Small Segment DNA Match on a Larger Shared Segment?

+17 votes
We talk a lot about the folly of small segment matches here, which I typically agree with. The one exception that always catches my eye is when I find a new small segment match on a larger segment that I have already identified across other tests.

I found an interesting new match today (through Wikitree) on a shared segment between my uncle (VJ), his first cousin (NB) and their paternal second cousin - 4 X removed (CE). On chromosome 15, we have the following:

VJ and NB - 17.6 cM/3,221 SNPs 65,011,539-79,894,101
VJ and CE - 26.7 cM/5,120 SNPs 65,011,539-87,909,765
NB and CE - 36.8 cM/7,096 SNPs 50,614,631-79,894,101

The only recent shared ancestors between VJ, NB, and CE are A.B. Jones (1836-1902) and Mary Coates (1839-1893).

VJ, NB, and CE each match with Kim (I found her Gedmatch number on her Wikitree profile), on a small part of this shared segment of chromosome 15 between 5.3-5.4 cM:

Kim and VJ - 5.3 cM/1,347 SNPs 69,709,089-77,270,126
Kim and NB- 5.3 cM/1,346 SNPs 69,709,089-77,266,102
Kim and CE- 5.4 cM/1,374 SNPs 69,642,036-77,268,138

Kim's tree is interesting, as her daughter is an active Wikitree user and has her tree well-sourced back to a Van S. Jones, who was born in Kentucky in 1799 and died in Indiana in 1877. In my tree, A.B. Jones had a younger brother who was born in Indiana in 1839, and Mary Coates's mother was born in Kentucky in 1820. Just by looking at our trees, the common ancestor between Kim and my three paternal tests would have to be at least 6-8 generations back.

I was just wondering how people generally react to a situation like this in which a new small segment match appears on Gedmatch for a larger segment that you have already identified in common between other tests.

I asked a similar version of this question on the ISOGG Wiki Facebook group.
WikiTree profile: Van Jones
in The Tree House by Ray Jones G2G6 Pilot (155k points)
retagged by Ray Jones
Interesting Ray.

3 Answers

+4 votes
IMO, The segment between Kim and VJ,NB, and CE are Identical by State (IBS), and not Identical by Coincidence (IBC) because of the multiple connections. In this case, I would say that this segment provides evidence indicating that VJ, NB, and CE are related to Kim via the track of the segment from each of them and those up to but not including the Common Ancestors.

If there are no IBD segments, then it is difficult to make any prediction as to the relationship distance between Kim and the others.

added: Just to be clear, I am not saying the common ancestor with Kim is within the genealogical time frame, only that there is a probable relationship to Kim, on this segment, via this side of the family.

Edit: I believe that "Probable" was a bit optimistic given IBS does not indicate a relationship when no IBD segments exist.
by Ken Sargent G2G6 Mach 5 (57.5k points)
edited by Ken Sargent
I might also suggest creating a Lazarus kit that included the descendants of the common ancestors of VJ, NB and CE in group 1, and Kim in group 2. This sort of phasing might reveal something you missed.
+5 votes
IMO Kim's matches at this location can only be IBS/IBC if Kim isn't matching (within the thresholds that are commonly approved and agreed upon) into any of the two triangulated groups that Ray has found for this location. That's the rule I apply in my web app.

If I do understand Ray correctly, then this matches into a part of a TG (usually at the beginning or end of a TG segment). This is quite common and indicates that the CA is actually further back than the MRCA of the original TG. Once you use smaller segments you will manage to break up many TG's into smaller TG's, representing a CA further back in time.

So in this concrete case with the numbers presented by Ray that's a proven TG for me, though I'd mark it temporarily if all matches are under 7cM and 700 SNP. In this case there are already the matches of the original TG which are way over that criteria.

Please also keep in mind that the number of SNP's plays a very strong indicator when it comes to rules out IBC. It's like the strength of your password, every character more makes it much more hard to crack. Same is true for the number SNP's, the more there are identical the less likely it can be IDC.
by Andreas West G2G6 Mach 6 (63.6k points)
+6 votes
IMO, the SNP counts are very indicative of a high potential for these to be IBD.  I would certainly use them as a goad to search for more documentation to extend the reach of both trees with the hope of eventual traditional confirmation of a relationship.
by Phil Kuhn G2G2 (2.8k points)
Fully agree with you Phil. Such a high number of SNP's in a row that matches exactly is a clear indication for IBD.

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