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 related?

In this article, find the answer to this question, plus learn more about the topic. You’ll find out:

• Whether it is possible to share DNA with someone and not be related
• 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. It can take a long time to thoroughly research a DNA match, after all.

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.

So, can you share DNA and not be related?

Yes, it is possible to share a small amount of DNA with someone and not be related. In other words, it’s possible to share genetic material and not share a common ancestor or any identifiable genealogical connection.

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. or identical pieces of DNA, with people that are coincidentally identical.

In other words, they just happen to be identical. This is different than shared DNA segments that are identical because they were inherited from a shared ancestor.

Coincidentally identical DNA segments are called “identical-by-state” segments. DNA segments that are identical because of a shared ancestor are known as “identical-by-descent” segments.

Identical-by-state (IBS) segments are small segments that happen to be identical. I like to call them “coincidentally” identical, though this is not their official name – it just helps in understanding the concept.

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.

The size of shared DNA segments is key

The size of the DNA segments shared between you and your DNA match, and to a certain extent, the total amount of shared DNA, can you help you determine whether you and your DNA match are truly related to each other.

As I mentioned previously, it is most likely to have a false DNA match when the shared DNA segment is small. How small is small enough to suspect that the DNA match may not be related?

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 false, or inherited from a common ancestor too far back to be identified using genealogical records or documents.

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.

Can you share 25% of your DNA with someone and not be related?

It is not possible to share 25% of your DNA with a match and not be related. Two people who share a high percentage of DNA with each other are almost definitely closely related.

As I mentioned earlier in this article, we can depend on the total amount of DNA that we share with our matches, or the percentage of shared DNA, to help us determine how likely it is that we have a shared ancestor. We don’t have to share 25% of our DNA with someone to be certain that there is a true family relationship, however.

You might be surprised to learn that we only share about 12.5% of our DNA with first cousins, 3% with second cousins, and an average of .78% with third cousins. All of these cousins share relatively recent common ancestors with us, however.

If we think about a third cousin, who might share less than 1% of their DNA with us, it means that we can share very small percentages of our DNA with more distant cousins. Is there a “minimum percentage” of shared DNA that we should look for that indicates a guaranteed genealogical relationship?

Not exactly. However, we can feel fairly confident that if we share at least 12 centimorgans with a match, along with another segment or two, even if they are smaller, then we likely could find a common ancestor in our family tree. That is, of course, if we both know enough about our ancestors to have built a family tree far enough back to find them.

A DNA match with a 12 cM segment, along with two smaller segments of 8 cMs, for example, would share a total of 28 centimorgans, or .38% of their DNA with us. I would consider this person to be a likely relative, though it could take some work to figure out the exact connection.

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

How can you tell if someone who shares DNA is really a relative and not a false match? There are a few things that you can do to determine whether your DNA match is truly related.

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. For example, do your siblings also share DNA with the DNA match in question?

You can easily examine shared matches on Ancestry DNA, and all of the other DNA testing sites.

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.

This feature is not available on all DNA testing platforms, such as Ancestry DNA, but some companies do offer the capability. If you have access to a chromosome browser, like the one on 23andMe or MyHeritage DNA, I would highly recommend learning how to use it.

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.

• Ancestry DNA
• 23andMe
• My Heritage DNA
• Family Tree DNA

Triangulation can help determine IBS vs IBD

DNA segments that are identical-by-state (IBS) were not inherited from a shared ancestor, and are thus false DNA segments. These are different from segments that have been passed down through the generations from a common ancestor, or identical-by-descent (IBD) segments.

It is possible to examine the exact locations of shared DNA segments using websites that have chromosome browsers, such as Gedmatch. Once you know the location of an identical segment, you can begin the work of determining whether the segment is due to a shared ancestor.

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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