Did your first cousin take a DNA test? In this post, learn how to know if your cousin is a full or half-first cousin to you.
Knowing whether your cousin is a full or half-cousin can be important in your genealogy research. For example, if your cousin is a half-cousin, you can use your shared matches to research your shared grandparent’s line of your family tree.
This article is intended to help people determine whether their known first cousin is a biological half-cousin, or whether they are a full cousin. In other words, if you know for sure that your relative is a first cousin, but you don’t know for sure whether they are a half or full-cousin, this post will help you.
If you aren’t yet sure how your relative is related, it is important to know that there are many different relationship types that fall within the same range of shared DNA that first cousins typically have. When researching how someone might be related, it is important to consider all options.
Other close DNA relationships that share a similar amount of DNA as first cousins or half-first cousins are close relatives such as great-niece/nephew, great-aunt/uncle, first cousin once-removed, grandchild, grandparent, second cousin, and sometimes even half-sibling or niece/nephew, among other less common possibilities.
If you are sure that your first cousin DNA match is a genealogical first cousin to you, which means that you both descend from at least one of your grandparents, keep reading to learn more.
As always, we should research our DNA matches with care and discretion, keeping their privacy in mind. Unless specifically asked about our cousin relationship, we should never disclose if we believe that we have discovered that they may be a half-cousin to us.
Check your total number of shared centimorgans (cMs)
The easiest way to know whether your cousin is a full or half-cousin is to see how much DNA you share., measured in centimorgans. This information is available on all of the major DNA testing companies’ DNA match lists, including Ancestry, 23andMe, Family Tree DNA, and MyHeritage.
For each of these relationships, there is a range of shared centimorgans that we expect to see. It is always possible for the DNA that you share with your relative to fall slightly above or below these ranges.
If your shared DNA falls squarely within the middle of the range for either of these two relationships, you can be pretty sure that you have discovered the nature of your biological relationship.
However, it is important to note that if you learn that you and your cousin are half-cousins, sharing only one grandparent, you will still need to do additional research to figure out whether it is you or your cousin who does not descend from who you know to be your grandparents.
How much you should share with a full cousin
Full cousins will share between 575 – 1330 centimorgans (cMs) of DNA, with the average being about 850. Amongst the first cousins who have tested their DNA in my own family, I have determined the average amount of DNA shared to be 879 cMs.
It is important to know that most full first cousins will share somewhere in the middle of the range, likely between 800-1100 cMs. A relative few will share less than 700 cMs, but it is possible and I even have an example of this further on in this article.
How much you would share with a half-cousin
Half cousins will generally share between 215-650, although it is certainly possible to have half-cousins share slightly over or under this range.
Can the number of DNA segments shared between first cousins help determine half or full relationship?
There is no set number of DNA segments that first cousins usually share. This means that we can’t come to any conclusions based on how many DNA segments we share with our cousins.
What if the shared DNA with your first cousin is inconclusive?
If you share somewhere between about 500-800 centimorgans with your first cousin, you might find yourself unsure about whether you are full or half-cousins based solely on the amount of DNA that you share. When this is the case, there is still a lot you can do to figure things out for certain.
You and your relative could be a “low sharing” match
It is possible for two relatives to be “low sharing”. This term is used to describe the DNA relationship between two biological relatives when the number of shared centimorgans is much lower than expected for their known relationship.
The image below is from my dad’s DNA match list where he has a few known first cousins. The cousin I’m showing you here is absolutely his first cousin, even though they only share 570 centimorgans.
It would be very easy to assume that this person is my dad’s half-cousin, since 575 cMs falls pretty neatly into the half-cousin range. However, both my dad’s mother and her brother (the parent of the first cousin) have tested their DNA, and their shared DNA is in line with that expected from full-siblings (2648 cMs).
Furthermore, this cousin shares DNA matches in common with my dad from both of their grandparents’ lines of the family tree. We can also rule out an unlikely, yet possible, three-quarter sibling relationship between my dad’s mother and his uncle because their father was an only child.
As you can see, the only way we can be sure that the amount of DNA shared indicates a low sharing DNA match and not a different relationship type is by finding additional data points to study.
Fortunately, we can find even these additional data points right on our DNA match lists.
You and your cousin could also be “high sharing”
It’s less common to hear about “high sharing” matches, but they happen just as much as low sharing matches do. All high-sharing really means is that the two relatives share a little more than is typically seen between two relatives.
As an example, I’ll show you a screenshot of my match with a full first cousin once-removed. My mother and my match’s father were full siblings, but I didn’t know who my cousin was when I first took my DNA test.
She was my highest match at the time, and so of course I was very interested in figuring out how we were related. This match is actually what inspired me to learn as much as I could about using DNA for genealogy!
Typically, we would expect first cousins once-removed to share between 215-650 cMs. My cousin and I share an amount that is right at the very top of the range, outside of what most first cousins once-removed share, and so we can classify this as a high sharing match.
The DNA amount that we share fits into ranges of shared DNA for other, closer relationships – even full first cousin.
Many people don’t spend too much time thinking about high sharing matches because they don’t stand out as much as a low sharing match would. Seeing lower than expected DNA often motivates people to try to figure out whether there is a half-relationship, or some other relationship.
If you believe that your first cousin is a half-cousin, you can still use the following two suggestions to narrow things down a bit.
Examine your shared matches
The first thing you should do when examining whether your DNA relationship with your cousin indicates a half-relationship is check out your shared DNA matches. If you and your cousin are full cousins, then you should have DNA matches who are descended from ancestors on both of your grandparents’ lines of your family tree.
This works best if you and your family are from the United States and your family has been in the US for several generations. Many millions of Americans have tested their DNA, and so there is usually a good number of close and distant cousins on our DNA match lists.
If you don’t live in the US, or your parents, grandparents, or great-grandparents immigrated to the United States from a country where DNA testing isn’t popular, your extended family might not be well-represented in your DNA match list. In this case, using shared matches to figure out whether a cousin is a full or half-cousin won’t work as well.
However, it only takes having one extended relative on each grandparent’s side of the family to use this method to confirm your relationship, so you don’t need to have a ton of cousins on your match list for this.
How much DNA do other relatives share with you and your match?
If you are able to find out how much DNA your first cousin shares with other known relatives, you will have additional DNA data points to help you narrow down whether you are full or half-cousins. You might be able to ask your relative for this information, or they may give you access to their DNA match list.
For example, if you share 650 cMs with your first cousin, you might think that you could possibly be half-first cousins. However, if your sister or another full cousin shares 900 cMs with that same cousin, you are definitely not half-cousins.
On Ancestry, Ancestry Pro Tools allows you to see how much DNA your match shares with your shared matches. So, if you tested on Ancestry, this is one option that you could use to get this information.
Conclusion
I hope that this post has helped you understand how you can use the information available to you on your DNA match list to determine whether your first cousin is a half or full-cousin.
If you have any questions about something that you read in this post, or if you would like to ask a question about a specific DNA relationship, please join in the discussion below.
Thanks for reading today!