Just to be upfront, I don’t have an answer yet to this only the questions. I have been working with genealogy for most of my life and with the DNA aspect of genealogy for several years. I already had my DNA in several genealogy banks, but I decided to do a test with 23andMe. At that time, 23andMe was beta testing a family tree created solely from information in our DNA – no records, no existing trees, just DNA.
The underlying genetic calculations on predicting how people are related assume that the parents are not genetically related to each other. Historically, in some areas, the population was so low that families intermarried multiple times over generations. And in those cases, the calculations give erroneous responses.
But Ireland is not currently considered one of those areas that had a large amount of intermarriage.
In the 23andMe tree, my Irish cousins from my mother’s family were all tangled up with cousins from my father’s family! While the tree was in beta testing, there was no way to change the tree to make it correct. But now it is live, and I am slowly moving people around to where they belong — except for the ones I don’t know.
Liz, Kerry, & Bill
I have never met Bill or Kerry, but I do know where they belong on the family tree. Nearly all of my great-great-grandparents were Irish immigrants. This pair, Michael and Margaret, had ten children and eight of those had children. I descend from their fourth child, Kerry descends from their sixth child, and Bill descends from their 10th child. We are third cousins. And our matching DNA reflects that same reality.
Liz and Danna
I have never met Danna, but again this is part of my family tree that I am very familiar with. William and Hanora had five daughters. All five daughters were married and had children. Danna and I are third cousins. My great-grandmother was William and Hanora’s fourth daughter. Danna’s great-grandmother is their fifth daughter.
Clustering – The 23andMe Way
Clustering is a relatively new concept in genetic genealogy to help genealogists overcome the missing family trees. There are several clustering techniques. Like all the big genetic genealogy banks, 23andMe gives you a list of people you are genetically related to, usually sorted in order of how closely you are related to them. It only includes people that have chosen to use their DNA for this purpose. The first person on my list is my first cousin’s daughter. When I click on her name, it shows me that we are predicted to be my first cousin once removed which is another way of saying my first cousin’s child. Since we have chosen to “share” our DNA, we can both see exactly where on each chromosome our DNA is matching. But I can also scroll way down and find the button called “View Relatives in Common.” I click on this and I see the people in the 23andMe database that are related to both of us. So everyone on this list should be related to either my maternal grandfather or my maternal grandmother. Since we are so closely related the list is huge.
Deborah
Deborah is the second person on my matching list and I have no idea who she is or where she fits in the family tree. She is predicted to be my second cousin! This would mean we share great-grandparents. In theory, everyone has four sets of grandparents. I have over 8,000 people in my family tree and Deborah is not in it. I click on the “View Relatives in Common” to see who else in my family tree Deborah is related to.
The first person in the list of common relatives is Danna, and the table shows that she is my third cousin and Deborah’s third cousin. Danna’s niece shows up as my third cousin once removed, but as Deborah’s fourth cousin. The clustering algorithm seems to use whole cousins unless it has specific data to show the removes. If this were all the data available, I would guess that Deborah is another third cousin under William and Hanora that I don’t have in my tree. But, the second person on this list is Kerry, and Bill is also on the list. How is that possible? Danna is related to me through my paternal grandfather’s MOTHER, and Bill is related to me through the same grandfather’s FATHER.
The only other clue I have from the DNA is that Kerry is predicted to be Deborah’s fifth cousin and Bill is predicted to be her fourth cousin.
Are my grandfather’s parents related? I need to do a lot more research before I know the answer to that question.
