GEDmatch Question: Matching chromosome segments but GEDmatch says "No Segments Shared"??

+4 votes
258 views
Recently, I have found matches with different people here on Wikitree. I compared our DNA through GEDmatch and every time that I do, GEDmatch tells me there is "No Segments Shared'. But when you look at the chromosomes for yourself, you can clearly see that we all match each other on the same Chromosome - number 16 - at the same segment. I've compared everyone to my own DNA as well as each other and we all clearly share the same segment. Most, if not all, of these people are descendants of Hugo Ernesto Krebs (b. 1714, d. 1776). He was from Germany originally and died in Louisiana where my family is from. I have no knowledge of being descended from him or any Krebs but it's still possible seeing as there is no other connection between me and these people and the fact that I have some ancestors with unknown parentage still. And the man DID have 15 children in total by his first and second wives.

Since I can't upload the pictures there, their kit numbers are as follows:

Match 1: A098006

Match 2: A778841

Match 3: A154154 (Match 1's mother who is not a confirmed Krebs descendant but on the "Are Your Parents Related" tool, matches the father on the same chromosome segment)

Match 4: A597227 (descendant of some Krebs but not certain who)

Match 5: M279410

Match 6: A952117 (not a confirmed descendant but matches on same segment)

Does anyone know if this confirms a DNA relationship between me and these people? And if so, how closely?
asked in Genealogy Help by S G G2G6 (7.3k points)
retagged by Ellen Smith

2 Answers

+7 votes
Don't forget you have DNA from both your mother and your father.  They both occupy the same 'address' on your chromosome, and the only way to tell them apart is by who they match with.

This means you definitely can have DNA at the same location as some other DNA tester, without it being shared DNA.  It is on the other side of your chromosome and will match with somebody else, someday.

 I'm sure someone with more skills will answer this more fully, but just thought i'd mention something for you to consider.
answered by Shirlea Smith G2G6 Mach 5 (54.3k points)
there is a clear description of this toward the end of this article:

http://smithplanet.com/stuff/gedmatch.htm
That article is excellent Shirlea.  Thanks for sharing it.
Thanks so much, Shirlea! This was so helpful to me!!
+9 votes
I checked a couple of those pairs on Gedmatch, and I in the images I do see bright green areas near the lefthand end of chromosome 16, an area where both people match on both chromosomes for most of the sequence and match on one chromosome in other parts of the sequence. The image is impressive, but whatever the length of that matching area is, it doesn't include nearly enough base pairs to meet the threshold for use in confirming ancestry. (It didn't even show up after I reduced the matching thresholds to ridiculously low values -- such as 3 cM.)
answered by Ellen Smith G2G6 Pilot (912k points)

You have to take the 'minimum segment cM size' down to 1 to see them, which is something I always do now (and recommend to others), so that I *can* see special segments like this.  This one is the PG16 segment I found, common to many of those who have a German ancestor that immigrated to Pennsylvania.  Most likely, it comes from someone farther back, from the German region itself.

I found it in all of your matches except Match 5.  If #5 has it, then it's too small or mixed to register.  Apart from the parent-child relationship of #1 to #3, the others do not appear to be closely related.

Small segments cannot be used for matching or proving recent ancestry.  The smaller the segment the more distant the relationship to the MRCA.  This one seems to point to German ancestry, and that's about as much as you can conclude.  I like to consider a segment like this as a spectral line, pointing to the existence of a particular population in the ancestry.  It points too far back to use it for proving anything, but is still useful for deciding which lineage a new but unknown relation might be on.  If you know it's *only* on those of your Krebs lineage, then it's easier to figure out how a new relation may fit in, by checking for it on a DNA comparison with them.  To identify a special small segment like this, you *have* to find multiple repeats of it among your DNA relations, to eliminate the possibility of it being only by chance (and the more repeats the better).

The smaller the matching segment the more likely it is not a real match.  Please see https://www.wikitree.com/g2g/199470/are-you-using-audna-segments-which-are-less-than-7-cm?show=245672#a245672
Yes, that's what I was talking about, Ellen! I'm glad you saw it, too lol. It must be a distant connection but I'm guessing it's really there? Since it DOES show up on the chromosomes every time. I found it curious though that we all have the same little segment in common with each other on the same spot.
Ah, my apologies - I hadn't seen this reply yet. Hm, that's interesting! Jm, well I'd found about 16 matches on that same segment which I'd say is conclusive to say that it's not just IBS. That's curious it does indeed come from the German region though because like I said, the Krebs family were from there. And Hugo was the progenitor of that line and he was originally from Germany! Of course that would make myself and these cousins be about 7th cousins which would make sense that it's farther back and not showing up except for that one segment.

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