How Much DNA Do Distant Cousins Actually Share?

Tracing Your Roots: We turned to a genetic genealogist to address a common issue for those using DNA-testing services.

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Dear Reader:

For this week’s column I decided to address a topic that comes up frequently in your questions: finding genetic relatives through one of the DNA-testing services that match people who share ancestry. In fact, in last week’s column a reader reached out to “a fourth-generation relative” that she met through such a service, and that person was able to give her information about a famous boxing champ believed to be a common ancestor.

We reached out to genetic genealogist CeCe Moore with a query of our own. Her replies follow.

Many DNA-testing companies have services that allow you to be matched with other people in their databases who share your ancestry, such as third and fourth cousins. But how much ancestry does one really share with a third or fourth cousin, and if you go further (fifth cousins, sixth cousins), do you end up sharing no DNA? Also, how many third and fourth cousins does the average person have?

There are three commercial companies currently offering autosomal DNA cousin matching: 23andMe, FamilyTreeDNA and AncestryDNA

The way it works is your autosomal DNA is tested and compared with that of everyone else already in the database. People who share long-enough stretches of identical DNA with you to indicate a genealogical relationship are listed as your DNA matches. Based on how much DNA you share with each of these individuals, the company predicts your most likely relationship.

By the Numbers

First-degree relatives—parents, children and full siblings—will share about 50 percent of your autosomal DNA (atDNA). With each relationship removal, the expected amount of shared atDNA is cut in half. Second-degree relatives (grandparents, grandchildren, uncles, aunts, nephews, nieces, half-siblings) are expected to share about 25 percent of their atDNA. Third-degree relatives (first cousins, great-grandparents, great-grandchildren) will share about 12.5 percent.

As for more distant relationships: Second cousins share about 3.125 percent of your atDNA, and third cousins are expected to share about 0.781 percent. A cousin relationship that is once removed (separated by one generation) or shares only one common ancestor instead of a couple further reduces the expected amount of DNA matching by half.

Do You Share DNA With All Your Distant Cousins?

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