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How Much DNA Do You Share With a First Cousin?

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.

How Much DNA Do You Share With a First Cousin

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 “coincidental” 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.

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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:

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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luella joyce kimble

Wednesday 2nd of December 2020

Hi, I am 75 yrs young still trying to find my bio Father. I just got results from Ancestry with a 1st or 2nd cousin who is 33 yrs (345cM across 13segments Could she be my half great niece with me being her half great aunt. Then her mother 60yrs old would be my half niece, then her 85 yr mother would be my half sister with her Father being my bio Father. I sure would appreciate your expert opinion.

Mercedes

Wednesday 2nd of December 2020

Hi Luella, Thank you so much for your question. A DNA match sharing 345 cM certainly could be a half-great niece. There are several other possibilities with that amount of shared DNA, however, including a second cousin or a second cousin once-removed, among others. If your DNA match's parent is available to test, it would help narrow down some of those relationship possibilities a bit. Geographic locations and ages could help eliminate others. Best of luck to you, Mercedes PS You might find "triangulation" of DNA matches useful, if you have access to family tree information for your DNA match and DNA matches that you share in common: https://whoareyoumadeof.com/blog/how-to-triangulate-dna-matches-to-identify-unknown-ancestor/

Dan

Monday 23rd of November 2020

Is it possible to have a first cousin share more centimorgans with me than the blood related aunt (his mother)?

Mercedes

Monday 23rd of November 2020

Hi Dan, Hypothetically, of course, the only scenario that I can think of where a true genealogical first cousin shares the same amount of DNA (or more) with you as an aunt would be if the cousin's father was also related to you. For example, if your first cousin was the child of your aunt on your mother's side and your uncle on your father's side (i.e. a double first cousin). Best of luck to you, Mercedes

Audrey

Friday 2nd of October 2020

Hi,I would like your thoughts on this if possible.... long story short is my siblings 6 including myself are all half brothers and sisters having the same mother... my dad or who I know as Dad supposedly found out he was “sterile”.... I have a 1st cousin according ancestry who I’m thinking may be my half sister... Her father is my supposed uncle but I’m thinking he’s my bio dad.... our numbers are 937 cm’s over 33 segments with the longest segment at 116.... is this possible?

Mercedes

Friday 9th of October 2020

Hi Audrey, A person who shares 937 cMs with you is definitely a first cousin instead of a half-sibling (assuming those are the only two options). This is a helpful calculator for figuring out relationship possibilities at a particular level of shared DNA: https://dnapainter.com/tools/sharedcmv4 We expect half-siblings to share at least 1300 centimorgans. I hope that this helps you! Sincerely, Mercedes

Robyn

Wednesday 2nd of September 2020

Hi there!

Genealogy is fascinating! I've always had a running joke with my cousin in that we share more DNA than the average cousins, more like sisters, really. Turns out this isn't too far from the truth. However I'm confused as to what the percentage would be. Our mothers are identical twins and the twins married half brothers. Our fathers share a mother but have different fathers. Wondering if you could help? We have discussed having our DNA looked at.

Mercedes

Thursday 3rd of September 2020

Hi Robyn, That's such an interesting story! Yes, you and your cousin likely do share much more DNA than the average first cousins. The identical twin mothers aside, you are "double first cousins" because you share grandparents on both sides of your family. You only share one grandparent on your father's side, but the fact that your mothers are identical twins will make up for less shared DNA (significantly). I would expect you to share DNA in the half-sibling range. It would be interesting if you both end up doing DNA tests. If you do end up testing, I would love to hear back from you. Sincerely, Mercedes

Roger Zapparelli

Sunday 30th of August 2020

Why is there such a disparity between Ancestry.com and 23andme.com ethnicity breakdown? (i.e.: 93% Italian on Ancestry. 72% Italian on 23and me?)

Monica

Wednesday 9th of December 2020

I share 1156 and 1115cm with two 1st cousins which are brothers. My aunts only kids. The rest of my 1st cousins, my other aunt and uncle kids, I only share from 859 to 701 cm shared. Question.... why is that? Why are my aunts two boys a lot more than other cousins? Thanks in advance

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