First cousins share around 12.5% identical DNA. My grandfather is not available to test, but we do have the next generation: his son. The expected amount of shared DNA is reduced in half, or about 6.25%. The actual shared DNA between these first cousins, once removed, is 6.71%. As the next generation, I could expect to share about 3.125% with a first cousin twice removed. Again, the amount of shared DNA is slightly more at 4.44%. [Extrapolate on these numbers for a few more generations and you can envision how some of your ancestors' DNA becomes undetectable.]
|Relative Finder at 23andMe|
My father's top three matches.
DNA comparison between First Cousins, once removed.
Last year, after testing my father's maternal third cousin, we discovered that he also matches my mother, though we do not know how. In this same spirit, my father's paternal first cousin matches my mother's brother. We do not know how.
This testing creates new questions to be answered.
This paternal cousin is somehow related to my maternal uncle!!!
So what is the point of testing these different family members? To narrow down the other matches to specific branches of the tree. When a distant genetic relation matches both my father and his cousin, the search for the most recent common ancestor is narrowed to my father's paternal grandmother's tree. Many of these lines have old roots in Monmouth County, New Jersey.
In upcoming posts we will examine some of my father's matches who also (surprise!) match this first cousin.