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Most Recent Common Ancestor (MRCA)

The Most Recent Common Ancestor (or MRCA for short) represents the closest in generation joint ancestor between two individuals.  If two individuals share an ancestor, they are designated cousins and likely share some DNA.  If the same number of generations occurs up to that common ancestor, then they are direct nth cousins withn being the number of generations less one.  If the generation count is different, then they are mth removed cousins, depending on how many generations m they are different by.  Often, they share common grandparents as ancestors. If only one grandparent is shared, they are termed half cousins.  Siblings have their parents as the MRCA and are such designated 0th cousins.  Aunts and Uncles are 0th cousins, once removed as the MRCA is the grandparent or parent; respectively.  Spouses are only considered, in Genetic Genealogy, when they contribute to the DNA of the descendant.

Using this measurable Degree of Relationship is important to understanding expected and analyzing actual DNA testing results.  The closer each testee is to the MRCA, the more of that ancestors DNA they still carry.  As the individuals share the ancestor(s), any ancestors of the MRCA are shared as well.

See the related Earliest Known Ancestor (EKA) also.