Friday, August 8, 2014

Autosomal DNA Matching 102, Class Two

In a previous post, I wrote about watching out for randomness in DNA matches. A new cousin match has helped me find a simple example of how just one match can be misleading. It's so important to find multiple matches to validate a conclusion.

This randomness in matching is called Identical by State or IBS. Matches due to family relationships are Identical by Descent or IBD. There are other complications in matching, but this is intended as a simple explanation of IBS.

This new match is from my father's Fry/Derrick family. On GedMatch, I've run a one-to-one match against our new cousin, "Helen", with each of the kits I manage. The blue shows the small area of chromosome 3 where we each match Helen.

Notice the tiny red edges that I added on the second and third lines. Those bits are where there is a random match, IBS, that is not a relationship match or IBD. How did I know that?

Let's explore the numbers. You don't need math, just logic.

On top is my Dad, who is a fourth cousin once removed to Helen. For now I'll assume that his entire match is Identical by Descent or IBD.

For me, any matching fragments lower than 22134900 are not part of the Fry/Derrick match, as those fragments did not come from my Dad. They came from Mom. Any match fragments higher than 29810802 are also not valid, as they came from Mom, too.

The same logic applies to my daughter's match and my granddaughter's match. I can only pass on what I have -- I can't add Fry/Derrick DNA and pass that on.

My matching fragments end too high and my daughter's matching fragments start too low. So my red area on line two is just a random fragment at the high end. My daughter's red area on line three is a random fragment at the low end. Those little bits are Identical by State to Helen, not Identical by Descent.

We need other people who are not members of my direct family to confirm the numbers and help weed out any other IBS randomness in this match.

You might think that I'm selling DNA tests! Honestly, I'm not. I am sharing the little lessons that I'm learning from administering multiple kits. I'm advocating for testing as much as we can and for using every tool at our disposal. I'm anxious to add my brother's kit to the mix, as I expect there will be many more lessons to learn.

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