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Segment Clarification Question - Easy

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  • Segment Clarification Question - Easy

    I just wanted to confirm I understand a few topics.

    1. If I take the end of a segment minus the beginning, should I expect that to be the size in centiMorgans of the segment? For example

    Start/End(Morgans) Created Size Size(cM) Mathamatical Size(cM)
    20.2M>60.1 46.2 60.1-20.2=39.9cM
    240.2M>247.2M 14.6 247.2-240.2=7.0cM
    5.0M>88.5M 102.2 88.5-5.0=83.5cM

    a. Is the created size generated by the software correct or is the mathematical size the true size of the segment?
    b. Why is there a difference between these two?

    2. I am working on creating a DNA Map and I see segments < 5.0cM often. Should I not include these as they are just noise?

    Thanks for straightening me out on this ahead of time.

    Regards

  • #2
    No, Mb and cM are two different concepts. Megabases are sometimes called physical distance and cM are called genetic distance. The cM is a probability measure -- how often you expect a segment to be split by recombination (one cM = 1% chance).

    For mapping purposes, you can use Mb to plot the segment boundaries. I'd avoid segments < 5 cM for sure. It might be better to limit your map to 10 or 15 cM segments, at least to start.

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    • #3
      Thank you so much for quick answer. One followup?

      How should I use the SNPs? What do they reveal?

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      • #4
        Originally posted by USMC7312 View Post
        Thank you so much for quick answer. One followup?

        How should I use the SNPs? What do they reveal?
        You can ignore the SNP counts. It's a case of sampling theory -- we want to be sure we've sampled enough SNPs to be reasonably confident we're not overlooking a contradiction to the match. FTDNA has set reasonable thresholds for that.
        Last edited by Ann Turner; 1 March 2018, 12:32 PM.

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        • #5
          Awesome. Many Thanks

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