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This is the same reply to you I posted at dna-forums.com:
A good, but rough, rule of thumb is that, at 67 markers, each differing marker represents 100 years to the most recent common ancestor (MRCA). So, a GD of 4 at 67 implies roughly 400 years to the MRCA. I have a match of 63/67, but we both have the additional 9 "SMGF" markers. We match on 72 of 76 markers total. So the MRCA for me and my match may have lived 350 years ago.
However, you mentioned that one of the markers you differ on with the other person is 492. This puts the match in a different light. That marker is a key dividing point in R1b1b2 subclades. You have your subclade as R1b1b2a1b, so you're P312+. Is 12 the count you have for 492? Most likely it is, since that's the strongly modal value for all the P312+ subclades; you also list that you're L21+, defining the largest subclade of R1b1b2a1b.
If your match has a different value for 492, it's probably either 13 or 14. If it's 14, then there's a good chance that he is also P312+, but is L21- and U152+. That would mean that you're in different subclades of P312. If your match has 13 at 492, then there's a very high likelihood that he's U106+, since 492=13 is strongly modal for U106 (brother subclade of P312) and its subclades.
Either way, this match would be alarming. That's because, even with R1b1b2 (a haplogroup whose subclades are hard to distinguish based on even 67 markers), it's expected that the GD at 67 markers of two men in different subclades should be at least 7 or 8. If you're in different subclades (the match is L21-), then there's no MRCA in a few thousand years! Of course, it's entirely possible that your match is one of those rare cases of someone who is L21+, like you, but doesn't have 492=12.
Does your match have a subclade assigned based on SNP testing? If so, what is it?
A 4th cousin twice removed is not the same as a 5th cousin - here is a link to the inheritance chart in the FAQ. But in terms of numbers of generations of separating the two people, I think it would work out the same, 5 and 7 generations versus 6 and and 6 generations for 5th cousins.
I think that 400 years is a good ballpark average estimate, but because of the random nature of mutations it could be considerably more or or considerably less than 400 years. It is close enough that you would want to look for a possible paper record of a common ancestor.
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