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?