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Saturday, March 10, 2012

More DNA tests mean more information (part 2)

In my last post, I looked at how I could use the “matches in common” lists on Family Tree DNA to pencil in which branch my DNA matches (probably) belong to.

With the addition of my father’s and two cousins’ match-in-common lists, I was able to label each of my matches as connected to my:

  • maternal side,
  • paternal side, or
  • a specific branch on my paternal side, “French”.

Up to this point, all that I have used is the lists of names. My next step is to start looking at the chromosome browser results. This is where you can see what segment(s) of DNA you have in common with each of your matches.

What I intend to do is much like the logic I have discussed before, only this time I want to take it a few steps further.

Matches-not-in-common with a known cousin

The key point to remember here is that for each stretch of DNA, half is from my mother’s side, half is from my father’s. If I match person “A” and person “B” at a particular location, but they don’t match each other, then I know that one is from my maternal and one is from my paternal side. Without additional information, I don’t know which! I now have that additional information for my own DNA matches, as I can see if my father (who happens to have contributed half of my DNA) matches.

For my father’s matches, I can look at his matches not-in-common with our “French” cousins to do something similar. If my father matches a “French” cousin at a particular location then anyone who matches him at the same location, but is NOT a match in common with my “French” cousins (who are on my father’s paternal side), must somehow connect to his maternal side, the “Bennett” branch.

Then it gets weird

So far so good. I started working away at my file on this basis and quickly found a match to my father at the same location on his DNA as my “French” cousin matches, who didn’t show as a match to them. Great! This person must match my “Bennett” family! Then I noticed that I had another match with the same surname and same contact email address on the same segment of DNA, who also did not match my French cousins.

No problem there? Well actually, there was a little problem. When I said “I” had a match I really meant “I” and not my father. My father’s matches not-in-common with my “French” cousins must connect to his “Bennett” branch. My matches not-in-common with my “French” cousins must connect to my mother’s side. Any yet – the two matches had the same surname and contact email address.

I wanted to find out more about these odd matches, so I sent off an email and asked. It turned out that one of our matches was the uncle of the other. My father matched the nephew, I matched the paternal side uncle. So, by using the same sort of logic, my father connects to the maternal side of one match, and I connect to the father’s maternal side of the other. Like a mirror image of how they connect to me. All on the one segment of DNA.

At least, I think that’s how it goes. It took a bit of getting my head around this. Another way of looking at it is this:


But what a funny coincidence that it’s all at exactly the same location on our DNA. It’s even possible that our relationship looks like this:


Ouch! My head hurts!

I think that the moral of this story is that when you pencil in a branch, you really need to pencil it in and keep your mind open to other possibilities.

I think I’ll have to leave it there for tonight…

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