Size Matters for Matching DNA Segments

The more large segments you share, the more closely related you are to a DNA match. To learn more about segments, read the new blog by one of my favorite DNA writers on the yahoo DNA-NEWBIE list, Jim Bartlett, called Segmentology.org
Brynne Ancestor MapHe explains why you share such large chunks of DNA with your closer relatives by showing how recombination works and how few cross-overs there usually are in a recent post at  http://segmentology.org/2015/05/24/segments-bottom-up/

While the total amount of shared DNA in centimorgans (cMs) is a fine indicator for your closest relatives out to second cousins, farther relationships can be more and more random in the amount of shared DNA. You can consult the charts and numbers at ISOGG here – http://www.isogg.org/wiki/Autosomal_DNA_statistics – to determine those closer relatives.

Although I like this new chart from Blaine Bettinger the best:

Shared DNA statistics from Blaine Bettinger

Shared DNA statistics from Blaine Bettinger, used by permission

Note that since you share about 25% with an aunt, uncle, grandparent, grandchild or half sibling the amount shared cannot tell those relatives apart. Sometimes the X can help distinguish those, see my post on that here.

Frankly the testing companies are not that accurate in predicting your more distant relationships, those past 2nd cousins. So when deciding which DNA matches to pursue, follow up on the ones with more than one large segment; those are your closer relatives, the ones whose trees may be easier to connect to yours.

14 thoughts on “Size Matters for Matching DNA Segments

  1. With my two successful matches, or matches with whom I found a connection, I know how unpredictable the size can be. And recently, after having a lot of cousins tested and two of them are 2nd cousins to each other and they share the largest segment 40.4cM, total 157cM, to me was a good indicator on what 2nd cousins should be sharing, more or less. Since I get pages and pages of 2nd cousins, if not predicted closer (FTDNA), it was good to go by this.

    My mother has had a 50cM largest segment match (total 122cM) and knew that this had to have been a close match. Took me over 7 months to figure that out (actually 11 straight days of non-stop research) to find out that my mother and this match are 1/2 third cousins. Pretty distant in a way, but that 50cM is pretty large. The next largest segment is 38cM.

    My mother gave my brother basically that same largest segment whereas I got only 14cM of that same exact segment. I actually share 28cM of that 2nd largest segment, but had I not got my mother DNA tested (I tested first) I wouldn’t have paid attention to that match, and actually I didn’t until almost a year after my mother tested.

    Definitely from 2nd cousins on it can be unpredictable.

  2. Kitty, a great post – and thanks for the plug. My true brother and I have such a mish-mash of DNA, that the Admixture/Ethnicity programs look like we are not related! DNA is random and very variable.

  3. Kitty – Great post. I’m so glad you like the chart! I’m still working on the Shared cM Project, there’s so much still to do, stay tuned!

    And I agree, Jim’s blog must be added to everyone’s must-read list!

  4. Hi Kitty,

    I am trying to understand the significance between “Total Segments” and “Largest Segments” on Gedmatch. If you have let’s say, 8 matching segments with someone, and the largest segment is maybe 4 or 5cM, and the total of all segments is maybe 17cM, is that of significance when trying to find a common ancestor down the line? I guess I’m confused with terminology because I keep reading in many places that matches over 10cM are pretty good ones, but I don’t know if this means 10cm on one segment alone or a total of multiple segments.

    • Reg,
      In your example the segments are too small for the relationship to likely be findable.
      If there is no endogamy in your tree then anyone who matches you on multiple segments of 7cM or larger is worth following up on. Segments smaller than that are often false, below 5cM they are false more often than true.
      Use total cM of 7cM and larger segments with the charts above which you can get on the one to one compare

  5. I am more or less the kit Admin for my adopted Japanese daughter in law. I noticed two of her matches, a mother and daughter have a lot of small segments. The mother daughter pair are probably 3c1r and 4th cousin. The daughter has the most looking at 3cm she has 130 cM total, other 4th cousins have maybe 30 to 40 cM some a little more. Does this mean anything? I was thinking maybe they are a little closer than they appear?

    • Alan –
      Small segments can be population specific. We tend to only look at matching segments greater than 7cM and total those. Also large segments greater than 20cM are indicative of a closer relationship not lots of little segments.
      Look at the numbers here after you total the larger segments.
      http://isogg.org/wiki/Autosomal_DNA_statistics
      Also if the daughter has matching segments that the mother does not, they would come from her father. Matching on both sides like that can make a relationship look closer than it is.

  6. i share546 centimorgans shared across 24 DNA segments with a match …
    trying to figure out the relationship ..she said its her moms side of the family and she is the half sister and her brother gave a baby girl up for adoption ??

    • Deanna – I am not sure what you are asking. That is on the low side, but the DNA match could be a half-niece relationship. If you are an adoptee or she is, then I recommend the DNAadoption.com web site plus join their mailing list at yahoo and ask questions there. Or DNA detectives on facebook.

  7. Hi Kitty,

    My Mom shares 89.8, 3 segments (chr 1,18,21) longest 74.4 (chr 1) with a male match and
    she also shares 187.2, 6 segments (chr 5,6,7,9,14), longest 78.4 (chr 9) and (chr 14 at 41.5 and 15.3) plus 26 “X” cMs with another male match. Any significance in 2 long segments within one chr?

    I’ve found the same MRCA in their trees but they don’t match each other on Ancestry or Gedmatch! It’s like they’re not related to each other (and not in common with my Mom) but I know they share this Hood line. Can you share any wisdom on this issue and both having long segment matches with Mom? Both are in their 60’s and Mom is in her 80’s…

    Mom’s 187 match thinks it’s possible his great Uncle or a 1/2 great Uncle may be my Mom’s great grandfather. Which means they would share a 2nd great grandparent. I’m having trouble with this because they share more DNA than 3rd cousin level? Thank you!

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