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On GEDmatch's autosomal one-to-one report, a match contains a column for cM and a column for SNPs values, for each of the matching chromosomes. Normally you look for high cM values as an indicator of genetic distance (and to some degree, generational distance). But I also understand that it's important to take into account the number of SNPs in relation to the cM value (the greater the SNPs, the better it is).

But, in my research, I've discovered high cM values with very low SNPs (for example, a common value that I find is: 14cM / 128SNP). On the other hand, I have very low cM values with very high SNPs (3.2cM / 4968SNPs).

I've not found these contrasting amounts useful for establishing valid matches so far (and often delete them from my database). But I'm wondering if I should take these "extreme" segment matches more seriously -- do they have any value? How should I interpret them?

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Ignore the SNP counts

Centimorgans are a weighted unit of measurement. The cM number depends entirely on the location of the segment and has little to do with actual length.

The SNP number is the actual length (more or less since all commercial DNA tests with matching only test a fraction of total SNPs).

Sure, the SNP number can be useful to advanced researchers, but it's not anything you need to worry about. Focus on the cMs. And ignore any under 5cM (preferably anything under 7cM), unless you're doing a very specific task where small segments can give you information (intermediate level stuff).

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  • Your answer could be improved by adding links to posts explaining why small segments are usually ignored, such as Blaine Bettinger's A Small Segment Round-Up: thegeneticgenealogist.com/2017/12/29/a-small-segment-round-up
    – Jan Murphy ♦
    Commented Nov 28, 2018 at 22:39
  • However, at GEDmatch it is important to note small SNP counts. Different companies test different SNPs, and if there is a low SNP count for a match, say under 500, then it is a low quality match with a good chance of being a false positive no matter how many cM the match might be.
    – lkessler
    Commented Nov 29, 2018 at 4:51
  • In one of Dianne Southard's webinar's, she suggested that 15 cM is actually a better threshold - that segments below 15 cM are about 80% likely to be IBC and above 15 cM about 80% likely to be IBD (I'm sure it's actually more subtle that that). This also seems to be in line with my personal experience.
    – cleaverkin
    Commented Dec 4, 2018 at 19:57
  • I'd be interested to see more on that. She is going against all the genetic genealogy wisdom and leaders that I've seen. 7 is pretty standard with 5 common. I may have seen something higher than 7, but not higher than 10. 15 is rather shocking, actually.
    – Cyn
    Commented Dec 4, 2018 at 20:03
  • Are you sure that's not the threshold for the largest segment? Most of us in endogamous communities say 20 for the largest, but 15 is not unreasonable, especially for non-endogamous. The 5 or 7 cM segment minimum I'm talking about is for any segment used in adding up the total cM match. For a close match, there would need to be one or more segments significantly larger than that.
    – Cyn
    Commented Dec 4, 2018 at 20:03

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