What is a double first cousin?
Short answerDouble first cousins share all four grandparents. It happens when two siblings from one family have children with two siblings from another, for example two brothers marrying two sisters. Because they are related through both parents, double first cousins share about 25% of their DNA on average, twice as much as ordinary first cousins.
How double cousins happen
Picture the Hale brothers marrying the Ortiz sisters. Each couple’s children have the same Hale grandparents on one side and the same Ortiz grandparents on the other. They are cousins twice over, once through each parent.
How Family Trees names them
Family Trees finds the shortest blood path between two people and names it, so double first cousins read as first cousin. The tree shows both connections, since each child is linked to both parents, and it draws the shared grandparents once.
Double cousins and DNA tests
Because they share roughly twice the DNA of first cousins, double first cousins can be mistaken for half-siblings or for a grandparent and grandchild in DNA match lists. A family tree with both lines entered clears that up quickly.
Let Family Trees name it for you
Every parent link in Family Trees is birth, adoptive, step or foster, and every couple is married, partners, separated or divorced. From that, the app names each relative in plain words on their page and under How are they related?, including half, step, foster and in-law relations. It’s free for up to 100 people.
Questions
How much DNA do double first cousins share?
About 25% on average, roughly the same as half-siblings, because they are related through both of their parents.
Are double cousins common?
They are uncommon today but were more frequent in small communities, where two families often married into each other more than once.