The Wonders of DNA

06/10/2015 04:54 pm ET | Updated Jun 10, 2016

My family has been doing family history for 150 years. Do I really need a DNA test? ~ LoAnne

Dear LoAnne,

If you are blessed to have a well-researched, extensive family tree, constructed on a solid foundation of detailed documentation, you may wonder what more you could possibly learn about your ancestry through an analysis of your DNA. But whether you have been trying to uncover your family history for decades or have only recently started searching for your roots, there are many delightful aspects of your distant and more recent ancestral past that can be recovered only through your DNA. We are tempted to say that DNA takes over where your paper trail ends, and that is certainly true about our distant ancestors.

But DNA can both confirm and unveil recent family relationships as well, including close and distant cousins with common ancestors back as far as 300 years ago. Sometimes, DNA may reveal unknown siblings, aunts, and uncles, and even previously unknown biological parents and grandparents, making it an ideal tool for adoptees searching for their biological roots. DNA is also often the only way that we can prove or disprove family relationships and family myths, such as whether two siblings are actually related or whether or not that proverbial Native American great-grandmother that so many Americans ardently believe resides among the not-so-distant branches of their family tree really existed. Once thought of as the proverbial "handmaiden" of genealogy, genetics has emerged in the last few years--especially with developments in analyzing one's autosomal DNA (and more about that in a minute)--as the co-equal partner in any sophisticated quest to find out as much as one possibly can about the shape and size and even color of the roots and branches on one's family tree. In other words, if you really want a full idea of your family tree, having your DNA analyzed is the other half of the genealogical equation. And if you haven't had your DNA tested, you haven't completed your research on your family tree.

Validating your pedigree

Every person inherits the DNA that makes up their genome directly from their parents. Each of us gets half of our genetic material from Mom and half from Dad. And here is where things get interesting: even our siblings (barring identical twins) don't receive exactly the same halves of our parents' genomes, which means that everyone's genome is unique. Similarly, each of our parents inherited half of their DNA from each of their parents. In other words, 50 percent of our DNA comes from each parent, about 25 percent from each of our four grandparents, and about 12.5 percent from each of our great-grandparents, and so on. DNA inheritance is mathematical, automatic, and guaranteed. (If only that were true of census records!)

Not only were biology's laws followed each generation down to you, they were also followed each generation down to every one of your living cousins (and you have a lot). So, that documented 2nd or 3rd cousin of yours should share some of the DNA that you inherited from your great-grandparents. In fact, if the 2nd- 3rd cousin relationship you've identified in your tree is correct, 99 percent of the time a DNA test will show that you share DNA. Some scientists estimate that you actually inherit some DNA from each of your ancestors on your family tree back six generations, or 180 years, but you also inherit DNA from ancestors much older than that, though not all of them.

Since DNA is independent of records (and family myths!), identifying your genetic relatives and common ancestors can provide independent evidence that your work is correct, or...incorrect. In the next season of Finding Your Roots, for instance, we reveal that a guest's mother--unbeknownst to her--had actually been adopted, which meant that the grandparents this person knew so well and loved so much weren't, in the end, his biological relatives at all. In fact, using both genealogical records and genetic testing, we were able to find one of this person's biological grandparents and share this astonishing news with his mother, who was deeply grateful, since she had had her own dark suspicions and heard "whispers" about this fact.

How did we do this? It's simple, if you take a minute to think about it: a child adopted into a new family to which she or he is not biologically related will not typically share DNA with his adopted family. So, if we test the DNA of two siblings or two cousins, we are almost guaranteed to find a lot of shared autosomal DNA. If we don't find any shared DNA, that is evidence that either one of the siblings or one of the parents of the cousins is not the biological child of the person they previously believed they were. That may seem obvious, but DNA can be used to determine if any of the ancestors on your family tree actually were adopted or were the offspring of "non-paternity events," as it is euphemistically put. And this isn't limited to just close relationships; DNA analysis helped Ross uncover an adoption in his family that happened more than 150 years ago!


DNA testing technology is advancing rapidly, and new, unprecedented tools are being developed. For example, late last year AncestryDNA released a new feature called DNA Circles. A DNA Circle uses genetics to bring together a group of people who already show they are related on paper. Accordingly, a DNA Circle brings together cousins who can collaborate on extending and refining each other's family trees. As DNA Circles grow, they can serve as powerful evidence that the genealogical research bringing all these people together is correct because of the shared DNA between members of the circle. For example, Ross has one DNA Circle around a 5th great-grandfather with more than 50 people in it. Fifty people, all with different paths descending from a common ancestor, and each person shares DNA with at least one other person in their DNA Circle. (Another feature, New Ancestor Discoveries, connects you to individuals with whom you do not share a paper trail.) This is just one instance of the marriage of genealogy and genetics, providing a researcher with a lot of confidence that the family trees of the members of the DNA Circle are correct. This also provides Ross 50 additional opportunities to collaborate with relatives about genealogy, which we'll get into later.

Making a personal connection with your ancestors through DNA

If you're like us, you've sat for hours looking at old photographs of your ancestors, searching for a physical connection with them. It isn't hard to figure out that Ross got his red beard from his 2nd great-grandfather. Also, Tom Joyner, the brilliant radio host and an African American, was born with blue eyes. The origin of those blue eyes had long been a family mystery. When we showed him a portrait of his white 3rd great-grandfather, a man named John Hall (1767-1833), it was obvious that he had inherited that eye color from this ancestor, an ancestor he couldn't know for certain that he had descended from without DNA testing. (We suspected that John Hall's son, Edward, fathered a child with his slave Jane Hall, who was Tom's great-great-grandmother. Using DNA, we proved that this assumption was correct!)

And the reason that you inherited physical traits from your ancestors is because you also inherited DNA from those ancestors. DNA is in almost every cell of our body; it is with us all the time, and it has been passed down over centuries and even millennia.

Your ancestors aren't just a picture in a photo album; a part of your ancestors lives in you today. So, when you see a geographical ancestry estimate (in the case of one of us, that is about 48 percent Sub-Saharan African) or a 4th cousin match, it is a reflection of the DNA that your ancestors on your fully populated family tree passed down to you--even ancestors whom you, as yet, cannot name.

For some people, that connection may take on a special meaning. For example, some may wonder if they inherited any DNA from a Native American ancestor or an ancestor from Nigeria. Seeing an estimate of their regional genetic admixture -- the places in the world where their ancestors might have lived as recently as half a millennium ago -- might give them a sense of connection to ancestors that wouldn't be possible otherwise. And, of course, this can take on extra special meaning if no paper records exist through which to trace your more distant ancestry, as in the case of African Americans eager to find out where in Africa their ancestors originated or some Jewish Americans from Eastern Europe. These estimates of geographical ancestry can be as rewarding and as exciting for a person in search of their roots as finding a written document -- and sometimes even more so!

Adding pieces to your family story2015-06-10-1433969474-3549465-ScreenShot20150610at4.46.35PM.png

Try as we might, it is virtually impossible to find every ancestor on our family tree, simply because the paper trail by which we document their relationship to us simply does not exist. Remember, with each generation, our number of ancestors doubles, so we have 64 4th great-grandparents, for instance, and 128 5th great-grandparents. And even for those ancestors we've found, not every name on our family tree is accompanied by a story about her or him. Some stories become popular along some branches of the family, while other branches of the same extended family tree have never heard of this person or their antics. So how do we find stories about these silent ancestors whose names seem to stare blankly at us from our family trees? By connecting with people who share the same ancestors that we have and who have been repeating stories about this unknown ancestor for generations! The biological connection through a DNA match is often one of the easiest ways to meet distant cousins we would have never met otherwise. And it takes only a few seconds to discover these cousins once you're both in the DNA database. In fact, the computer does it for you! Once you contact them, you can start sharing stories as you try to figure out how you are related.

For example, Ross has spent hours learning about his great-grandparents, studying their life stories, and talking to his grandparents about them. So imagine his surprise when a message arrived from a DNA cousin telling a story about his great-grandfather Bernard that he'd never heard before! This DNA match's father was Great-grandfather Bernard's 1st cousin Ammon. As it turned out, Ammon had stolen the girl that Grandpa Bernard wanted to marry. It was another detail to add to the collection of stories about Grandpa Bernard, and the story added color to how Grandpa Bernard's parents felt when he did get married later.

DNA analysis doesn't stop at connecting you with ancestors whose identity you know. Recently, AncestryDNA released New Ancestor Discoveries, an addition to DNA Circles that can connect you to potential ancestors and relatives that you don't have in your tree-using just your DNA. So now you can find out about possible relatives starting with just a genetic connection. These kinds of connections could help you break through those seemingly impenetrable brick walls where records seem to have disappeared. Through a DNA connection, you can also learn about close relatives who might have been a big part of your ancestor's life but whose names you could never ascertain before because no paper records of your relationship have been found.

Being part of new technology

DNA can help us connect to our ancestors in a way that a name on a census record can't by connecting us to living cousins, descendants of our ancestors, people who are living today, people we can contact and share research with, people who can help us in our common search for knowledge about our shared ancestry. Think of DNA as those tiny yet highly identifiable pieces of our ancestors that are living in us today. These identical segments of DNA that you and your cousins share are the result of your relationship, even if your relationship cannot be documented through written records. And if it can be documented, that shared DNA is powerful evidence that your family history research is on the right track. In other words, DNA testing is an essential addition to any well-researched family tree.

Try it! You will be astonished at what you will discover.

Do you have a mystery in your own family tree? Or have you wondered what family history discoveries you could make with a DNA test? Send Henry Louis Gates Jr. and his team of Ancestry experts your question at