Barton Whenever a Match has more than one shared segment, most of the time it will be from the same Common Ancestor, but not always. Each IBD segment should be treated in its own right - as if it came from any of the Common Ancestors between the two. Often, with Colonial American ancestry, Matches will actually share several Common Ancestors. Usually the closest one will be the one who provided the shared segment, but not always. That too is random. So keep your options identified and open until you can find another Match on that segment who agrees on the Common Ancestor = genealogy Triangulation. Your second question was how close do segments have to be to make it likely they are from the same Ancestor? This is an unusual question for me, because all segments from ancestors abut each other. One SNP will be from one ancestor, and the next SNP will be from a different ancestor. Somewhere on each chromosome there is a crossover point (technically between two SNPs; in practice, the algorithms look at blocks, so the actual crossover point is a little fuzzy). Actually, as discussed before, on Chr 1, there are usually 3 such points (on Chr 20 usually 1) that separate your grandparents on that Chr. So the much smaller segments that are adjacent at that point are from much deeper ancestors from your grand parents. Similarly all crossover points from many generations will create segments from different ancestors. Now if you have, say, a 20cM segment that was passed along several generations, you my find Matches from the first part of that segment (say 12cM) with a different Common Ancestor than Matches from the other part (8cM). These two ancestors would be closer than usual, with one ancestral to the husband and the other to the wife who provided the 20cM segment. In reality, of course, the DNA comes down from many distant ancestors. But a way to look at it in the last paragraph, is like a branching tree, starting with a main trunk from your father, and splitting into say 4 large large branches (2 for each of your father's two grandparents); and then in the next few generations each of those branches splitting into two more branches. In practice each branch is not split in half at each generation going back; instead, on Chr 1 for example there will be an average of about 3 branchings each generation. I need to make a picture of this with Kitty's mapper (for her to show in a blog) to show that each segment is not divided at each generation.... Jim - Sent from my iPhone - FaceTime! On Oct 9, 2013, at 3:50 AM, Barton Lewis <bartonlewis@optonline.net> wrote: > Hi list, > > My aunt matches her fifth cousin on chr 17 as follows: > > 25354798 30027010 7.84 1249 > 33868089 47167537 12.67 2900 > > Is it likely these 2 sections are from the same ancestor? How close do > segments have to be to make it likely they are from the same ancestor? What > accounts for a section being "broken up" in this way? What's a good > layman's way of understanding "how big is big" when talking about these > kinds of break up on a chromosome? > > Thank you. > > Barton >