DNA matching segment [closed]

Question

Mystery in a DNA segment match. Size of segment here is 20 cMs +

What to make of:

If you’re A, and your two 7^th cousins are B and C, who themselves are 2^nd cousins, when A & B share the exact same segment that A & C share.  But B & C don’t share that same segment.

Comments

It might be useful to provide the kit numbers to help with investigating the issue (I'm assuming B&C are both on Gedmatch?)

Answer 1

DNA donors A and B share a segment that overlaps a segment that B and C share.  That situation is called an overlap.  If C and A share a segment that overlaps with a segment that A and B share and overlaps with a segment that B and C share, then you have a triangulation.  A triangulation indicates that A, B and C have a shared ancestor.  If genealogical evidence identifies a shared ancestor among the three donors, the triangulation supports the validity of the genealogy,

Sometimes, even when the three donors are known to have the same ancestor, the DNA for overlapping segments may not all come from that shared ancestor.  That doesn't mean you have disproven the genealogy.  Keep looking for overlaps and triangulations.  Sometimes they form clusters which give even more validity.

Comments

Gary, thank you.  I've had a response from John Trotter with whom I shared the kit #s and he has pointed out the overlap in segment length within the three of us.  Admitting I had only looked at exact segment length and never noticed the one was within the others!