DNA Matches and Common Ancestors

Figuring out DNA matches and who might be our possible common ancestors is the most difficult thing to explain to a newbie.

When our DNA matches are group by level of confidence and then we look at our shared matches, I find that having a visual representation sometimes helps.

Thus I decided to create a tree of sorts that shows where our closer matches might fit onto our tree.

Let me explain some terminology that I use. DNA Matches are any match that shows up as sharing DNA with you. Share matches are those DNA Matches that both you and another DNA Match might share. Usually there is a button within your DNA application that you can click after viewing a DNA Match. This button will display all the DNA Matches that are the same people in your DNA Match list and one DNA Match's list that you were displaying. 

Below you will find the tree that I created, and perhaps I have made this more confusing than clearer, however, I wanted to save this image somewhere that I could direct those that I am helping.

Let me explain my graph:

Siblings share common parents, yes I know that half siblings will only share one parent, however full siblings will share two parents.

The next level are your 1st cousins, these are the kids of your aunts and uncles. Your aunts and uncles are siblings of your parents. These 1st cousins have the same grandparents as you. The same one set of grandparents. Okay, perhaps they only share one grandparent, this just makes it easier, however lets talk about both sets of grandparents. Thus your father's brother's children most likely won't match your mother's brother's children unless they share another set of common ancestors. There are so many situations that could be explain where other common ancestors could come into play, that I am not going to even try to explain any. Therefore, I am thinking of the most common situations, your first cousins on your father's side (Paternal side) will not match DNA wise to your first cousins on your mother's side (Maternal side). This is where looking at shared matches come in handy. If you identify all the share matches between your father's brother's children and you, you can at least place that shared match onto your paternal side and those share matches between your mother's brother's children and you will place the shared match onto your maternal side. Again, disclaimer, this is guessing that the share match only is related to you and your cousin with one set of common ancestors.

Going up to the next level are your 2nd cousins. These are simply your full 2nd cousins, no half, or once or more removed. These 2nd cousins will share a set of Common Great Grandparents. As you can see from my graph, you will have ideally,  four sets of 2nd cousins. Each set should match within themselves. So once you identify who your common great grandparents with a 2nd cousin, you can again place shared matches into one of four buckets. My disclaimer, again this is based on that you only have one set of common ancestors between you, your 2nd cousin and all your shared matches.

Up through this point, you should match everyone who are your siblings, 1st cousins and 2nd cousins. Thus if your paper trail shows a sibling, 1st cousin or 2nd cousin and they tested, and share their test results with the world, and you don't match, then there is a NPE. A Non-paternal event. Of course this opens a whole different can of worms that I am not going to cover here. I am just trying to make understanding your DNA closer matches more clear.

Okay, the final level of my graph gets to the fun part. Your 3rd cousins and you will share a sent of common 2nd Great Grandparents AKA Great Great Grandparents. However, it is said that if all your 3rd cousins would test their DNA, you probably will only match 90% of them. This means you won't match 10% of your 3rd cousins. Therefore, if a known 3rd cousin test and they don't match you, don't panic. You might still be related. For example, if your sibling and the 3rd cousin match and you don't, it just means you both didn't get the same DNA segment from your 2nd Great Grandparents. You may match a different 3rd cousin of that 2nd Great Grandparent. This is why testing siblings and 1st and 2nd cousins really help you find all your relatives. My siblings and I, and I have both full and half siblings match a lot of different descendants of our common ancestors. Its all good.

Going back to our 3rd cousins, thus, if you can identify a 3rd cousin, again a plain 3rd cousin, not a once or more removed cousin, this will help narrow down all those unknown shared matches. This can be especially helpful for lines that are currently your dead ends. I have a few unknown 2nd great grandparents and even a few that end with my 2nd great grandparents. I am hoping to use DNA to sort out and find my ancestors.

Therefore, as you step back each generation to 4th and more cousins, you are going to match fewer and fewer of all the possible 4th and more cousins that you have. Plus, you will probably start to find more than one set of common ancestors, or that you only share one common ancestor while the cousin might come from your common ancestor's other spouse. This actually makes it easier, because you have just reduce the number of ancestors your share matches will fall under. Just like with Genealogy research we are told to start with the known and work backwards, I try the same principle with my DNA matches, I start with the known and work backwards.

Have fun!