Somewhat less than 2 people sharing 7cM on the same segment - with just two people the 7cM segment might be IBS (not from an Ancestor). If three people all match each other on that segment, the odds are slim that the segment is IBS. If it's an IBD segment, that means it came from a Common Ancestor. You can not have segments from two different ancestral lines using the same space on a chromosome. The only way three people can have the same segment from one of your chromosomes is if they got it from the same ancestral line. Now - two of you might have a closer MRCA, and you and the other Match a more distant MRCA, but both MRCAs would be on the same ancestral line and you would all descend from the more distant CA. The likelihood in your question is close to zero. This is even the case with double cousins. The shared segment came from one ancestor - who is ancestral to one of the 4 persons in a double cousin relationship. Double cousins have twice the possibility of getting segments, so you sometimes get 2 or 3 or more, but each one is from only one ancestral line. Jim Bartlett 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