Eric, Yours is the same aha moment that everyone interested in segments or mapping needs to understand. You have two of each atDNA chromosome - one from Mom, one from Dad. Each one is filled with segments that came from the respective parent's ancestors. When you have a shared segment with a Match, or see a segment on a chromosome browser, you cannot tell which one of the two chromosomes it is on (the one you got from Mom or the one you got from Dad) Each the two chromosomes (one from Mom, one from Dad) has a different random arrangement of segments - some are overlapping, some are in roughly the same location (the location is noted by a start position and a stop position) So, just because two shared segments have the same (or similar) start and end points (or are closely overlapping), they could be on the same chromosome (from Mom or Dad), or each one could be on a different chromosome (one on the chromosome from Mom, one on the chromosome from Dad). If the Matches who share the two segments above match each other, then the two segments are on the same chromosome (it would be a very, very unlikely happenstance to have such segments on different chromosomes match each other). As such you and the two Matches will have the same Common Ancestor. This is called genetic Triangulation, and the three of you are in a Triangulated Group. If the Matches who share the two segments above, do not match each (and the segments with your are long enough and overlap enough that they reasonably should match if they were on the same chromosome), then it is safe to assume that they are from different chromsomes (one is from the chromosome from Mom, the other from Dad) If any of the folks in a Triangulated Group is a close relative, then that TG, and all the segments in it can be assigned to the appropriate parent (Mom or Dad); and the segments which clearly don't match anyone in the TG (but should if they were on the same chromosome) can be assigned to the other parent. Triangulated Groups are particularly important to adoptees and folks with brick walls because if any two people in the TG can determine the MRCA for the TG, then that ancestral line will apply to everyone in the TG. Notice I said "ancestral line" - some in the TG may not have the same MRCA, but all will have a Common Ancestor whose descendants include all the MRCAs in the TG. In other words two 2nd cousins will have great grandparents in common; another Match who is in a TG with these 2nd cousins, may be a 4th cousin and have 3G grandparents in common, but the 3G grandparents will be ancestors of the G grandparents the 2nd cousins share. For shared segments, there needs to be a path from a Common Ancestor down to each Match in a TG. Hope this helps your aha... Jim Bartlett On 11/20/13, Eric S Johnson<crates@oneotaslopes.org> wrote: > From: a[1]utosomal-dna-bounces@rootsweb.com [[2]mailto:autosomal-dna- > [3]bounces@rootsweb.com] On Behalf Of Tim Janzen > Sent: Wednesday, November 20, 2013 03.32 > Then about 3 weeks ago my mom got another match on this same segment > from someone who has Bigelow ancestry. So it's possible to have an "identical" match with two different "genetic relatives" on (more or less) the same "chromosomal segment," but have these matches be a result of two different MRCAs--one on one half of the chromosome (HIR), one on the other? Sorry ... I'm having one of those "aha" moments ... Best, Eric References 1. mailto:utosomal-dna-bounces@rootsweb.com 2. mailto:autosomal-dna- 3. mailto:bounces@rootsweb.com