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All righty now, two posts in a row focusing on a DNA question!

I’ve posed this question in one of the Facebook groups dedicated to DNA and genealogy to see if anyone there had an opinion.

While looking at my latest GEDmatch results, I found a new match near the top of my results with an estimated distance of 3.9 generations between us. Still exciting to see that after all these years swimming in the genetic genealogy ponds. I have a few others of the same distance, but haven’t gotten anywhere with figuring out the shared ancestors with most. (Due to either lack of info far enough back, or just the usual non-response to any reaching out.)

Gedmatch single segment match of 68.3 centimorgansBut, something caught my eye on this match: the “Total cM” and “Largest cM” are both 68.3, so it is all in a single segment! Over 8,341 SNPs. Count ’em.

(By the way: this is using default search settings.)

So, I am now wondering: At what amount of shared cM does it become unusual for the entire match to be in a single segment?

Looking at all of my matches, I have:

  • 1,301 matches that are in a single segment.
  • 1,300 of those are at 32cM or below.

After that, way up at 68.3cM is my last single segment match.

Is the fact that it is one segment with that many cMs just chance, or is something else going on? It is more striking when you see a chart of all of my single segment matches.

Anyone have any ideas?

chart: Count of Single Segment DNA Matches by Match Length

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    Comment by Jerry — March 18, 2021 @ 3:12 am

    The higher the shared dna, the higher the probability that a single segment will be broken. So you will have fewer matches with larger single segments.

    I have a match of 65 cM, with largest of the TWO segments being 35 cM. You do the math for the second one. Pretty serious match at first. But because of endogamy, a dna expert told me I should likely dismiss this match – and also because reportedly some companies – like MyHeritage – take or process dna matches on different chips or something like that. I guess this is why I see the same person appearing more than once on my list. Annoying.

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