A Segment-ology TIDBIT
In genealogy if we look for Thomas BARTLETT in the census and find three entries with that name, we don’t discard them all if we cannot immediately figure it out. We record them all and look for more evidence. The same concept applies to DNA. Record all the Common Ancestors you find with a Match. Even if the Match is a 2nd cousin (2C), she may also be a 5C or 8C on a different line. One of her shared segments may go back to that more distant ancestor – it’s happened to me! Don’t disregard a “pile up” of shared segments which Triangulate (just because they may be from a very distant ancestor). Science tells us that some of those 7-20cM shared segments will be with closer cousins – it’s happened to me, often. Don’t disregard a distant cousinship beyond 5C with a Common Ancestor. Save the clues. As new Matches come in, you may find supporting evidence for that CA in the TG – it’s happened to me. I have 11 out of 35 Matches, on one TG with the same 7G grandparents (some of them are 6C and 7C, the rest are 8C).
[22E] Segment-ology: Save the Clues TIDBIT by Jim Bartlett 20170103
segment-ology 
I have a huge pile-up.. I haven’t counted, but well over 500-1000+ people. I had been discarding it because I couldn’t make heads nor tails out of it, Once I started conquering my Colonial Soup, it was becoming clear that I had three 7GGFs who were brothers on three different lines making my 8GGF (their father) the likely common ancestor (and probable cause of the pileup.) The fun part will be trying to place each segment on the correct line (the pileup crosses many segments)
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Clark,
I’d be very suspicious of such a pile-up. Please read my blogpost about pile-ups to see if you have a good one or a bad one. I’ve got two, similar to your description, that are very BAD.
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Thanks Jim. I have read it before, and these massive (i didn’t say large) pileups are in the 8-10cM range and also hover around the centromere on Chr19. I still consider the majority as bogus, but there are a few that seem good or that I would consider good if the pileup wasn’t all around them. By bumping the cM to 15, I tend to weed out the pileup noise.
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P.s., while not the best indicator of a pileup, I’ve noticed the SNPs are rounded to the nearest 100 in many cases:
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500 <–pileup?
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 35756431 9.74 1500
20190111 36206662 10.33 1600
20190111 36707544 10.82 1700
20875787 35756431 9.10 1400
20875787 35756431 9.10 1400
20875787 35756431 9.10 1400
20875787 35756431 9.10 1400
20875787 35756431 9.10 1400
34103313 46393368 16.12 3184 <–Good matches?
38702129 50085313 16.48 2861 <–Good matches?
38921659 45801296 9.10 1684 <–Good matches?
38921659 46127576 9.45 1784 <–Good matches?
38921659 53030647 22.83 3514 <–Good matches?
38921659 55971694 31.19 4447 <–Good matches?
39742673 50085313 15.12 2561 <–Good matches?
41311232 54350202 21.91 3314 <–Good matches?
Hard to say without further investigation, but the "exact identicalness" raises red flags for me.
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Clark, As I indicated in my Pile-Up post, a very narrow, very tall, group is usually a BAD pile up. Have you compared any of these to each other? That is an important step. If they do not match each other, they are IBC.
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Re; ” Even if the Match is a 2nd cousin (2C), she may also be a 5C or 8C on a different line. One of her shared segments may go back to that more distant ancestor – it’s happened to me!” Excellent advice. It’s happened to me as well.
Lou
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Cora,
Working with atDNA is like working logic puzzles – it sure keeps you on your game;>j
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