I agree with Kathy that the scenario she described is very rare. This would involve three 7cM segments that were exactly alike over the whole area from 3 separate ancestors. It is possible. It is possible to have 20 and 30cM shared segments that are IBS. But very unlikely. With the randomness of DNA, unusual events occur (a few people exist with 3 sex chromosomes: XXY - and other combinations) For general spreadsheet analysis and chromosome mapping, I think it's safe to consider all of your matches on a particular segment to come from one of two of your ancestors (one paternal, one maternal), and any who match each other are Triangulated, and come from the same ancestor. I'm now curious as to why the original question came up: what is > >> the likelihood they don't all descend >> from the same ancestor... It seems to me that if a fit cannot be found, there is a much more likelihood that the answer is behind a genealogy brick wall (or mistake), than a rare genetic event. IMO. But with DNA strange things will happen... Jim - Sent from my iPhone - FaceTime! On Nov 17, 2013, at 4:17 PM, KATHRYN JOHNSTON <kathryn.johnston@sbcglobal.net> wrote: > To answer Peter's first question below. You asked about finding shared segments without phasing which means you don't know which one out of the two segments you are sharing on. So that means three people can share with each other without all sharing on the exact same segment even though the segments line up in exactly the same places. Each person can match one person on the segment coming from his/her mother and also match the other person on the segment coming his/her father. I call it a three-way but not exact match. > > Example, John, Kate and Mary all match each other at approximately 7 cM on a chromosome browser, in the same spot and the segments are IBD. John and Kate match the German ancestor here. John and Mary match the Irish ancestor here. Kate and Mary match the English ancestor here. There are three ancestral lines. Each person only matches two out of the three. Mary may not have any German ancestry and does not descend from that ancestral couple yet she still matches the other two people. > > It is like holding hands in a circle with two other people. There are three different matching possibilities and you don't match where the two other people are holding hands. But you all connect with each other. I think this is very rare but can certainly happen in communities where the ancestors were all living in nearby circles (pun intended) but not all related to everybody else through exactly the same ancestors. > > > Kathy J. > > -- >> On 11/15/13, Peter J. Roberts<peterebay@yahoo.com> > wrote: >> >> If three or more people share a 7cM segment >> and they all match each other on >> that segment then what is >> the likelihood they don't all descend >> from the same ancestor or >> ancestral couple? >> What would be other exceptions >> besides double cousins? >> Thanks and sincerely, >> Peter >> Peter J. Roberts