How rare is it to match someone on EVERY single gene?

Question

Searched for a rare name on my mother's side Boerjan and found someone whose gedcom on gedmatch has it.  

I ran his gedmatch number through with a minimum of 1 cM and was very surprised!  He matched for a total of 324.5 cM (9%) but with no single match over 5.7cM.  What was even more surprising was that he had at least one segment on all 22 chromosomes (everything but the X).  I have never seen that before.

So my question is even with this amount of shared DNA - are none of them eligible for a DNA confirmation because less than 7cM?

That would seem to be counter to reason.  I still do not know which Boerjan he is from (or even if that is the real link) but at 9% (324cM) he is most likely a second cousin.

Comments

I ran his and my gedmatch kit number to find common matches and found 47 who all seem to match 9-11% of my DNA in mostly small chunks with a few over 7cM.

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Have you contacted any of them? Checked to see if they are PM at Wikitree?

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working on it now

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The 324cM = 2nd Cousin relationship is only for totals computed using a reasonable minimum segment threshold, like 7cM. It happens a lot that the total will increase by several factors dropping the threshold as you have done. So you need to be careful with the relationship estimate process and only use totals computed the way the big companies do. Even then, the companies all do things differently and so those relationship ranges are just very rough guides.

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Good to know.  The totals are much higher if counting everything.

Answer 1

Short "matches" are often not real, just an artefact of the testing process.  The system can't tell whether two segments actually match, it can only tell if they're a bit similar.  Then you say, if they're similar over a long enough stretch, it's probably a match.  But random noise will often look similar over short stretches.

The testing companies mislead people by using the word match.  But then, misleading people is what they do.  If people understood the limitations of what they're selling, people wouldn't buy.

Putting it another way, if your parents were also tested, and compared with the same third party, you would find that you sometimes have a "matching" short segment where neither of your parents matches.

Answer 2

My mother has a known 6th cousin on GEDmatch (who shares at least two dozen common ancestors; this person is related to my mother through both of her parents and my mother and I are related to 7 out of 8 of their great-grandparents); at a 1cM threshold they match on 21 out of 22 autosomal chromosomes for a total of 202cM, and 31.4cM on the X chromosome (with a total over 7cM of 39.6cM in three segments on different chromosomes).

Comments

Is this an example of endogamy back in the line somewhere?  It seems almost unreal.

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2 dozen common ancestors between 1650 and 1750. No single relationship closer than 6th cousin but also 6C1R, 7C, 7C1R, 7C2R, 8C, etc. on different lines. On edit: I would guess that if your ancestors were from small farming villages you may find that a degree of endogamy lays on the other side of your brick walls, based on the shared small segments.

Answer 3

Be careful with matches less than 7cM.  https://www.wikitree.com/g2g/199470/are-you-using-audna-segments-which-are-less-than-7-cm?show=245672#a245672