Help:Most Recent Common Ancestor

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Anyone who is related shares common ancestors. To know how you are related to someone, you need to know your most-recent common ancestor (MRCA).

For example, the most-recent common ancestor of two siblings is a parent. This is what makes them siblings. The most-recent common ancestor of first cousins is a grandparent. This is what makes them first cousins.

WikiTree's Relationship Finder names cousin relationships by finding the most recent common ancestor that two people share. This relationship chart illustrates how it's done — how the most-recent common ancestor is used to name those confusing "X cousin X times removed" relationships.

Significance of MRCAs in Genetic Genealogy

Genealogists who have taken DNA tests are very interested in most-recent common ancestors (MRCAs). Matches may be able to confirm their relationships back to their MRCA.

For y-chromosome DNA tests the MRCA is a paternal line ancestor, e.g. a father, or father's father, or father's father's father, etc. For mitochondrial DNA tests the MRCA is a maternal line ancestor, e.g. a mother, mother's mother, or mother's mother's mother, etc.

For autosomal DNA tests the MRCAs are an ancestral couple. Any given segment of DNA only came from one of the two parents, but we can't know whether it was the father or mother (except in the case of half-cousins that only share a mother or a father of half-siblings).

Relationships beyond MRCA(s) cannot be confirmed. Confirming deeper ancestry requires a more distant cousin to be tested. For example, a first cousin match can only confirm relationships to grandparents. Confirming relationships to great-grandparents requires second-cousin matches.

See this ISOGG page for more information.

MRCA vs. EKA

You'll often see Earliest Known Ancestor (EKA) mentioned in the context of yDNA and mtDNA testing. It's important to know the difference between EKA and MRCA.

Your earliest-known ancestor in a line, of course, is the most-distant ancestor to whom you've been able to trace your genealogy.

On WikiTree, if you take a yDNA or mtDNA test, we attach your test to all the profiles of your direct-line paternal or maternal ancestors back to the EKA in that line. Your test is applicable to all these ancestors and to all their direct-line paternal or maternal descendants. And it's of great interest to all genealogists who are descended from these ancestors and interested in confirming their family tree.

When you're deciding who could take a yDNA or mtDNA test in order to help you confirm a particular parent-child relationship, you want to look at the DNA Ancestors page for the starting person and look for the EKA in their Y-chromosome or mitochondrial line. You can then click to the EKA's DNA Descendants to see everyone who could be tested.

It's important never to confuse the EKA and MRCA because relationships can only be confirmed back to the MRCA shared by two test-takers. Even though the yDNA or mtDNA presumably matches all the way back to the EKAs that cannot yet be confirmed.

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This page was last modified 15:50, 17 March 2019. This page has been accessed 4,789 times.