A unit measuring consanguinity: the extent to which two persons are related to one another by common descent ("by blood," as the usual saying is). Essentially all cultures, societies, and nations have prohibited marriage between closely related individuals, and the determining factor is often the number of degrees of consanguinity between the two persons. If X is an ancestor of Y, the number of degrees of consanguinity between X and Y is the number of descents; thus parent and child are separated by 1 degree, grandparent and grandchild by 2 degrees, and so on. If persons X and Y are not linked by direct descent but do share a common ancestor A, the usual definition is that the number of degrees of consanguinity between X and Y is the total number of descents, counting up from X to A and then back down from A to Y. For example, siblings (brother and sister) are separated by 2 degrees, aunt and nephew by 3 degrees, and first cousins by 4 degrees. This calculation is backed by modern science, because we have learned that persons separated by n degrees of consanguinity share about 1/2n-1 of their genes if they have two common ancestors or 1/2n if they have only one common ancestor. See also: Cousins