If you are reading this post, you probably have a sister who shows up as a cousin, instead of as a sibling, on your DNA match list. Alternatively, you are just curious about how this might occur. What does this mean? Is there any way to tell if you are full or half-sisters? How much DNA should sisters share? In this post, I’ll answer these questions and more.
How common is this? It is very unusual for half-siblings to show up as first cousins, and it is not possible for full siblings to show up as first cousins on a DNA match list.
Why do the DNA results show you as cousins instead of sisters?
DNA testing companies use the amount of DNA that you share with your DNA matches to estimate your relationship. In the case of DNA matches who are sisters, they will also measure the amount of DNA that you share and estimate the relationship that you might have.
For each type of relationship, there is an expected range for shared DNA. For example, a parent and a child will show between about 3300-3700 centimorgans (cMs)* of DNA with each other.
*(Centimorgans is genetics term used to describe the length of DNA segments or the amount of DNA shared between two people)
How much DNA should sisters share?
Full sisters (and brothers, for that matter) will share between 2300-3900 cMs and half-sisters will share between 1300-2300 cMs. It’s possible for full siblings to share a little bit less than 2300 and for half-siblings to share a little bit more than 2300 cMs. As you might notice, it is possible for the expected range of shared DNA for both full and half-siblings to overlap to a certain extent.
What does this have to do with siblings showing up as cousins?
Just like with full and half-siblings, there is also an expected range of shared DNA between first cousins. The range of shared DNA for two first cousins is approximately 575-1300 cMs. It’s possible for first cousins to share DNA slightly above the top of the expected range, and it’s possible to half-siblings to share DNA at slightly below the expected range.
So, there is a slight overlap. If the amount of shared DNA between half-sisters falls into the range of expected DNA between first cousins, then there is a chance that they will be labeled as first cousins on the match list.
Determining whether you are actually biologically half-siblings or first cousins with the person who you know as your sister is a matter that you can explore if you wish.
Note: Each DNA testing company has their own criteria for how they determine an estimated relationship. The numbers that I communicate here in this post are not officially used by any testing company, but they are generally accepted by genetic genealogy experts as accurate.
Sister shown as cousin, or cousin shown as sister?
You might not be surprised to learn that this phenomenon can be reversed, meaning that it also would be possible for first cousins to show up as half-sisters.
In my own family, we have a unique situation which makes this exact question important. My mom has a first cousin match who is the child of one of her uncles. One of the uncles has a daughter, and the other uncle never had any children, according to family lore.
Suddenly, a new DNA match popped up for my mom. She was in the first cousin category for my mom, but in the half-sibling category for one of my mom’s cousins.
The amount of DNA that the cousin shares with the daughter of one of my mom’s uncles is 1303 cMs. It’s so close to the first cousin range that we felt that we shouldn’t rule out the possibility that the two women were cousins and not sisters.
Fortunately, since the two DNA matches in question are female, there was something that we could do to resolve our question once and for all.
Sisters with same father will share full X chromosome
Two sisters who have the same father will always share an entire X chromosome since they inherit a full copy of the X chromosome from their father (intact) and a recombined copy of the X chromosome from their mother (usually fragmented). You can compare your X chromosome with that of your sister on Gedmatch Genesis.
To read more about siblings matching on the X chromosome, including full and half-sisters, read this post by Kitty Cooper titled, “How can the X chromosome help with maternal versus paternal?”.
In the case of the DNA match that my mom and her cousin shared, we were able to determine that they did not share a full X chromosome. They were not half-siblings, and instead were first cousins. My mom’s uncle had had a child that the rest of the family never knew about.
No matter the relationship, we were all glad to welcome a new member of the family.
I feel like it’s important to mention that sisters are sisters, no matter how they show up on our DNA match list or how much DNA they share. My childhood was enriched by three sisters, and while our fights were pretty intense, our love for each other is stronger.
If you have any questions about something that you read in this post, or if you would like to add your own experience with your sister showing up as a cousin on your DNA match list, I would love to hear from you in the comments below.
Thanks for being here today!