What are double first cousins?

When it comes to family relationships, there are many different terms to describe the various types of cousins, siblings, and other relatives. One such term is "double first cousin," which refers to a specific type of cousin relationship.

A double first cousin is a cousin with whom you share two sets of grandparents. In other words, they are the child of your parent's sibling, who is also the child of your other parent's sibling. This means that a double first cousin is genetically equivalent to a half-sibling, since they share roughly 25% of their DNA with you, as opposed to the roughly 12.5% shared by regular first cousins as you can see on this handy chart at ISOGG.

To give an example, let's say that your mother has a sister, and your father has a brother. If your mother's sister has a child with your father's brother, that child would be your double first cousin. This is because they are the child of both your mother's sister (who is your parent's sibling) and your father's brother (who is your other parent's sibling).

Double first cousins are relatively rare, since they require two sets of siblings from each parent's side of the family to have children with each other. They are more common in certain cultural or geographic regions where intermarriage between close relatives is more common.

In conclusion, a double first cousin is a cousin with whom you share two sets of grandparents. While relatively rare, this type of cousin relationship can occur when two sets of siblings from each parent's side of the family have children with each other. Understanding the nuances of different family relationships can be helpful for tracing one's family history and understanding genetic inheritance.