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I ran a GEDmatch multi-kit analysis and got this report for Matches with Joseph's kit, for chromosome 12:

enter image description here enter image description here

And putting a different kit in first position, I get this report for matches with Charlotte:

enter image description here enter image description here

The strange thing I notice is on Chromosome 12, in the first report I get several matches that span as much as from 1,359,590 to 5,316,226 (about 12.3 cM). In the second report, I get Edna matching Charlotte from 1,517,192 to 3,941,530 (7.58692 cM).

The segment matches Charlotte from the first report, so why doesn't Edna appear in the first report? Is this a bug?

NOTE: All kit numbers have been removed, and names changed to pseudonyms for ease of discussion.

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    Would it be possible to obscure the names associated with these kits? See this post from The Legal Genealogist: legalgenealogist.com/2015/01/25/9381 "...we should not take a screen capture of DNA results from a testing company and post it in a blog post or on Facebook with the names or pictures of our matches still attached unless we’ve asked those matches specifically if we can post it." Linking to results isn't the same as a screenshot, but I think the same caution should apply.
    – Jan Murphy ♦
    Commented Apr 10, 2017 at 23:25
  • William – I've edited the question to remove the names and kit numbers, and are happy to reopen it now there is no identifying information. I've replaced all the names with pseudonyms.
    – Harry V. ♦
    Commented Apr 11, 2017 at 12:51
  • @ColeValleyGirl I think this warrants further discussion on Meta as to what our policy on this is. Under the current revision someone could still argue that it contains DNA data without their permission. Providing a useful answer may now be difficult because it doesn't contain enough detail.
    – Harry V. ♦
    Commented Apr 11, 2017 at 13:45
  • @WilliamKF What cM threshold did you use for each analysis? Might a simple explanation might be that the match between Edna and Joseph is below the threshhold you used?
    – user104
    Commented Apr 11, 2017 at 13:54
  • 2
    So little information remains in the edited question that I doubt much can be said about the particular situation.
    – RobertShaw
    Commented Apr 11, 2017 at 19:46

1 Answer 1

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There are 46 chromosomes that come in 23 pairs, one from mother and one from father. You are talking here about chromosome pair 12.

The way matching works is that it cannot determine whether the match is occurring from the maternal chromosome or from the paternal chromosome. It only determines that either the maternal or paternal chromosome is matching at each point along the chromosome pair.

In your case, on that segment on chromosome pair 12 you have:

  • Joseph matches Laura
  • Joseph matches Charlotte
  • Charlotte matches Joseph (good, that verifies)
  • Charlotte matches Laura
  • Charlotte matches Edna (7.6 cM)

There are two possible reasons why Edna does not match Joseph.

  1. Charlotte matches Joseph on one of her chromosomes (maternal or paternal), and Charlotte matches Edna on the other.

  2. Charlotte and Edna match not because it is a common segment, but because it is a by-chance match where matching base pairs crisscross between the paternal and maternal chromosomes and appear to the matching algorithm to line up. The smaller the matching segment, the more likely this is to happen. Normally you need a 15 cM segment or longer before by-chance matches become improbable. At 7.6 cM, you always need to be suspicious that this might be a by-chance match.

Laura matches both Joseph and Charlotte so they triangulate meaning the 3 of them match each other on the same segment. Triangulation (without a by-chance match in there) is good assurance that they are matching on the same chromosome.

To verify whether Edna is matching on the other chromosome, or whether Edna's match with Charlotte is a by-chance match, you need other people to compare to. You will have to find at least one other person who matches Charlotte on that segment but does not match Joseph and Laura. Then on GEDmatch, you can do a one-to-one compare of that new person with Edna. If the new person matches Edna, then Charlotte, Edna and the new person all match on the same segment and triangulate. They will be matching on Charlotte's other parent's chromosome from the one she is matching with Joseph and Laura.

If the new person does not match Edna, then either Edna, or that new person, or both are a by-chance match with Charlotte and you'll need to find yet another person to compare with.

The key is to realize (1) there are two chromosomes in each pair that a person can match to, and (2) by-chance matches on small segments are possible. Knowing this allows you to logic through the possibilities.

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  • It was my first thought too that they matched on opposite chromosomes, but looking at the data (which has since been edited out of the question) I came to conclusion this was unlikely
    – Harry V. ♦
    Commented Apr 12, 2017 at 14:54
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    @HarryVervet interested to know how you could tell that from the 2D chromosome browser.
    – user104
    Commented Apr 12, 2017 at 17:35
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    @WilliamKF - Kitty Cooper describes it quite well here: blog.kittycooper.com/2014/10/… She gives an example of Dad giving her AAAAAAAAAAA and Mom giving her CCCCCCCCCCC and how she would appear to match to anyone with either A's or C's on that segment, e.g. ACACCAACCAC or CCAACCCACA. So using that example, people with AAAAAAAAAAA on that segment will match to each other on her dad's side, and people with CCCCCCCCCCC on that segment will match to each other on her mom's side.
    – lkessler
    Commented Apr 14, 2017 at 4:44
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    @WilliamKF - Roberta Estes gives a similar example here: dna-explained.com/category/ibc-identical-by-chance and talks about an identical by chance match due to "zigzagging back and forth between your Mom's and Dad's DNA strands."
    – lkessler
    Commented Apr 14, 2017 at 4:53
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    @lkessler Thanks, didn't realize the technology was doing an OR and thus allowing a match to hop back and forth between your mom and dad. Hopefully they will improve the sequencing in the future to map the two sides of a chromosome independently, which would make a match transitive.
    – WilliamKF
    Commented Apr 15, 2017 at 2:38

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