Sunday, April 1, 2018

Deceiving DNA Test Results

A new DNA match appeared for me the other day. I recognized the name as someone I already had in my database. He is my fourth cousin on my mother's side. He's also descended from my great-great-great-grandparents, Johan Herm Theodore Menke and Maria Catharina Schirren, from Schwagstorf, Osnabrück, Lower Saxony, Germany. Family Tree DNA estimated him to be my 2nd to 4th cousin, so the estimate lines up. I started looking at some of our common matches and since he's more closely related to my mother, I brought up their common matches. That's when I noticed. Family Tree DNA estimates that my mother and this fourth cousin of mine are 5th to distant cousins. How can this be? Unless he's related to me on my father's side, which would be a complete surprise, this doesn't make any sense.

I posted my question on the Family Tree DNA users group on Facebook. I can't encourage you enough. If you have genealogical questions, be sure to post them in the appropriate groups on Facebook. There are knowledgeable people out there that are happy to answer your questions!

I brought up this cousin's DNA in the chromosome browser along with mine. I also brought it up with my mother's and my aunt's DNA. I wanted to compare the matching segments. The segments that are interesting are on chromosomes 8 and 21.

Chromosomes 8 and 21 between this fourth cousin and myself
Chromosomes 8 and 21 between this fourth cousin and my mother
Chromosome 6 between this fourth cousin and my aunt
You can see the blue, non-matching stripe in the middle of the segment on chromosome 8 on my mother's test and, while you can't see it very well, the segment on chromosome 21, is longer on my test than on my mother's. It's 9.74cm on mine and 9.3cm on my mother's. In addition, the matching segment on my aunt's chromosome 6 encompasses a centromere, which is a "pinched" area that cannot be tested.

The helpful person on Facebook described these as "no calls". I had never heard this term before. No calls, in regards to DNA tests, are small segments of DNA that, for whatever reason, did not appear in the results of a DNA test. These are technically errors in the test where I inherited segments of DNA from my mother that don't appear on her test results. As long as less than 1% of your DNA results are no calls, the company considers the test valid. I'm told that if we would test at another company, it's probable that we would not see these same no calls. We'd likely have no calls at other points of our DNA.

The thing that is going to make these particular no calls challenging is that they appear in segments in common with a known cousin and makes one long matching segment with my mother appear as two shorter segments, resulting in a drastically reduced estimate of how closely she's related to people on this branch of her tree.

So, keep "no calls" in mind when you see DNA matches that don't make sense. It might not be that this person is a double-cousin. It could be that one of the people you're comparing just didn't have that segment of their DNA show up in their results.


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