What is a Third Cousin DNA Match?

Do you have cousin matches that show up as a potential third cousin to you?  In this post, learn what a third cousin DNA match really is.

You will also find out:

• what it means to have a third cousin DNA match
• how much DNA you share with a third cousin
• whether or not your “third cousin” match is really a third cousin
• how to know how your third cousin match is related to you.

The information in this post is most helpful to those who have tested with Ancestry DNA, but anyone who has done a DNA tests and wants to better understand their DNA match list will find helpful information here.

In this post, we will refer to “true third cousins” and third cousin DNA matches.  A “true third cousin” is a person who is a third cousin to you. 

A third cousin DNA match is a person who is estimated to be a third cousin to you by your DNA testing company.  If this distinction seems confusing, don’t worry. 

I will explain everything below.

What is a true third cousin?

A true third cousin is a person who is also the great-great grandchild of at least one of your great-great grandparents.  You are descended from your great-grandparent, and your third cousin is descended from a sibling of your great-grandparent, and the shared ancestor is one or both great-great grandparents.

Your most recent common ancestors (MRCA) are your great-great grandparents.  Your parent and the parent of your third cousin are second cousins. 

You are second cousin once-removed to your third cousin’s parent, and they are second cousin once-removed to your parent.

What is a half-third cousin?

A half-third cousin is just like a third cousin, but instead of sharing both of your great-great grandparents on that line of your family, you share only one.  This is a very common situation in many families, and it has a pronounced effect on the probability of sharing DNA. 

Half-third cousins are less likely to share DNA than full third cousins.

How much DNA do third cousins share?

It is important to know that it is possible to share no DNA with a third cousin.  In fact, there is a about a 10% chance of sharing no DNA with a third cousin. 

This means that the bottom end of the range of shared DNA between two third cousins is zero.  The top end of the range for shared DNA between two third cousins is as high as 225 centimorgans (cMs), though most third cousins will share between 25-150 cMs.

It’s possible to share slightly more DNA with a third cousin, especially if there has been extensive intermarriage, also known as endogamy, in your family – even if it was many generations back.  Intermarriage can mean a higher chance of sharing many smaller DNA segments with a third cousin because of many distant ancestors. 

Some DNA testing companies claim to be able to recognize more third cousins as DNA matches than others.  Ancestry DNA is one such company, and says that they can detect as many as 98% of third cousin matches.  

Other companies say that they can only detect as many as 88% of true third cousins as matches.

How much DNA do half-third cousins share?

Since there is about a 10% chance of sharing no DNA with any given third cousin, and the chance of sharing no DNA with a half-third cousin is slightly higher, shared DNA between two half-third cousins normally ranges from zero to about 55 cMs, though it is entirely possible to share much more DNA with a half-third cousin – as much as about 175 cMs in rare cases.

What is a third cousin DNA match?

A third cousin DNA match is a cousin with whom you share an amount of DNA that falls within the range of DNA typically shared between two third cousins. 

Sometimes, a third cousin DNA match is a true third cousin, and other times, it is a person who is slightly more closely related, or more distantly related.

Here are some of the relationship possibilities for a match who shows up in your third  cousin category on your DNA testing site:

• second cousin twice-removed
• second cousin three times removed
• second-cousin four times removed
• third cousin
• third cousin once-removed
• third cousin twice-removed
• fourth cousin

In rare cases, especially if you share a higher-than-average number of centimorgans with your third cousin match, the relative could be more closely related than those relationships in the list above. First cousin twice-removed, or half-first cousin once-removed are other possibilities.

The takeaway is that a third cousin match could be a true third cousin, a similarly related cousin, more closely related, or slightly more distantly related.  You’ll have to use more traditional genealogical research in order to really know how a match is related. 

In the following section, I’ll give you some ideas to perhaps help you figure out your exact relationship to your third cousin match.

How to know how you are related to a third cousin DNA match

The two easiest ways to find out how you are related to a third cousin DNA match?

• Check out their public family tree, if they have one posted on their DNA profile.  If they don’t have one, then you might consider:
• Contacting your DNA match.  If you have a nicely-developed family tree, and you want to know how you are related to a particular cousin, you might consider researching out to your match.  Try to have a basic idea (see below) about how you might be related, and offer a few surnames as suggestions, if possible.  There is a chance that your DNA match might know a lot about their family history, but just doesn’t have a family tree posted online anywhere.

It should be noted that comparing your family trees is only a guaranteed method if both of you have all of your great-great-great grandparents included on your tree, since there is always a chance that a true fourth cousin has been placed in the third cousin category because they share a higher amount of DNA with you than is typical. 

Fourth cousins share great-great-great grandparents as the most recent common ancestor(s).

If you don’t want to contact your match, or you have tried but they haven’t responded to you, all hope is not lost.  There are still ways to determine your relationship, and even learn from your DNA match.

Don’t ignore shared matches

Check out your shared matches.  Depending on who your shared matches are, you might be able to determine which line of the family your third cousin DNA match is on

Find your third cousin DNA match’s family tree

Your shared matches might have family trees posted on the site, or somewhere else online.  You can also consider contacting them to discuss your connection, which could help you determine how you are related to your mystery third cousin DNA match, too.

Check out your match’s profile

Don’t forget to view your DNA match’s profile (their main profile, not just their DNA profile), since they might have an additional family tree or other information on their profile

See if you can learn about your DNA match’s family tree on Google

Do a Google search for their name + genealogy.  For example, my third cousin’s name is Mayhem Applebogger III,  so I will search for “Mayhem Applebogger III genealogy” in Google, with or without quotations.  Many of my DNA matches are pretty deep into genealogy, and some of them even have blogs or websites dedicated to their research and sharing information with family.

Note:  I don’t really have a cousin named Mayhem Applebogger III, which is unfortunate, since he sounds like a riot!

Conclusion

I hope that this article provided insight into what it means to have a third cousin DNA match, and gave you tips to help you figure out how your third cousins DNA match might be related to you.  If you have any questions about something that you have read here, I would love to hear from you in the comments.

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