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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, 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!




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.


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?


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.

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