Measuring Segments

For autosomal DNA, segments are measured 3 ways:


Example: Start at 23,500,000; End at 46,300,000


Example: The segment is 23.5-46.3Mbp



Example: A segment may be 15.4cM


Example: A segment may include 2,451 SNPs


So, in summary:







03 Segmentology: Measuring Segments; by Jim Bartlett 20150513

19 thoughts on “Measuring Segments

  1. Hello Jim,
    At GEDmatch on the one to one match when specifying graphics and full resolution the chromosome bars have two scales. One is Mbp, what is the other scale? Is it a metric scale?


    • Bill, It’s a cM “scale” of sorts. Our chromosomes have hot spots and deserts for recombination crossovers. The cM data is determined from lots of data. There is no conversion factor from cM to SNPs or cM to Mbp. You’ll note the carrot marks are not uniformly spaced like the Mbp marks are. So we cannot interpolate between the cM carrot marks – they are just a guide.


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  5. Jim, what do you make of a match that shows a single large segment match but not much else. Several times I have found a match of over 20 cM on one chromosome but nothing else over the default (7cM) threshold level anywhere else.


  6. Cheryl

    You say you used the Chromosome Browser and found all of these segments “matched”. Yes, you can use the CB and determine they all match you at the same location; but no, this does not mean that they match each other. At FTDNA it is impossible to check two of your matches to see if they match each other. You have to check them using In Common With to see if they are ICW each other.


  7. Hello Jim,
    I am a newbie to all of this but I found your article very interesting and understandable to someone with limited knowledge of DNA.
    I tested at Ancestry and 23andme, then uploaded to Family Tree DNA and Gedmatch. Last night I was reviewing some information.
    I was just reviewing mt DNA results on Family Tree DNA. I noticed that 21 of my matches matched me on Chromosone 19 at the exact range and they all had 8.15 cM. I also noticed that another seven matches were 9.14 cM at another spot on Chromosone 19, I have many more matches to compare but stopped when I noticed this pattern emerging. Is any of the information relevant? Does it means anything. My father is unknown to me. Thank you in advance for your help. Cheryl Glenn Bradley


      • Hello Jim, I am not sure what you mean about a pileup. I have checked the matches with me in the chromosone browser on FTDNA and we all match. I am not sure what to do next. Thank you for answering me.
        I do have a higher person on FTDNA who has a total of 127.61 and the longest is 60.22 on Chromosone 11 and 49.99 on Chromosone 17. There are two other matches who are on Chromosone 10 and one is 28.14 and the other is 30.73. From what I read and if I understand it, I might be able to find a common ancestor with them as their matches are higher in number. Is that correct? Since I do not know who my father is and have no information on him, is it possible to determine how I am related to these matches and how? Thank you so much for answering me.
        With Warmest Regards, Cheryl


  8. The SNPS may sometimes be a secondary measure, but not often. The 16cM segment is always good. The 9cM segment with the fewest SNPs is suspicious. I’d make sure that one matched at least 3 others.


  9. Hi Jim,
    I’m struggling a bit with the SNPS, so based on your comment, if I have results like below, that does not tell me anything other than the 16cm match is closer (in terms of generations) than the others? Here are a few examples,

    Match A with 16 cm 700 SNPS
    Match B with 9 cm 2800 SNPS
    Match C with 9 cm 500 SNPS

    If SNPS aren’t useful, to determine something, I’m wondering why this info is included Does it matter if the matches above were on the same chromosome (other than Match A is a closer cousin)? Told you I was confused!



  10. Thanks, Jim. I was informed of your blog via Roberta’s. This “Segmentology” series are excellent! I’m able to apply immediate the methodology you’ve outlined. I’m also currently in a family project in which triangulation is proving to be an exceptionally helpful tool in deconstructing and reconstruction the “yellow brick road” to our common ancestor.

    Liked by 1 person

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