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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

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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.

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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?

Keep in mind that all of these percentages just mentioned, except the 50 percent shared by a parent and child, are averages. In actuality, you may share a little more or less than expected because of the random nature of autosomal DNA inheritance. There’s more about that on my blog.

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Furthermore, how much ancestry you share with a cousin is not necessarily the same as how much DNA you actually share. For example, full third cousins always share a second-great-grandparent couple, but they may not share any DNA at all. There is about a 90 percent chance that a pair of true third cousins will share enough atDNA to be detected by the company algorithms as relatives. The other approximately 10 percent of third cousins will not appear on each other’s match lists.

Does this mean that one or both of the cousins did not inherit any DNA from their common second-great-grandparents? No, it just means that they did not in inherit the same DNA from their common ancestors.

As my good friend Blaine Bettinger has explained on his blog the Genetic Genealogist, we all have a genealogical family tree and a genetic family tree. Our genetic family tree is a subset of our genealogical family tree and consists only of those ancestors from whom we inherited DNA. Back to about our third-great-grandparents, this will include all of our ancestors, but after that level some of our ancestors will start to “fall off” of our genetic family tree. Because of this, it becomes increasingly possible not to share any DNA with our more distant cousins. Fourth cousins will match each other about 50 percent of the time, but when you get out past the fifth-cousin level, the odds of sharing any DNA are less than 5 percent.

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How Many Third and Fourth Cousins Might One Have?

The number of third and fourth cousins a person has varies widely based on individual family structures and the culture of their ancestors. That being said, as Brenna Henn explains in a blog post for 23andMe: “Under a simple model where a family has two to three children, you would have 190 third cousins, 940 fourth cousins and a whopping 4,700 fifth cousins. … Now, this model is a simplification because some families have a dozen children and some have none; but [it] does help illustrate just how many potential distant cousins are out there.”

From my experience, the average person will only receive a couple of predicted third-cousin matches through a DNA testing service but will typically find many more possible fourth-cousin matches. Regardless, with rare exceptions, everyone will be matched to someone through these testing services. Most of us will have hundreds or even thousands of predicted cousins, albeit predominantly distant ones.

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To date, thanks to these relative matching services, there have been many genealogical and birth-family search success stories. With the databases growing every day, these will only increase over time and we will all see closer and more meaningful matches.

Henry Louis Gates Jr. is the Alphonse Fletcher University Professor and founding director of the Hutchins Center for African and African American Research at Harvard University. He is also editor-in-chief of The Root. Follow him on Twitter and Facebook. CeCe Moore is the genetic-genealogy consultant for two PBS television series, Finding Your Roots With Henry Louis Gates, Jr. and Genealogy Roadshow. She is co-founder of the Institute for Genetic Genealogy and author of the blogs Your Genetic Genealogist and Adoption and DNA.

Send your questions about tracing your own roots to TracingYourRoots@theroot.com.