Facets of the world

DNA tests don't always say the same things about your ancient ancestry. Here's why.

CONTENTS
How is genetic ancestry calculated?
Sometimes, tests get it wrong
What do your ancestry results mean?


Have you ever wondered why the sky is blue? If so, you’re not alone. Great minds like Einstein and his predecessors also puzzled over this question. What they found is that white light from the sun collides with small molecules—like oxygen and nitrogen—in our atmosphere. This collision causes the blue light to bounce off in every direction, while the other colors continue mostly on their way. With blue light bouncing all around, the sky appears blue (this is true for violet light as well, but our eyes are less sensitive to it). At sunset, when the sun is farther away from us, the light has to travel through more atmosphere to reach us. As a result, the dominant blue light is all but scattered out, leaving the reds and yellows to light up the sky.

 

Everyone’s genetic history is a rich tapestry of different peoples and cultures

Aside from being a fun fact, learning about the colors of our sky is reminiscent of how we learn about human history and, more specifically, our genetic ancestry. In its most superficial viewing, our ancestry can seem pretty straightforward, like the sun’s combined light. But, when we look a little harder, you find that everyone’s genetic history is a rich tapestry of different peoples and cultures.

Helix offers a peek at your ancestral tapestry through a report that’s available to anyone who has their DNA sequenced with us. In this report, you’ll learn about your distant ancestors and where they may have lived relative to 26 possible regions. To many, the results will shed light on a new aspect of their distant past—but for some, their results may come as a surprise.

How is genetic ancestry calculated?How is genetic ancestry calculated?

So why might your results be different from what you expect? This is where our blue sky analogy comes in handy.

Though sunlight appears monotone, it’s actually composed of many different colors. In order to see them, you have to use the right tools. One of the most simple tools is a prism which, when light shines through it, will cause the light to split into different components that are visible as a rainbow. More sophisticated tools let us see more shades and nuances to each color.

In this same way, depending on the tools that scientists use, your ancestry can be described in varying levels of detail. In general, calculating a person’s genetic ancestry is done by looking for DNA sequences that are characteristic of specific populations. Through most of history, people were fairly stationary, meaning most people lived in (or near) the town where their parents and grandparents grew up. This led to some sequences of DNA becoming more common among the descendants of one population compared to the descendants of another.

 

The more people you look at, the more nuance you can see between populations, and the more detail you can provide in an ancestry analysis

When you look at enough people from different populations, you can start to identify these sequences and which populations they’re associated with. The more people you look at, the more nuance you can see between populations, and the more detail you can provide in an ancestry analysis.

This is all a bit abstract, but fortunately you can see this same concept play out in language. A good example of this is the word “pop.” In the Midwest, you’ll often hear people refer to carbonated soft drinks as “pop,” while people living on the East or West Coast tend to call it “soda.” Of course there are people around the nation using both terms, but there’s a clear pattern where the term “pop” is more common in the Midwest and could therefore be used to estimate—but not guarantee—that someone grew up in that region. Combine this one phrase with tens of thousands of others, and you can start to get pretty specific in figuring out where someone may have grown up. The same is true with DNA.

 

Sometimes, tests get it wrongSometimes, tests get it wrong

Of course, mistakes can happen in the telling and retelling of our families’ stories across generations. But even if we assume that your understanding of your family’s history is accurate, there are two main technical reasons for why your ancestry analysis might seem off.

The first has to do with the difficulty of identifying genetic factors that allow you to distinguish between two closely related populations. Scientists use complex tools to identify specific DNA sequences (like the word “pop”) that are associated with specific populations (like the Midwest). In some cases, there are DNA sequences that are very specific—only people from that one population have it. When that’s the case, you can be pretty confident that a person who has this DNA sequence also has a shared ancestry with that population.

 

This flow of people can cause the genetic patterns of neighboring populations to overlap

In many cases, though, the association is not that strong. Humans do migrate—sometimes around the globe, sometimes just to the neighboring town. This flow of people can cause the genetic patterns of neighboring populations to overlap and appear very similar at the genetic level. This overlap can make it hard to link a specific genetic sequence to one group. For example: People living in northern Italy may share many genetic markers with people living in southeastern France. If you haven’t studied many people from France, you may not be able to accurately tell these two populations apart—meaning someone with ancestry from southeastern France may incidentally be reported as having ancestry from northern Italy.

The second reason your results may not match with your true ancestry has to do with something known as “reference populations.” In essence, a reference population is a group of people whose DNA helped us link a specific DNA sequence to a specific region.

For example, if we look at DNA from many people living in the Korean Peninsula, we’d likely find specific patterns that occur more frequently in their DNA than in the DNA of people from other populations. We would then define those sequences of DNA as characteristic of people with ancestry from the Korean Peninsula. So, if you have patterns in your DNA that look very similar to those found in our Korean Peninsula reference population, we would conclude that you likely have ancestors from that region. That doesn’t mean with 100 percent certainty that you have ancestors from Korea—they could have been living in the nearby region of Manchuria in Northeastern China. If this were the case, the DNA they passed along to you would likely show similar genetic patterns to people living on the nearby Korean Peninsula.

When you have your DNA sequenced for an ancestry analysis, it means we’re looking in your DNA for specific patterns that we know to be associated with specific populations (we know this thanks to research on reference populations). However, some patterns in your DNA may not perfectly match any reference population. In this case, part of your ancestry would be reported as the population that most closely matches with yours, even if it’s not a perfect match.

A good analogy for this is the struggle that comes with naming a new color. Suppose for a moment that you’ve discovered a new color, one that you’ve never seen before. How would you go about describing it to other people? Do you say it’s a shade of green, an unequal mixture of crimson and turquoise, or mauve layered atop an earthy brown? Likely none of these descriptors could accurately portray the color’s unique look, but you have to make due with what you have. Similarly, when we trace your ancestry, we have to match the patterns in your DNA to the reference populations we have. It likely won’t describe your ancestry perfectly, but it’s the closest we can get with the reference populations we currently have.

 

Ancestry results calculated by different tests may be slightly different

These two related factors can partially explain why you may get ancestry results that are different from what you’d expect. Likewise, ancestry results calculated by different tests may be slightly different because they may be using different reference populations.

For Ancestry Basics by Helix, we’re able to explain your ancestry with some combination of 26 different regions. You may learn that your ancestors lived in (or near to) Gujarat, India, where Mahatma Gandhi once lived. It’s possible that the bulk of your ancestry can be traced to the Senegambian region—encompassing Senegal, The Gambia, Guinea-Bissau, and parts of surrounding countries—which was once part of the Mali empire. Your results may not be perfect, no ancestry test can always be, but we hope that learning about your ancestry can help as a first step towards learning about the many other cultures and peoples that exist on our diverse planet.

What do your ancestry results mean?What do your ancestry results mean?

When we say that you have a “percentage” of ancestry from a given region, what we’re saying is that you likely have an ancestor in common with modern people living in that region. For example, if our test determines that you are 83% Persian, both you and some modern people living in Persia are likely descended from a common ancestral population. In some cases, this shared ancestry is from very far back in time—your last common ancestor with these populations could have lived tens of thousands of years ago!

 

It’s entirely possible that, thousands of years ago, these two populations were one

While our ancestry test was carefully designed, no ancestry test can be perfect. For example, it’s likely that people who share ancestry with modern day Sindhi peoples in India will receive ancestry results indicating that they have Punjabi and Pathan (Pashtun) ancestry. It’s entirely possible that, thousands of years ago, these two populations were one. However, it’s also possible that a person with Sindhi ancestry would be incidentally grouped with Punjabi and Pathan (Pashtun) because the reference populations we used don’t enable us to differentiate these two populations. The same holds true for many other ancestral backgrounds. In the future, we hope to find greater resolution in these areas.

It’s also important to note that these results are not reporting on your culture. Culture is a complicated concept that joins people through shared experiences, beliefs, locations, and many other aspects of life. Your genetic ancestry, as calculated in these tests, is very different. These tests tell you where your ancestors may have lived, but they cannot tell you what their humor was like or whether they were a dog or cat person.


It took great minds to figure out why the sky is blue. Likewise, after decades of research, we’re beginning to understand our own history. Ancestry Basics is just the beginning: it’s the prism that will help you illuminate the colorful history in your past. But there’s always more shades and nuances to be discovered, and we’ll keep developing tools to help you see more.

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