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 analyzing DNA tests spend a lot of time talking about most-recent common ancestors.

With Y-Chromosome DNA Tests they're talking about the most-recent direct-line paternal ancestor, e.g. a father, or father's father, or father's father's father, etc.

With Mitochondrial DNA Tests they're talking about the most-recent direct-line maternal ancestor, e.g. a mother, mother's mother, or mother's mother's mother, etc.

When test-takers have a genetic relationship it's important to discover their MRCA. Depending on the test, the test-takers may be able to confirm their paternal or maternal relationship back to this most-recent common ancestor, but no further.

Confirming deeper ancestry requires a more distant cousin to be tested. For example, two first cousins could confirm a relationship to a common grandparent. Two second cousins could confirm a relationship to a common great-grandparent.

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 DNA Ancestor Confirmation Aid|confirming their family tree, regardless of line.

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 13:14, 29 April 2018. This page has been accessed 2,182 times.