Thursday, November 29, 2018

When GEDmatch and The Testing Company Are Far Apart

I have autosomal DNA tests with four companies. I am not going to say which one this refers to. They can all say "This isn't us. Must be someone else."

When you test with the DNA companies, you know that you will have matches available to you. Some send you notices, some do not but allow you to look for yourself. When I see such matches for my own test results, I contact the owner of the matching kit and ask if the kit is on GEDmatch. That would give me a possibility to see who else in my family matches this kit. Is this new match on my father's side or my mother's side. Which grandparent or great-grandparent? The Hungarians, the Slovakians, the Galicianers or the Russians.

Some of these new matches reply, some do not. When they do, I take their GEDmatch kit and see how it lines up with my family. Usually nothing comes of it, but sometimes we get some sense of geography, if not specific family connections.

But sometimes the match does not show up on GEDmatch at all. And this problem is very common - perhaps 30% of the time - with one particular company. I have lost count of how many times I have written:
Unfortunately, despite what the company says, GEDmatch does not show a match with me at all. Or my brother. Or my sisters. Or my father's sister and brother. Or my first cousins.
Here is a recent example. The company labelled the match as a suggested "second cousin to fifth cousin." They said that we had shared 70.5 cM spread across nine segments, with the longest 13.5 cM. It looked worth an inquiry, though not particularly promising.

I wrote to the man and his wife came back to me with his GEDmatch number. I looked at his GEDmatch results andand I was nowhere on his one-to many. I tried on the Tier1 one-to-many, searching all his matches, not just the 2000 that is the GEDmatch limit. And there I was. A match with a total of 18.5 cM and a longest segment of 11.2 cM. There were only two segments.

I dropped the search threshold to 3 cM and found our match with four more small segments bringing the total to 32.5 cM. This is less than half of what the company showed. And the company's longest segment is 13.5 cM while GEDmatch shows only 11.2 cM.

But it gets worse.

The longest segment on GEDmatch is on chromosome 7, a segment where the company shows only 6.0 cM.  The company's longest segment is on chromosome 15; GEDmatch has nothing at all on chromosome 15.

In total, GEDmatch has six segments to the company's nine. Most of those segments are on different chromosomes entirely.

We know that there is much to be done as DNA for genealogy emerges from its infancy. Basic consistency would be a good place to start.

6 comments:

  1. Replies
    1. If I had wanted to ID them in public, I would have.

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  2. This is very disturbing. Why aren't you disclosing which company? And which do you think is more reliable---GEDmatch or the testing companies?

    ReplyDelete
    Replies
    1. I don't care to challenge a specific company in public.

      I have a bias to GEDmatch but I acknowledge it as a bias - if only because I can do more with them than with anyone else.

      Delete
  3. Have you tried lowering the SNP threshold on one-to-one at gedmatch? One company has a 128 SNP minimum, which is way below gedmatch's default of 500.

    ReplyDelete