Finding Unknown Biological Ancestors with DNA

This is a topic that comes up regularly in genealogy circles, because DNA testing often reveals cases of unknown adoptions, or what we call "non-paternity events" (NPE), when someone's father is not their biological father. Once there's enough suggestion that something like this has happened, the question then becomes, how do I identify this unknown biological ancestor? It can be done, although the further back on your tree it occurred, the more difficult it will be (far enough back and it might not be possible). Whenever possible, it's best to have someone from the oldest generation descended from this event to test. Like if you're looking for Grandma's unknown biological father, have Grandma take the test, or if she is unable or unwilling, have your relevant parent take the test. At the same time, if the person you're looking for is actually still living (like if you're adopted and looking for living biological parents), it will be difficult to research since lots of records on living people are private (that's a whole different ballgame and you often have to rely more on information and communication from your DNA matches). Additionally, if you're working with an endogamous population, you may be out of luck. With all that in mind, here's how it works.

Step 1: Look for your closest DNA match that you can't identify as being from another known branch of your tree. If they don't have a family tree added, that's okay because first you want to look at their Shared Matches, and open any matches that do have family trees (the bigger, the better).

Step 2: Compare the family trees of those Shared Matches, looking for ancestors any two or more (the more, the better) of them have in common with each other (especially if those matches also match each other) - ancestors who are not found in your tree. Yes, this may take some time because you have to manually compare the trees - I find it best to start with the surname list on the match review page and find surnames they have in common with each other, then see if those surnames actually lead to a common ancestor among them. If the ancestor is found in your tree, then you know this group isn't from the branch you're looking for and you can label them and move on.

Step 3: Build a descendant tree for the ancestor you found. Make a note of any descendants who were in the right place at the right time at the right age, but we're not done yet.

Step 4: Repeat this process with the next closest match you can't identify (who isn't a part of the first group).

Step 5: Look for a descendant who appears in both the trees you've built - so someone who descends from both the ancestors you've identified. This is probably either the person you're looking for, or a close ancestor of theirs, like a parent or grandparent. If you don't find one, keep repeating this process until you do.

Chart showing the two different DNA matches groups and their shared ancestors.
Click to enlarge.

For example (shown above - these names are made up but the situation is real and came from my tree): I was looking for my grandfather's unknown biological father, so I had my grandfather take the test before he died. I first found a group of his matches (who mostly all matched each other) who were all descended from a colonial ancestor named John Smith (I told you I changed the names, lol), so I built a descendant tree for John Smith. I then found another group of matches who all descended from another colonial ancestor called Christopher Jones, and built a descendant tree for him. By building those trees, I found a descendant of John Smith - named Isaac Smith - had married a descendant of Christopher Jones - her name was Carrie Jones. This suggested that the man I was looking for was probably a descendant of Isaac Smith and Carrie Jones, and based on the dates, it could only really be one of their sons, specifically one of their four oldest sons. Eventually, a close descendant of one of the four sons tested and confirmed which of the four sons was my grandfather's biological father (below).

Chart showing the closer matches that eventually showed up and allowed me to figure out
which of the 4 brothers was my grandfather's bio father.

Granted, there could have been another descendant of John Smith who married a different descendant of Christopher Jones, and that could have led me to the wrong family - this is why too much endogamy can throw you off. But as long as there's not too much of it, you can document each case of it and using your DNA matches and how much DNA you share with them, you should be able to figure out which descendants are the ones you're looking for. But a highly endogamous population might be too complex. If I was looking for an unknown bio ancestor on my mom's Mennonite branch, I'm not sure it would be possible. I can sometimes share up to about 5 ancestor couples with matches on my Mennonite branch. And the unknown father of my Italian ancestor who was from a tiny, highly endogamous town in Italy where everyone there is related to everyone else somehow? Forget it.

However, this is the same type of method that professional Genetic Genealogists like CeCe Moore employ to identify individuals from DNA left at crime scenes (either suspects or unidentified victims). It can be done (for the most part), it just takes work, and sometimes some patience for the right matches to come in.