Double-fun arises when siblings marry another set of siblings. This special connection between two families produces a special set of cousins — the double cousin.
What is a Double Cousin, a Half Cousin, and a Double Second Cousin?
When two siblings from one family, marry two siblings from another family, then their children are double first cousins.
Double first cousins share both sets of grandparents, and are as genetically related as half-siblings.
Usually first cousins share about 1/8, or 12.5% of DNA, but double cousins share around 1/4, or 25% of DNA — just like half-siblings. Hence, the term ‘double’ is used, since twice as much DNA is shared relative to a first cousin.
While half-siblings share either a mother or father, double cousins share neither a mother nor a father. Yet, like half-siblings, double cousins share 25% of DNA.
Similarly, half cousins, are cousins from half-siblings. The term ‘half’ is used because half cousins only share 1/16, or 6.25% of DNA, which is half the amount as much as a standard first cousin.
The children of double cousins are called double second cousins, and they are as genetically related as first cousins, sharing 12.5% of DNA.
By understanding how much DNA is genetically shared between cousins, you can better understand the type of cousin relationship. Read more here: How Much DNA Do Family Members Share?
Some Wacky Double Cousin Scenarios to Ponder
If you and your cousin’s fathers are brothers, and you and your cousin’s mothers are sisters, then your cousin is your double cousin.
If your cousin’s mother’s brother is your father, and your cousin’s father’s sister is your mother, then your cousin is your double cousin.
If two identical twins marry another pair of identical twins, their children would be double cousins and genetically as similar as true brothers and sisters, sharing 50% of their DNA on average. Yet, they would share neither the same mother nor the same father.
That’s some wacky genetics, folks!