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I have foolishly accepted to act as an advisor to my uncle (on my mother's side), and find that I am quite unprepared to do so.

He is trying to follow up a family tradition that his x-GF (all on his paternal line) emigrated to southern Germany from Switzerland in 1589. His last name is concentrated in one village in Germany, whereas the name is quite prevalent throughout Northern Switzerland (with concentrations around Zurich and around Winterthur/St. Gallen). I have been in email contact with a person in Switzerland who shares my uncle's last name and participated in a largish DNA study on Ancestry.

According to this gentleman, there are two unrelated groupings of the family name, which correspond to the geographic distribution. The Winterthur members of the family are related to each other, but not to the Zurich group, and vice versa. The Southern German group was not represented in the study.

My uncle would like to test the family tradition, but he asked me for advice about the DNA tests. It sounds like a Y-DNA test would be most appropriate to prove descendance along the male line. If there is a match with either the Zurich or the Winterthur strain, then that would confirm the family tradition. If not, the tradition looks a bit shaky.

Through the aforementioned gentleman, I think I can get access to one person from each group, but I am not sure if that is a good idea.

On one hand, is it possible to link into the existing study? Since most likely the genetic split would have happened more than 400 years ago, I am not sure what a match or absence thereof would prove.

On the other hand, I have done my own DNA through 23AndMe and was told my Y-haplogroup. The analysis says that this particular haplogroup arose about 10,000 years ago and is shared by something like 20% of all European males. This seems to indicate that a Y-DNA test might not be all that useful, either.

I have been reading that familytree.com has a proper Y-test, but I do not understand the differences, and I am totally confused at this point.

What is the best DNA test to study whether a male 400 years ago belonged to one of two particular families if all one has is 21st century data?

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The proper test to take in this situation is the Y-DNA test. Y-DNA as you have noted gets passed down the straight male line only.

Why not autosomal tests.

The 23AndMe autosomal test you did tests the DNA that you inherited from all your ancestors, not just the male line. There are two main problems with using that autosomal test for your situation. The main one being that you can't directly tell from the autosomal test which ancestor you inherited a particular segment of your DNA from. By painstaking genealogical research on your line and the lines of your DNA matches you can in some cases figure out which ancestor a particular DNA segment came from, but the more generations back you want to do this, the more difficult it becomes. This brings up the second problem with the autosomal test. For each generation you go back in time you lose roughly 1/2 the information about a particular ancestor in that generation. On the male line you are talking about you get 1/2 DNA from your father, 1/4 from his father, 1/8 from his father etc. More than 3 generations back there is a possibility that you will not inherit anything at all from that straight male line.

Why Y-DNA tests

With the Y-DNA test you bypass both of the problems discussed above. You only get the Y-DNA from the line your are researching. You get 100% of the Y-DNA from your father minus any mutations that take place between you and your father, the same between you and your grandfather and on back the male line, 100% minus any mutations. These mutations are the most crucial part of the Y-DNA test, but they happen very slowly such that not many of them have occurred in the 400 years you are talking about. You certainly are not throwing away 50% of it in each generation as happens for the autosomal tests.

Which Y-DNA test

Which test to take is pretty straightforward, familytreedna.com has the largest database of men that have already taken the test. The larger the database, the larger the probability that someone descended along the line you are interested in has already taken a test. Otherwise you need to recruit someone on that line to test. The basic Y-111 test could be enough to answer your question. As you increase in cost in the Y-DNA tests you get better resolution about how many generations apart your matches are. You could have close matches at Y-37 whose common ancestor is many many generations back. Whereas a very close match with a sequencing test like the Big Y-700 usually only occurs when you have a recent common ancestor.

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