DNA Match That Doesn’t Match Close Relatives on Either Side?

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One of the first things that we do when we are researching a DNA match is check the matches that we share in common.  This strategy can help us figure out if our match is on our maternal or paternal lines. 

But what does it mean when a DNA match doesn’t appear to be related to close relatives on either side of our family?

There are a few different scenarios that can occur, and every case is a little different.  In this post, I’ll explain in detail to you, using examples, some ways in which this is totally possible:

• You might be more closely related to your DNA match than your relatives are
• Your close relatives might not have inherited the DNA segments from your common ancestor(s) that you share in common with your match
• You might share a DNA segment with your match that is “identical by state” and you are not really related

Less common, but still important to mention:

• Your close family members that have done DNA tests might not be related to your DNA match, perhaps you really are the only one related.

This post primarily address reasons that a DNA match that is a 3rd cousin match or more distant might not genetically match your close relatives on either side of the family.  Our close family members should all match each other (refer to my post, “Beginner’s Guide to Shared Centimorgans” to learn about expected levels of shared DNA). 

For the purpose of this post, we will define a “close family member” as a first cousin once-removed, first cousin, aunt, uncle, grandparent, sibling, niece, nephew, etc – someone who you would consider to be a close relative.

Are you more closely related (genealogically) to your DNA match than your other close relatives?

If all of your known close relatives on your DNA match list are younger than you and are on the “next” generations (i.e. not your generation) of your family tree, there is a chance that your DNA match doesn’t match any of them because you are more closely related to the DNA match than they are.

What do I mean by this?

Let’s imagine that you have a first cousin once-removed on your dad’s side of the family and a first cousin once-removed on your mom’s side of your family on your DNA match list.  Most of the time, you can use whether or not you share DNA with these relatives to help you figure out which side of your family your DNA match is on. 

What happens when your DNA match doesn’t match either first cousin once-removed?

Let’s give your DNA match a name and call him Ken.  If Ken is a half-second cousin once-removed to you, then he is either a half-third cousin or a half-second cousin twice-removed to your cousin’s children on either side of your family. 

Because you are a generation more closely related to him, there is a higher possibility that you will share DNA with him than there is that your other cousins will.

There is a little over a 2% chance that second cousins twice-removed or third cousins won’t share DNA with each other, and because half-second cousins or half-third cousins only share one common ancestor (instead of two), they generally share less DNA overall.  Of course, the applicable statistics and reasoning will be different depending on your particular situation, but you can now see how this is possible.

Note:  We don’t inherit any DNA that our parents didn’t pass down to us.  If your parents have done DNA tests and you have a DNA match that doesn’t match either of them, then please read the section that is immediately below this. 

For the rest of this post, I am assuming that you have close relatives who have tested that are not your biological parents.

Maybe the DNA segment that you share with your match is identical-by-state and you aren’t really related

More often than we would like to think, we have DNA matches that share small segments with us who are not actually our relatives.  This is because, occasionally, our DNA can recombine in a way that seems to match someone, but it’s really a random coincidence. 

When we find out that we have a DNA segment that is a false positive match to someone, we call it an “identical by state” match.  This is different than a DNA segment that is “identical by descent”.

If you have a DNA match that doesn’t match either one of your parents, assuming that they have done DNA tests and you have access to a chromosome browser, then this is what has happened. 

It’s most likely to happen with segments that are under 10 cMs in length, but it is theoretically possible for a single segment that is much larger than that to be only coincidentally identical and not inherited from a common ancestor.

This is information is relevant to all of us, whether or not our parents have tested.

It’s possible that your close relatives didn’t inherit the DNA segments from your common ancestor that you did

A parent passes down 50% of their DNA to each of their children, but they don’t give each of their children the same identical 50%.  Right from the start, descendants of the parents will only share 33-50% of their DNA with each other. 

When those children have children, the same phenomenon happens, meaning that their descendants will share much less DNA.  Since the process of passing down DNA is fairly random, there is no set amount of DNA that descendants of a particular ancestor will share with each other. 

After a set number of generations, there is no guarantee that the descendants will match each other at all.

Consider, for example, a Bob and Sandy.  They have two great-great grandchildren.  One great-great grandchild is descended from their son Tom, and the other great-great grandchild is descended from their daughter Mildred.  

Tom and Mildred only shared 33-50% of their DNA with each other to start off with, and each time a generation passed down DNA, it was a randomly selected 50%.

By the time the great-great grandchildren were born, there is a 100% probability that they both inherited DNA from Bob and Sandy.  The problem is that the DNA that they inherited that matches Bob and Sandy, their great-great-grandparents, might not match each other. 

Through the generations of descendants, the DNA that Bob and Sandy passed down became less and less, and different.

This is how two legitimate descendants of a particular person, or persons, end up not sharing any DNA wither each other.

It’s possible that your close family members aren’t related to your DNA match

I left this option for last, primarily because I don’t want to encourage people to assume that they have uncovered something surprising by noticing that a DNA match doesn’t match their close family members.  Even so, this occasionally can be the explanation.

For example, let’s pretend that you have a first cousin once-removed who has done a DNA test.  Her name is Polly, and she is the daughter of your first cousin Sally.  When your new DNA match (let’s call him Dan) pops up on Ancestry, you notice that Dan is estimated to be your third cousin.  

The problem is, he doesn’t match any of your known close matches on your dad’s side of the family, and he doesn’t match Polly, who is your only close match on your mom’s side of the family.

Just to make sure, you check to see how many centimorgans that you share with Polly, and you notice that you share 325 centimorgans with Polly.  This seems like the right amount of DNA for first cousins once-removed, until you notice that half-first cousins once-removed also can share this same amount of DNA.  This means that you can’t be sure that Sally, Polly’s mom, is a full sibling or a half-sibling to your mom.

Let’s also say that you have determined that your DNA match, Dan, is a third cousin, and he is descended from your great-great grandfather on your mom’s side – your mom’s dad’s side.  If Sally and your mom aren’t full siblings (maybe they only share a mother), then Polly would not show up as a DNA match for Dan because she is not related to him. 

She is only related to you on your maternal grandmother’s lines (and Dan is not related on those lines).

In this scenario, however, you could NOT use the fact that Dan doesn’t match Polly as proof that Sally is your mom’s half-sister.  If Sally and Dan are related in a family tree way (i.e. genealogically speaking), they would be third cousins once-removed.  There is a greater than 10% possibility that we wouldn’t share DNA with a third cousin once-removed, so it could be completely normal.

This means that you can’t jump to any conclusions – you need more information.  If Sally and your mom could test, it would be ideal.  Otherwise, having additional family members do DNA tests can help provide more data points to help you figure out what is really going on. 

For example, if Polly has a brother (David), and he tests and does show up as a match to Dan, it’s most likely that Sally and your mom are indeed full sisters and Polly just didn’t inherit the DNA segment that matches Dan,

Conclusion

I hope that this post helped you understand some of the reasons that you might have a DNA match that doesn’t appear to be related to close relatives on either side of your family. 

Sometimes, it’s because there is a logical explanation, and other times we need to do more research into our family tree to decipher the DNA.  If you have any questions about something that you read here, I would love to hear from you in the discussion below.

Thanks for being here today!

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