Why would GEDmatch report no relationship to my 3rd cousin?

Question

I'm quite new to looking at DNA, but I thought I had a basic understanding, ... until today...

According to GEDmatch, my 1st cousin and I compare as follows:  Largest Seg = 70.1 / Total cM = 758.8 / Gen = 2.1

That makes sense.

My 1st cousin compares to our mutual 3rd cousin as:  Largest Seg = 33.1 / Total cM = 67.5 / Gen = 3.9

That makes sense too.

And yet, when I compare with the same 3rd cousin, there is apparently: "No shared DNA segments."

I thought I understood things well enough that this shouldn't happen.  I suppose it's POSSIBLE that he isn't really my 3rd cousin, but that would imply that his relationship to my 1st cousin is not what we think it is, but still has the same degree of separation.  That seems like too big a coincidence to accept, especially as we span two continents and our presumed mutual ancestor was in a third.

What do I need to look into here to understand this better?

Thanks! 

Comments

Try entering a number lower than the default (7) in the minimum segment size field in the One-to-One screen. The first cousin relationship looks very solid so it's probably just a lower than average amount shared.

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Thanks, I tried 4 and got something more interesting:

Largest segment = 7.0 cM

Total Half-Match segments (HIR) = 23.2 cM (0.647 Pct)

4 shared segments found for this comparison.

611785 SNPs used for this comparison.

55.615 Pct SNPs are full identical

I obviously need to do some basic learning about all this stuff. I'll be looking for some good intro resource.

Thanks.

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Blaine Bettinger's Family Tree Guide to DNA Testing and Genetic Genealogy is a good primer.

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Thanks!

Answer 1

3rd cousins have about a 10% chance of not matching with the usual matching threshold. You may have gotten unlucky.

Comments

Thanks, it does appear you are right.  Clearly I need to educate myself more about what all this means.

Answer 2

50% of DNA is not passed on from parent to progeny. Eventually relationships are far enough apart that they haven't inherited any DNA (from the reported areas) that is common to both people. Beyond second cousin, you won't always share DNA with your relatives.

Comments

Thanks, I guess I thought that would take longer.  Good to know.