How Much DNA Do You Share With a First Cousin?

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Are you looking at your DNA match list and trying to figure out if one of your matches could be a first cousin to you?  Did your first cousin do a DNA test, and now you want to know if you share the right amount of DNA based on your relationship? 

In this post you will learn how much DNA you should share with a first cousin match, and how to figure out if you are full or half-first cousins.

Do full and half-cousins share the same amount of DNA?

You will share more DNA with a full first cousin than a half first cousin.  Full first cousins share two grandparents, and half-first cousins only share one grandparent. 

For example, if your grandfather divorced after your father was born, remarried and then had your uncle with his new wife, then a child of that second marriage will be a half-first cousin – since you only share one grandparent (your grandfather).

How DNA is typically measured for the purposes of genealogy

In genetic genealogy, we prefer use the term “centimorgans” as a unit of measurement for describing the length of shared DNA segments between two family members.   Family members of different relationship distances will share at least one segment of DNA – and sometimes many more than one. 

Each DNA segment is measured in terms of centimorgans.  We usually only pay attention to segments that are larger than about 6 or 7 centimorgans, since segments that are smaller than that are usually only “coincidentally identical” matches. 

Valid DNA segments can be as small as 6 or 7 centimorgans, and as long as about 280 centimorgans (cMs).

Your DNA testing company looks at the size and quantity of shared DNA segments in order to predict your relationship with your DNA matches.  First cousin matches are fairly close relatives, and for this reason, there is a specific range of shared DNA  – measured in centimorgans – that is typically found between two first cousins.

How much DNA do you share with a first cousin (full)?

Note:  You will always, always, always share DNA with a first cousin – even if they are a half first cousin.

These are the generally accepted shared cM amounts between two first (full) cousins.

Average:  850 cMs (centimorgans)

Low end of the range: 575

High end of the range: 1330

Most of the time, the shared DNA will fall towards the middle of this range.  From the cases that I have worked, verified full first cousins have ranged from 730-1081. 

The average amount of shared DNA between first cousins from my own personal research is 897 cMs.

An important note about these numbers:

There are other relationship categories that have ranges of shared DNA that overlap the first cousin range, so you will have to use shared DNA, along with other information, in order to determine your exact relationship.  The other relationship categories that have a similar range of shared DNA are:

• half-aunt
• half-uncle
• half-niece
• half-nephew
• great-grandparent
• great-grandchild
• great-aunt
• great-uncle
• great-niece
• great-nephew

How much DNA do you share with a full first cousin (half)?

As you might expect, half-first cousins will share approximately half as much DNA as full first cousins.  They will always share a substantial amount of DNA, however.

The range of shared DNA between two half-first cousins: 215 – 650 cMs

Average: 425 cMs

Just like with full first cousins, there are other relationship categories that fall within the same 215-650 cM range for shared DNA. The other relationships that you can expect see to share similar amounts of DNA (like the amounts shared between half-first cousins) are:

• first cousin once-removed
• half-great uncle
• half-great aunt
• half-great nephew
• half-great niece

As you can see, the top of the range of shared DNA between half-first cousins overlaps with the lower end of the range of shared DNA between two full first cousins. 

• If you share 600 cMs with someone who you believe to be a first cousin, you can’t use shared DNA to determine if your parents were half-siblings. 
• If you share only 300 cMs, however, then you could know for sure that your parents were half siblings. 
• If you share over 700 cMs, then you can be reasonably sure that your parents are full siblings.

So how do I know if someone is a first cousin, a half-first cousin, or some other relationship to me?

DNA is so awesome and useful, but it definitely has limits.  We can’t expect to get all the answers so easily – what fun would that be? 

Once you know that your cousin is most likely a full first cousin, you’ll want to make sure that they can’t be any of the other relationships to you.  The same goes for the half-first cousins.

Here are some ideas on what to do you if you just can’t figure it out for sure:

Have multiple family members do a DNA test

If your shared centimorgans fall in one of the overlaps between the ranges, you’ll have to see if you can get other family members to do a DNA test to see if you can narrow it down further.   

The following companies offer high-quality DNA tests. You can use any of the sponsored links below to order an test for a relative:

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

For example, if you have a first cousin match at 600 cMs, your brother does a test and shares 400 cMs, then you know that your cousin is a half-first cousin. 

If your brother’s test comes back and he shares 800 cMs, then you know that you are just in the “low sharing” range for first cousins – but that you are, indeed, full-first cousins.

If you have a mystery DNA match who you think might be a first cousin, then by having aunts, uncles, and grandparents do a DNA test, if possible, you can determine which line of the family a first cousin falls on. 

The amount of shared DNA between your mystery match and other family relationships will be different, and depending on who they really are to you, you might be able to learn who they are for sure by comparing your results to those of other family members.

Use basic genealogy strategies

• How old is your match?  Too young to be a great-aunt and too old to be a half-niece?
• Where was your match born? Was it in a place where only one relative lived, far away from everyone else?  This could be a clue!
• Contact your match and compare what you both know about your families – you might be surprised.

Conclusion

I hope this article has helped you understand how much DNA you should share with a first cousin, whether full or half, and some of the ways that you can try to determine your relative’s relationship to you.   Do you have something to add, a question, or a story you’d like to share?   I would love to hear from you in the comments.

Thanks for stopping by!

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