Jim's statement about the chances being slim that 3 have an IBS is only partly true.  My sister has recently gotten her data, and as expected at 1/2 of my matches at the 5th cousin level do I match her.  But of that 1/2, 1/2 of that is at places where our parents each gave each of us the exact same DNA, so at that 1/4th of our genomes we are like identical twins.  So we only count as 1 person at that 1/4th of our genomes.  (It actually is a slight bit of corroboration since my data was V2 and hers is V3 chip.)  You need to use the Family Traits app on 23 to see if you are Full IBD there instead of HIBD. On Sat, 16 Nov 2013 01:00:02 -0700, autosomal-dna-request@rootsweb.com wrote: > > Today's Topics: > > 1. Re: Finding shared atDNA ancestry without phasing (Jim Bartlett) > 2. X chromosome matching (Linda Boorom) > 3. Re: Managing autosomal DNA projects (Jim Bartlett) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 15 Nov 2013 16:10:42 -0600 (CST) > From: Jim Bartlett <jim4bartletts@verizon.net> > Subject: Re: [AUTOSOMAL-DNA] Finding shared atDNA ancestry without > phasing > To: peterebay@yahoo.com, autosomal-dna@rootsweb.com > Message-ID: <13162610.1870934.1384553442310.JavaMail.root@vms170015> > Content-Type: text/plain; charset="UTF-8" > > > 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 > > > ------------------------------ >