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.
- From the relationships of two first-cousins with two other first-cousins.
- 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
This is what a set of first cousins looks like in a family tree. John Clemens is the brother of Marie Clemens. Mary Horst and Theodore 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.
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. UPDATE: In speaking with Schneble descendants of Anna and Marie and seeing pictures of them, they were not identical twins.
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!