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What Are Double Cousins?

In working on a family tree, you may have seen cousins that aren’t just simply first cousins (children of siblings who marry unrelated people), they may be what is called “double cousins”.

How Does This Happen?

Double cousins are when two siblings of one family have children with two siblings of another family (brother/brother, sister/sister or brother/sister, sister/brother).

The children are related to each other through both of their parents and are considered double related.

Double second cousins can happen in two ways.

  1. From the relationships of two first-cousins with two other first-cousins.
  2. From the relationships two double-first-cousins with two other persons.

Double first cousins share both sets of grandparents and have twice the degree of kinship of ordinary first cousins.

How Does This Affect Genetics and DNA?

Genetically, double cousins could test with numbers consistent with half-siblings.

Normal first cousins (cousins with one parent of each as a sibling while the second parents are unrelated), can share roughly 12% of their inherited DNA. Double cousins, because both parents are siblings to each other, can share roughly 25% of inherited DNA.

This is the same amount of DNA that you would share with a grandparent, a half-sibling or an aunt or uncle.

Real Life Examples

First Cousins

This is what a set of first cousins looks like in a family tree. John Clemens is the brother of Anna Clemens. Mary Horst and Joseph Schneble, their spouses, are not related. Their children are simply first cousins.

These cousins will share only one set of grandparents through the Clemens side of the family.
first cousins

Double Cousins

The family tree below is an example of double cousins. Anna and Marie Clemens are sisters, twins as a matter of fact. Joseph and Theodore Schneble are brothers.

Their children are considered double cousins.

They will share both sets of grandparents through the Clemens and Schneble family.

Because Anna and Marie are twins, they may even share more than 25% shared DNA. However, we would need to know if Anna and Marie are identical or fraternal twins.

Double Cousins

What if Joseph and Theodore were twins as well?

According to the website FamilyPedia.wikia.com, when identical twins have children with identical twins, the children are genetically indistinguishable from full siblings. When identical twins reproduce with siblings the resulting children are more related than half-siblings but less related than full siblings.

 

Do you have double cousins in your family tree? Are you a double cousin yourself? Tell us about your double cousins in the comments!


Comments

22 Comments

John

I am, kind of. My dad and my cousin’s dad are paternal cousins, while my mom and my cousin’s mom are full siblings. Because my parents are cousins, his grandpa (my great uncle) is as related to me as a normal grandpa. Basically, it looks like we share three grandparents on a DNA test. We share four out of the six great grandparents that we have. We’re maternal first cousins once, and second cousins from three different paths, as a result.

Adding all these relationships up, we get 21.875%. It’s in the range of a half sibling or double cousin, and we may get mistaken for either one on a DNA test. We both have the same Y chromosome, and same mitochrondia.

John

It’s like I have my own “genetic double cousin”. I don’t think GEDMatch can distinguish between my cousin and a real double first cousin. We also most likely share fully identical regions, so we’re going to most likely show up as double cousins.

I do actually have a paternal cousin who is double cousins with my other paternal cousin. Their dads married two sisters. They’re only first cousins, second cousins, and half second cousins once removed to me, though.

Erin Harris
Erin Harris

Thank you for sharing! That’s very interesting.

I would agree that GEDmatch can’t always tell those DNA differences between cousins.

Have you had your other relatives take DNA tests?

John

No. Not yet. It’s interesting how my cousin can possibly show up as a double first cousin even though he’s not. There are no twins involved, yet he manages to share that much DNA.

Looks like you really can produce “genetic double first cousins” after all! I don’t think my cousin can show up as a half sibling because we share BOTH our mitochrondia, and our Y chromosome.

The only other case of producing a genetic double first cousin would be if two pairs of identical twins had kids, and those kids married a pair of siblings. I’d guess those double second cousins/first cousins would be like if they were normal double first cousins.

Carol Lovell

My mother and her sister married brothers. I am an only child, but mother’s sister had 3 children. We, of course, shared all the same relatives. When we were children, sometime one of my double cousins would tease me and exclaim that her best friend was HER cousin but not mine!

Karen Bullock

I and my two sisters married three brothers. We have two children, the second set has two children, and the third set has three children.

All seven double cousins resemble each other, and now they are grown and having children of their own.

We were wondering if a study of our family’s DNA would be of any genetic use to researchers. How could I find out about such studies currently in process?

Thank you.

Erin Harris
Erin Harris

Hello Karen,

That is a handful of double cousins!

I personally don’t know of any genetic studies but that would be a great case study for researchers. Perhaps you could try reaching out to one of the DNA testing companies or even a medical school? Those students could take this on for themselves as a project.

Lola White

My maternal grandmother and her sister married brothers. Their children are my mother’s double cousins. What is their relationship to me?

Erin Harris
Erin Harris

Hello Lola,

That’s a great story.

So is your mother one of the double cousins?

I think in your case, you are a generation following the double cousins? If I understand it, you’re their second, double-cousin. If your father isn’t apart of that family, then you will share less DNA that the original double cousin group but I am not sure on the exact math.

Joe

That would be your double first cousin once removed. You would share 12.5% of your DNA with them.

allison

I am one of ten double cousins. my mother, her brother, and her sister married my father, his sister, and his brother. my parents have four kids, one set of aunt and uncle have four kids, and the other set has two kids. it’s so confusing to explain to other people how we’re all related and who is who. it’s even trickier because we almost all look like we could be siblings.

Cate Wilkins

Paul & Sallie get married. Years later, their younger siblings (Paul’s sister Annie AND Sallie’s brother Harold) get married also. Their collective offspring share uncanny similar DNA. I am one of those offspring. All of us (offspring) have tested our DNA. We match as if we were half-siblings. Our ethnic origins are also similar with two outliers. We are all 99% European. However, we have 4 great great grandparents who were Native American of the Cherokee tribe. This Native American doesn’t show in our DNA tests. We did not specifically ask for NA DNA testing.

Chameka Dionne Ezell

My Grandmother and her sister married and had children with first cousins… so I have always called their children( my mother’s first cousins) cousin and Aunt/Uncle… but now looking at the definitions I’m not sure if that’s “correct”

Joe

That is normally not considered a double cousin by geneological standards. If you were using the term loosely, then sure, it counts. If you were going by its strictest definition, only double first cousins would count.

I am actually in this situation, only I have the cousin that is first and second. We might as well be first and half first cousins because his grandfather is 25% related to me due to my parents and his parents being cousins.

There is actually a term for this, which would be irregular double cousin. There are different kinds, such as first on one side and half first on other, second and first, second and double second, etc.

It’s fine to call them aunt and uncle if you consider them to be.

Dan Abrahams

My maternal grandparents were double first cousins. Two Persian Jewish brothers married two sisters, and their offspring married, in Shanghai of all places. The odd thing was, my grandparents, who had a difficult marriage by all accounts, separated around the time I was born and later divorced…but because they were part of a fairly tight-knit extended family, they could never be totally rid of each other! Over the years, I’ve had a lot of fun playing the “if you weren’t my [sister, child, mother etc.] you’d be my [eg. “first cousin once removed” etc.] game with various members of my family. I must confess, it does kinda gross some people out.

David

What would you call a third cousin whose grandparents were second cousins to each other? The cousin of mine is technically a fourth cousin as well since we share a great great grandparent, and her other great great grandparent is my great great great uncle, brother to our great great grandparent.

Shawna Cheney

I am trying to locate my birthfather. Dna matched me to a large family, so far 30 cousins. One of those is a first cousin to me, the others are 2nd cousin. The first cousin match has a set of double cousins, her Dad and his brother married sisters. Since none of her brothers is my birthfather can this mean MY birthfather could be one of her double cousins? Would that give me the 1st cousin match with her?

Stephanie

I have the same situation as Carol above; my mom’s sister married my dad’s brother, and they have three kids – while I was an only child. I just took the Ancestry DNA test and I wonder how much of the result would be relevant for my double cousins. Shouldn’t all of our ‘origin’ ultimately be the same?

Judy Adkins Hill

while not *technically* “double cousins, I have two first cousins on my dad’s side who are third cousins (or second cousins once removed) on my mom’s side. their father is my father’s younger brother, and their mother is my mother’s first cousin. 23 & Me identifies us as “half-siblings” due to our shared DNA.

Sandra Prince

My mother and her brother have two double first cousins. Their moms are sisters and their dads are brothers. How are all of their children related?

Jan Thomas

I have that in my mother’s family. I have a first cousin who is also my third cousin!. His parents are double second cousins. Three sibling married another set of 3 sibling in the same family. Yes his match to me is the same as a first cousins.. I have another cousin who showed up with a lot high match,She was adopted and we are trying to see how she is my first cousin. Weird!

.

Erin Harris
Erin Harris

WOW! That sounds complicated and cool at the same time!


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