Most of my maternal heritage is English and Irish. My mother has many Family Tree DNA Family Finder matches for her autosomal DNA but I cannot tell whether they came from her mother’s side or her father’s side. In order to try to sort this out, I need to test as many known cousins as possible.
My mother has a first cousin who is the child of her father’s sister. By testing K, as I will refer to her, I can find matches in common, which is often referred to as triangulation. The only known common ancestors between my mother and her cousin K are James and Helen, their common grandparents. Therefore, any segments that match between them should be inherited from James and Helen, their common ancestors.
Family Tree DNA offers a tool called a chromosome browser, which can be used to examine common segments of autosomal DNA. The chart above depicts the 22 autosomes of K. The areas that are highlighted in orange are areas that my mom and K share identically in common. The areas in blue are the areas that my mother’s brother shares identically in common with K. As you can see, some of the shared areas are the same and some are different. This is because of the random way in which autosomal DNA is inherited. My mom and uncle both received a random shuffle of autosomal DNA from each of their parents.
Any place where my mom or my uncle matches with K can be attributed to James and Helen. If I had only tested my mom or only tested my uncle, I would have lost the information about the areas that my mom and uncle do not share in common with each other that they share with K. This is why it’s important to test as many siblings and cousins as possible.
When my mom or uncle have other matches in the database who match in an area on a chromosome where they match K, I will now know that I should look for the common ancestor to be somewhere in my maternal grandfather’s tree. As I test more cousins, I will be able to “sort” out matches as maternal and paternal, and hopefully into quadrants or even further, by testing second cousins and beyond.