If you have a DNA match that shares about 1300 cMs with you, they could be your half-sibling or a half-uncle or aunt, among other relationship possibilities. In this post, learn how to know whether your match is your half-sibling, or that of one of your parents.
DNA results are an amazing way to find relatives and connect with distant cousins who can help you learn about your ancestors. But, sometimes we find a match that we aren't sure about.
When I first did my DNA test on Ancestry, the person on my match list who shared the most DNA with me was a complete stranger. I had no idea how we were related, and my quest to learn about our relationship was what got me interested in learning about DNA.
When it comes to someone who might have a very close relationship to you, such a sibling, aunt, or uncle, it is understandable that you would want to know absolutely for sure which relationship they have to you.
The difference between a half-sibling and a half-uncle / half-aunt
Before we dive into this subject further, it's important to clarify the definitions of the two relationships we discuss in this post.
A half-sibling would be a biological child of only one of your parents. If you have a sibling who is the child of both of your parents, they would be a full-sibling and will share much more DNA with you than 1300 cMs (centimorgans).
If your parent has a half-sibling, they would be a half-uncle or half-aunt to you. Your parent's half-sibling shares only one of your grandparents in common with you.
How much DNA would they share with you?
Half-aunts and uncles could share as much as about 1300 cMs with you, with the typical range of shared DNA for this relationship being from about 575-1300 cMs. Shared DNA can fall slightly above or below this range.
Half-siblings would typically share more DNA with you than half-aunts/uncles would, with the range for this relationship being about 1300-2300 cMs. Very occasionally, we might see shared DNA for half-siblings fall slightly below this range.
This means that if your DNA match shares far below 1300 cMs, and you know that the only relationship possibilities are half-sibling or half-aunt/uncle, you can be sure that your match is a half-uncle.
Alternatively, if your match shares far above 1300 cMs and the only two possibilities are half-sibling or half-aunt/uncle, you can be sure your match is a half-sibling.
Shared DNA for these relationships can fall just below or just above the typical range
It's normal to see variation in the exact amount of DNA that half-aunts, uncles, and half-siblings might share with you. The amount of DNA that you actually share with these relatives could be a little under or above the ranges that I mention here in this article.
The Shared cM Project 4.0 had 1266 submissions for data about shared DNA between half-siblings, and only 26 of those submissions were below 1300 cMs. None were below 1100.
Ancestry DNA has many millions of customers who have tested their DNA on their site, which means that they have a large amount of data to work with when it comes to estimating relationships and determining the probability of any given relationship based on the amount of shared DNA.
For the below DNA match who shares 1338 cMs with my mother, Ancestry estimates that 24% of the time, people with this amount of shared DNA might have a relationship to each other that includes half-uncle as a possibility, among several other relationships.
This means that a 1300 cM match is in a "grey area", and we need to be careful about making assumptions based only on shared DNA.
Who else could a 1300 cM match be to you?
It is very important to know that there are several categories of relatives who could share around 1300 cMs with you. As I mentioned previously in this article, we know that the ranges of expected shared DNA may overlap with those of over relationships.
Aside from a half-sibling or half-aunt/uncle, the match could also be any of the relationships below.
Relatives that can share as little as about 1300 cMs (typically range between about 1300-2300 cMs):
- half-nephew or half-niece (a child of your half-sibling)
- double-first cousin
- grandparent
- grandchild
Relationships that can share as much as about 1300 cMs (typically range between about 575-1300 cMs):
- full first cousin (a child of your aunt/uncle)
- great-grandparent (a parent of one of your grandparents)
- great-grandchild (a child of your grandchild)
- full great-aunt/uncle (a full-sibling to one of your parents)
- full great-niece/nephew (a grandchild of one of your full siblings)
Fortunately, we can use lots of additional information about our DNA matches to figure out how we are connected to them. Shared DNA is merely an important clue about how we are related to our matches.
How to figure out whether the match is a half-sibling to you or to your parent
If your DNA match shares right around 1300 centimorgans with you, and you have eliminated all of the other relationship possibilities that I mentioned above, you'll need to learn more about your DNA match to know for sure which relationship you have to them.
Below are a few strategies you can use to find more details about your relationship to your match.
Examine your shared matches
Take a look at the matches that you share in common with your DNA match. If your match is your half-sibling, then you would share two grandparents in common with them.
This means that you will share DNA matches who are descended from ancestors of both of your grandparents.
Half-aunts or half-uncles, on the other hand, will only share DNA matches who are descended from ancestors of a single grandparent.
Ancestry Enhanced DNA Matches
If you did your DNA test with Ancestry, you have the ability to upgrade to Ancestry Enhanced DNA Matches (a part of the Pro Tools bundle). This feature allows you to see how the matches you share with your DNA match are related to each other.
This is very helpful if you have lots of close relatives who have also tested their DNA with Ancestry. By knowing how much your DNA match shares with all of your other close relatives, you can more easily place them in your family tree.
Compare your X-DNA
If both you and your DNA match are female and you suspect you share the same father, you can find out if you are half-siblings by comparing your X-DNA. Gedmatch is a website to use for this.
Gedmatch has a free tool called the X-DNA One-to-One Comparison tool that allows two people who have uploaded their DNA data to the site to compare their X-chromosome.
Two females with the same father will always share a full X-chromosome with each other. This is because fathers always pass down their fully-intact X-chromosome to their daughters.
Test additional relatives
If your parents, grandparents, aunts, uncles, or siblings are available for a DNA test, their results will be very helpful in figuring out exactly how your DNA match is related to you.
Get in touch with your match
Don't underestimate how much you can learn from contacting your DNA match to see the type of information that you can share with each other. You might be able to learn from your conversation with them where they are from, where they were born, whether they are looking for family, and any other details that they have that they can share with you.
For example, by finding out where they were born, you might be able to completely eliminate certain relationship options. I'm reminded of one of my surprise DNA matches who was born in another country; only one of my parent's uncles had ever traveled there.
Conclusion
I hope that this article has helped you understand how to know whether your DNA match might be a half-sibling or half-aunt or uncle to you. It's important to know the difference in these two relationships, and understand how DNA can help you.
If you have any questions about something that you read in this post, or if you have a DNA match that you are wondering about, please feel free to join in the discussion below.
Thanks for reading today!