We will call him "PK." He is from my maternal grandmother's branch. He is a second cousin of my grandmother. They share a pair of great grandparents: Peter ODONNELL and Margaret GALLAGHER, say born around 1820, in Ireland. They had four children (so far discovered): Katherine, Rose, Patrick, and Cornelius, all born in Ireland, but came to the United States. PK is a grandson of Rose ODonnell, while everyone else in these DNA comparisons are descendants of Patrick ODonnell.
At 23andMe, PK supplied the names of his four grandparents and their locations. I knew which of his lines to pursue when I saw the surname ODonnell and the location of Bayonne, New Jersey, United States.
My grandmother is long gone, but her first cousins have kindly supplied their DNA to help our family history research. They are sisters and second cousins of PK.
PK matches one of his second cousins 192 cM over 9 segments and the other 264 cM over 14 segments. This is within the expected range for second cousins.
|When 23andMe expanded the comparison function from 3 people to 5 people,|
the corresponding chromosome chart also expanded and does not copy well.
To see how the DNA passed from one generation to the next, we can compare PK to the children of one of these second cousins. Notice that one of these children shares a tiny segment on chromosome 5 that is not shared by his mother. This can be because of a misread in the DNA or can indicate that this child is also related to his mother's cousin on his father's side.
PK has five people from my branch who are second cousins, once removed. You can see the variation in the amount of shared DNA. Different branches inherit different pieces of DNA in smaller amounts. This is why third cousins and more distant will share little to no DNA. That is the design.
Below is the comparison of PK to my mother and to my sister and me. Genetically, a second cousin twice removed is the equivalent of a third cousin. Note that my sister shares only a very tiny segment with PK. This is very important, as a lot of people (myself included), might disregard such a small match as "too distant" to realistically identify in a family tree that only extends to the 1800s. Yet uncovering PK's relation was not difficult at all.
Now that we have a cousin from the ODonnell/Gallagher branch in the genetic pool, we can compare him to our plentiful DNA cousins to narrow down where we must look for a common ancestor.