How Many DNA Segments Do First Cousins Share?

Do you want to know how many DNA segments first cousins share? In this post, I’ll show you some examples and discuss shared DNA segments between first cousins.

If you and a first cousin have both done a DNA test with the same company, you should see each other’s names on your DNA match list. This list is also sometimes called a Genetic or DNA Relatives list.

When you are trying to determine whether the amount of DNA that you share with your first cousin is within the expected range, you might look to the total number of shared DNA segments for clues.

Any little detail can often help us figure out how we are related to a DNA match or further evaluate the relationship.

How many DNA segments do first cousins share?

There is no set range for how many DNA segments that first cousins should share, but we do know that full first cousins should have a couple of dozen DNA segments in common.

Take, for example, the DNA match list that we see below. Our test taker has five first cousins who have taken Ancestry DNA tests:

We can see that our test taker shares between 38-46 DNA segments, likely of various sizes, with her five known first cousins who tested their DNA with the same company that she did.

It’s possible to share even few DNA segments with first cousins. As a second example, we can examine a second DNA test taker’s results.

She, too, has five known first cousins who have done DNA tests. The number of DNA segments that she shares with her first cousins ranges from 35-43.

You might notice that both of our test takers shares the fewest number of DNA segments with the first cousin who shares the least total number of centimorgans with her.

We’ll go into this in further detail below.

How much DNA should first cousins share?

More important than the number of shared DNA segments is the total amount of DNA shared measured in centimorgans (cMs). This number represents the total length of all of the shared DNA segments added together.

We typically expect first cousins to share between 575-1330 centimorgans, with the average being about 850 cMs.

Half-first cousins, on the other hand, can share as little as about 215 cMs and as much as about 650 cMs.

It’s important to note that these are ranges and we do occasionally see numbers slightly below or above the averages.

An an example, check out the screenshot below. This is from yet another DNA test taker who has four known first cousins on Ancestry DNA.

We notice that the range of shared DNA segments between this person and their first cousin goes from 28-28.

These two first cousins only share 570 cMs across 28 DNA segments. 570 cMs is on the very low side of shared DNA for full first cousins.

For those who are curious, the parents of these first cousins share 2648 cMs, and so they are very clearly full siblings. Thus, their children are full first cousins, even though they only share 570 cMs.

Can number of DNA segments determine full or half-cousin?

As we noted earlier, half-first cousins can share as much as about 650 cMs, which happens to overlap with the range of shared DNA for full first cousins.

If you have a first cousin or a DNA match that falls within this overlap range, you might wonder if you can use additional details, such as the number of shared DNA segments, to determine definitively whether your cousin’s parent was a full or half-siblings to your own.

We cannot make a determination about whether someone is a full or half-first cousin based only on the number of shared DNA segments.

Instead, we must rely on the the total number of shared centimorgans between ourselves and our first cousins.

If we have other family members who have also done tests, we can use those data points as additional evidence of our relationship with our DNA match or relative.

One of the examples that I used in the explanation above about the two full first cousins who share only 570 cM cMs is a perfect example of how using additional information about shared DNA between other relatives can help us figure things out further.

If we use the Shared cM calculator on DNA Painter, we see that two people who share 570 cMs have an 83% chance of being half-first cousins vs. a 15% chance of them being full-first cousins.

If we didn’t have any other family members to test, we might feel unsure about whether they are full or half-cousins.

By having their parents test, we were able to determine for sure that they are full-first cousins, even if they share a low-ish amount of DNA for their relationship.

Conclusion

I hope that this post has helped you understand more about how many DNA segments are generally shared between first cousins, how the number can vary, and the best way to evaluate the amount of DNA that you and your cousin share.

If you have any questions about anything that you read in this post, or if you would like to share the number of DNA segments that you share with your first cousin(s), please join us in the discussion below.

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