28
May

Can You Share DNA and Not Be Related?

Do you ever wonder if you are really related to all of your DNA matches? In other words, is it possible to share DNA and not be truly related?

Yes, it is absolutely possible to share small DNA segments with a person and not be related in a genealogical sense.

In this article, learn more about this topic, as well as:

  • Why the size of the shared DNA segments is important
  • How common “false” DNA matches are on our match lists
  • How to tell if you are really related to your DNA match

There is nothing more frustrating than researching a DNA match and finding no evidence of a connection going back as far as you can trace in both family trees.

This is why it is important to know how likely it is that a DNA match might not be related to you in the first place. This can save you a lot of time and help you focus your efforts where they will be most effective.

Does sharing DNA always mean you are related?

If you share only a very small amount of DNA with a person, there is a possibility that you are not related to each other. This is because sometimes we share tiny DNA segments with people that are identical-by-state and not identical-by-descent.

Identical-by-state (IBS) segments are small segments that happen to be identical. I like to call them “coincidentally” identical.

DNA segments that are identical-by-descent (IBD) were inherited by each DNA match from their shared ancestor, or shared ancestors.

IBD DNA segments were passed down from the original shared ancestor to their children, who passed them to their children, who passed them through any number of generations until they got to you and your DNA match.

If you share a small DNA segment with a match that was not inherited from a common ancestor, it means that the DNA segment is a “false” segment and you and your DNA match are not related.

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The size of shared DNA segments is key

As I mentioned previously, it is most likely to have a false DNA match when the shared DNA segment is small. How small?

DNA segments that are less than 6-12 centimorgans (cMs) in length have a likelihood of a common ancestor within 6-8 generations of only about 5%.

This means that segments of this size are usually either a) false or b) inherited from a common ancestor too far back to be identified using genealogical records.

Larger segments, however, can often indicate fairly recent common ancestors. For example, a DNA segment larger than 30 cMs has a 90% probability of being inherited from a common ancestor from within 6-8 generations.

How common are false DNA matches on our DNA match lists?

The people at the top of our DNA match lists are always related to us. Our closest DNA matches, usually sharing more than 100 centimorgans, share ancestors with us. In other words, they are relatives.

While the DNA testing company’s estimated relationship might not be accurate, we can be sure that those people sharing DNA segments larger than about 20 cMs are likely related to us in some way. Even though the ancestor could be far back in our tree, we are likely related.

Some DNA testing companies show us matches who share very small (6 cM) segments. Additionally, some companies include even smaller segments (less than 6 cM) in the total tally of shared DNA when there are larger segments present.

This means that we might find a great deal of false matches on our list, especially as we scroll past the closer matches. We are most likely to find people who are not really related to us towards the end of our DNA match list where people share less than 10 centimorgans of DNA with us.

As I mentioned before, the smaller the segment, the higher the chance that the DNA match is not really related. That’s why I always recommend focusing your research efforts on closer matches first.

How to know if you are really related to your DNA matches

There are a few things that you can do to determine whether your DNA match is “truly” related to you and not a “false” match.

Do you share an ancestor in your tree?

First, examine your family tree. If you share common ancestors from within the past 6-8 generations or so, then there is a good chance that your shared DNA was inherited from those ancestors.

If you haven’t yet started building a family tree, definitely check out my book which is a guide to family tree building basics.

Keep in mind that it is possible to share small false DNA segments (smaller than 10 cMs or so) with people who are really our matches. This is true even if we know that some of our DNA segments were were passed down from common ancestors.

Do you share close matches with your DNA match?

You might also check to see if your close DNA matches, such as siblings and first/second cousins, also share DNA with the person in question.

The most conclusive way to figure out whether the DNA segments that you share with your DNA match were inherited from a share ancestor is to examine the shared DNA segment in a chromosome browser. Ideally, you would be able to compare your mother and/or father’s DNA to your DNA match.

If one of your parents shares the same DNA segment with your DNA match, then the likelihood that you share a common ancestor increases dramatically.

Asking your close relatives to take DNA tests is a fantastic way to learn more about your ancestry, including how you are connected to your DNA matches.

You can use any of the sponsored links below to to order a test for a relative. It’s best if you have your relatives test with the same company that you tested with.

Triangulation can help determine IBS vs IBD

You can further study the shared DNA segment that you share with your match by using triangulation techniques. If you share the same DNA segment with multiple descendants of the same common ancestor, this is additional evidence that the segment is identical-by-descent and not a false segment.

Conclusion

I hope that this post has helped you understand how it is possible to appear to share DNA with someone who you are not really related to, as well as how to spot these matches in your DNA match list.

If you have any questions about something that you read in this post, I would encourage you to join us in the discussion below.

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