We receive DNA from our mother and father, who received it from their mother and father. And for most of us, those ancestors going back several generations are unique. But what happens when they aren’t unique?
Most of us have a family tree that branches out, growing in size with each generation, and our DNA tree does as well to a certain extent. However, many people find in their heritage that in some cases those branches are the same branch.
So what happens to your DNA when your tree doesn’t branch because your family kept on marrying within the same group? Whenever you are related to the same person in multiple ways, you have a multiple relationship. You may be related to the same person in different ways.
Since DNA can accurately predict relationships out about 5-7 generations, this is the timeframe we are looking at. Almost everyone will have their tree collapse as they go out 10 generations or more, simply because the population groups we lived in just a few hundred years ago weren’t that large.
Let’s look at the easiest one to understand, double 1st cousins. Double first cousins occur when siblings marry siblings. So, you end up related to your cousin through both sets of your grandparents. With DNA, 1st cousins share on average 12.5% of their DNA. But as you can see with this example, all of the shared segments are just half matched, because they are only related through one parent.
With double 1st cousins, the first thing we need to do is double the amount of shared DNA, so 25%. Next, because they share a relationship through both of their parents, double 1st cousins also share some portion of fully matched DNA. About 22% is half-identical and 1.5% is fully identical. None of the other 25% relationships have fully identical regions, not grandparents, not aunts/uncles, not nieces/nephews, and not half-siblings.
After double 1st cousins, there is any myriad of double relationships. Albert Einstein married a woman who was a 1st cousin on his mother’s side, and a 2nd cousin on his father’s side. I don’t know what you call that, relationship: wife+1st cousin+2nd cousin?
Because of the multiple relationships, figuring out how you are related can get really tricky. The most basic thing to remember is that you add the expected amount of DNA for each relationship. So for double 2nd cousins, you would expect to share 3.1%+3.1% or 6.25%. Except that DNA doesn’t follow the average value, so at the lower extreme, double 2nd cousins may share 190 cM (85cm +85cM) which kind of looks just like regular second cousins. On the other hand, double second cousins may share 960 cM which kind of looks like 1st cousins.
Once you get out to 3rd cousins, it gets even worse. Because these relationships potentially share 0 cM, you could have double 3rd cousins who don’t show up as a match at all or share 350 cM and look like a 1st cousin once removed. When you are trying to find unknown relatives, figuring out the right generation to look in can be daunting.
It’s possible to have a triple relationship or more, you just have to keep adding DNA for each relationship.
And then there is endogamy, which is a subject for another video. But one way to look at it is when everyone in a group is related to everyone else in a group in multiple ways (more than just 2 or 3). In other words, siblings and cousins were marrying other siblings and cousins ad infinitem.