Do you have two relatives who are DNA matches to you, but not to each other? Are you wondering what this really means, and whether or not it could mean a mistake? In this post, I’ll explain to you when you should expect relatives to match each other, and when it’s normal for two people who are related genealogically to share no DNA.
This topic comes up most frequently when someone is trying to figure out how a person is related to them by using shared matches, or matches in common. Sometimes, people are surprised that two of their relatives don’t seem to share DNA. Other times, a known relative does a DNA test and doesn’t show up as a match to people who they expect to match.
DNA can be a tricky business – but don’t worry, I’m here to help you figure it all out!
First, are you sure your cousins are really related to each other?
Family trees can get really twisted and tangled – trust me, mine is gnarlier than most. When trying to figure out why your two cousins don’t match each other, the first thing that you should consider is that they don’t actually share a common ancestor. Most of the time, you’ll be able to figure this out by looking at a posted family tree. While they might both be related to you on the same side of your family (i.e. paternal or maternal), it doesn’t necessarily mean that they are related to each other.
The first step is to identify who the ancestor that they share is common is – not the one that they share with you. It could be a different person (or persons) entirely.
It can be normal for two relatives to not match each other on DNA results
Now that you know for sure that your two cousins really do, or should, share a common ancestor, you can continue to research whether or not they *should* share DNA – or not.
We don’t share DNA with all of our relatives. It’s completely and 100% normal for two people who are related genealogically to each other at a second cousin once-removed distance* or more distant to share no DNA. This means that people who are closely related to each other will always share DNA, and people who are more distantly might share DNA. The closer the relationship, the higher chance that two family members share DNA and the more distant, the high the chance that they don’t.
With this said, it’s important to mention that everyone who is related to each other at a second cousin once-removed distance or closer will always share DNA. Almost always. The amount of DNA that they might share could vary drastically, but there always is some detectable genetic connection.
*There is a .10 probability that two second cousins once-removed share no DNA, and it also means that there is a 99.9% chance that you WILL share DNA with any given second cousin once-removed. The end of result of this is that you will just about always share DNA with a cousin at this level.
Note: A second cousin once-removed is someone who is the second cousin of one of your parents. Two second cousins share great-grandparents, so a second cousin once-removed to you is someone who is the great-grandchild of your great-great grandfather. It could also be vice-versa, where you are the great-grandchild of your relative’s great-great grandparent. Another way to look at second cousins is the grandchildren of two siblings. The second cousin once-removed would be the child of one of those grandchildren and the grandchild that they are not descended from.
How to know whether two relatives should be a DNA match to each other
Now that we’ve got some background to our topic, we can figure out whether or not two of our relatives should match each other on our DNA match list. It’s important to figure this out first before we assume that we’ve discovered something untoward or secretive in our family history.
So, should your two relatives share DNA? They are both related to you, so shouldn’t they match each other?
Figure out how your two matches are related to each other, not to you
The way to figure out whether your two DNA matches really should match each other is to calculate their relationship with each other, not their relationship to you. For example, if I have a fourth cousin once-removed and a second cousin and they both match a particular line of my family, how should I know whether or not they should match each other?
If I look at both of their family trees, I realize that the fourth cousin once-removed is also my second cousin’s fourth-cousin once-removed. We are related to this person at the same relationship distance. And since I know that it’s normal for two people who are related more distantly than second cousin once-removed to not share DNA, I don’t think twice about whether or not my second cousin is really related to this fourth cousin.
(Want a quick way to calculate cousin relationships? I really like this Cousin Relationship Calculator)
How is it possible for two relatives to share no DNA?
Now that you know how your two cousins are related to each other, and whether or not they should (or probably would) share DNA, you might be wondering how it is possible for two verified relatives who are biologically related to their common ancestor to share no DNA with each other.
The explanation has to do with how DNA is inherited. Two parents only pass down 50% of their DNA to each of their children. While each of their children does inherit 50% of their DNA from each of their parents, the 50% of the DNA that they inherit won’t be the same 50% that their siblings inherited. Siblings will share a lot of DNA, but no more than 50%, and sometimes as low as about 33%. This little bit of information is key to why their descendants may, or may not, share DNA segments.
If you imagine that every generation this same pattern of random inheritance occurs, the amount of DNA that they share with any given distant common ancestor could vary greatly. Eventually, some descendants will no longer carry DNA from a distant ancestor. They’ll have some DNA from some of their distant ancestors, but not from all of them.
If you take the example of my fourth cousin once-removed that I mentioned in the previous section. I will share at least one great-great-great grandparent with my fourth cousin once-removed. My great-great parent and their great-great grandparent were siblings, and those siblings likely shared between 33-50% of their DNA with each other. When those siblings got married and had children, they passed down a randomly selected 50% of their DNA to their children.
My parent who is descended from my great-great-great grandparents (in this case, my mother) likely only shared an average of 6.25% of their DNA with her 2nd great-grandparents. Since there are two common ancestors (the set of great-great-great grandparents), my mother might share an average of 12.5% with them as a couple. This means that it is likely that 87.5% of my mom’s DNA did not come from either of these ancestors.
It is key to note that since DNA is inherited in a random way, my mother might share much less than 6.25% of her DNA with any of her great-great grandparents, or a little more. It all has to do with how the DNA was passed down each generation. In other words, which 50% of DNA each generation inherited.
Since I inherited 50% of my DNA from my mom and dad, and I don’t know which 50% I inherited from my mom, I could have inherited only a tiny amount of that 12.5% of DNA that she shares with her great-great grandparents (who are my 3rd great-grandparents), half of it, some of it, the entire 12.5%, or none at all. Statistics come into play in figuring out what is most likely to happen, but all of these scenarios are possible.
Now that you understand how this all works, it’s easy to imagine how more distant relatives are less likely to share DNA with each other. Each generation, it becomes less likely that two descendants of the same distant common ancestors will share genetic material, even though we do share DNA with many of our distant relatives.
I hope that this post helped you figure out whether or not your two relatives should match each other on their DNA lists, and I also hope you understand a little bit more about how DNA is passed down. If you have any questions about something that you read here in this post, or if you wonder about two relatives of yours who don’t match, please feel free to join the discussion below.
Thanks for stopping by!