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Britannica Sources used in this answer

About how much genetic material do first cousins share?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
(Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above
consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

First cousins are considered consanguineous kin of the third degree. From a genetic perspective, they share approximately one-eighth of their genes in common.1

While the term consanguinity historically referred to "common blood," it is more accurately understood today as a genetic concept involving the inheritance of DNA from common ancestors. The specific fraction of shared genetic material can vary due to the chance elements involved in meiosis, the process of cell division that creates reproductive cells.23