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Old 4th July 2017, 01:30 PM
AKH AKH is offline
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Join Date: Jul 2017
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Can 1cM segments be useful in phasing with close matches?

Hi. I'm trying to do some phasing on an account I manage, specifically to solve a mystery branch that is in a recent generation. For that branch, I only have a surname, but little else in figuring out who this individual ancestor is.

Anyway, I know that we generally discard segments smaller than 7cM due to the chance of IBS. My question is...if I have a 2nd cousin match, with very high cM, high enough for FTDNA to flag it as "close family", are tiny cM segments useful in widening the net to determine other triangulated matches to this person? In other words, if I widen the filter on this close family match to show all those little 1cM bits, can I assume that those are true inherited DNA segments due to the fact that the match is a bonadfide close match? So if I find overlap on those tiny segments with unknown matches (and I have a lot of "who the heck are these people?" matches), can I safely assume that this triangulation is valid enough to be IBD?

I want to see who matches on these 1cM bits from the close family match and if THEIR segment is larger than 7cM, be able to say that the match surely must have a common ancestor with the close cousin one. I'm not sure if I'm saying this clearly.

Any thoughts on the validity of this?
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Old 4th July 2017, 01:55 PM
John McCoy John McCoy is offline
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The key word seems to be "assume"! I don't think you should assume, but I think you should investigate with the intent of testing whether the short segments behave as an IBD segment should. If you happen to find a short segment with a very high SNP count, and it matches in all the right combinations, you may be on to something. But you should keep looking for additional evidence. You would not want your conclusions to depend solely on a 1 cM segment.
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