What is a Third Cousin?

Are you interested to know what a third cousin is and whether you are genetically related to them? In this article, we'll help you understand what a third cousin is, whether they are considered family, and what DNA results might say.

So you have taken your Ancestry or 23&Me DNA tests and you are going through your lists of matches. If you have close family members who also tested you likely recognize those matches. There may be a few first or second cousins in the list that you recognize as well.

As you work your way down the list you start to get to the third, fourth and fifth cousins. Some may have the same surname as you but many will not. You don’t recognize anyone from the pictures or can even imagine how they are related to you. You ask yourself then how is this third cousin related to me?

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What Is a Third Cousin?

The third cousin connection is a little complicated but let's try and explain this as simply as possible. A third cousin is an individual who is the great-great-grandchild of your great great grandparents. You share that great-great-grandparent as a common ancestor.

One of your third cousin's great grandparents is the sibling of one of your great grandparents. Their respective offspring would be classified as first cousins. The children of those first cousins are considered second cousins to each other. Finally, the children of those second cousins would be you and your third cousin.

Are Third Cousins Family to You?

There is undeniably a decent amount of separation between third cousins and it is highly likely you may not even know any third cousins. Generally speaking, most people will never have met their great great grandparents who are the common ancestor we share with a third cousin.

Sometimes families remain close for several generations and huge family reunions are not uncommon. You may meet and even know a third cousin personally. Scientifically, a third cousin is family because they descend from one of your ancestors. They may be a complete stranger to you but, obviously, if you are seeing them in your DNA matches they share some of the same blood that is in your veins.

Do We Have a Lot of Third Cousins?

The answer to this question is quite variable and dependent on many different factors. It is estimated that on average most people have around 190 potential third cousins. The assumption behind this math is that each generation between the great-great-grandparents and the third cousins has 2 to 3 children.

Obviously, some families may have more children on average especially if their religion forbids the use of contraception or there are other social factors that lead to bigger family sizes.

Generally speaking though most people will likely have a large number of third cousins as they come from both sides of our family tree.

Are We Connected to All of Our Third Cousins Through DNA?

This is an interesting question because biologically we do not share a huge amount of DNA with our third cousins. The average third cousin match is 74 centimorgans with the expected range for this relationship being 20-230 centimorgans.

The effects of recombination in DNA over the generations, however, does mean that the third cousin relationship is not always recorded in our DNA. In a perfect world without DNA recombination we should inherit about 6.25% of our DNA from each of our great great grandparents.

Recombination of DNA is random though, so that small amount we receive from our common ancestor may not be the same as what our third cousin receives. This essentially means that on paper we are third cousins but it is just not reflected in our DNA.

It is estimated that only about 90-98% of our third cousins will have matching segments of DNA in common with us.

Other Variations of Third Cousins

There are other relationships that use the third cousin term and they are also worth noting. In terms of DNA matches sometimes a third cousin match may also be one of the following.

  • Third Cousin once Removed: This is the child of your third cousin.
  • Half Third Cousin: This is when your great grandparents are only half-siblings. You only share one great-great-grandparent instead of both.

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  • "What is a Third Cousin?". NameCensus.com. Accessed on May 29, 2022. http://namecensus.com/blog/what-is-a-third-cousin/.

  • "What is a Third Cousin?". NameCensus.com, http://namecensus.com/blog/what-is-a-third-cousin/. Accessed 29 May, 2022

  • What is a Third Cousin?. NameCensus.com. Retrieved from http://namecensus.com/blog/what-is-a-third-cousin/.