMe says their Ancestry Composition feature covers the last 500 years). Read up on colonialism and the trans-Atlantic slave trade.
"That trace of African just means you have DNA that is similar to people of African descent, and since many of them also have European ancestors, your so-called African admixture is probably just a misreading of European DNA."
No, that is not probable at all. Non-Africans are more closely related to each other than to present-day Africans, making African admixture very distinctive compared to the rest of the world. 23andMe's precision and recall rates for Africa and its subregions are very high. This, by the way, is another reason not to assume that African admixture is statistical noise.