Genetic Ancestry: What tests can really tell us

With direct-to-consumer genetic testing growing rapidly in popularity and genetic testing for ancestry frequently popping up in news headlines, an understanding of what these tests can actually tell us matters now more than ever.

Our genetic history is an ocean of immense depth and complication. For example, going back about 200 years in your family tree identifies 128 great-great-great-great-great grandparents who contributed to your genetic code. But humans are a map-making species — a race of cartographers. Naturally, we want to chart out even the most minute details of our being. Genetic sequencing gives us an additional tool to fill in some of the landmarks and boundary lines of our personal history.

So here’s how we draw the maps, what they mean and why they matter.

How they do it

It’s important to note that companies performing basic tests for ancestry aren’t offering whole genome sequencing (WGS), where they examine your entire genetic code. Instead, they’re essentially doing “spot checks” of certain parts of your DNA that often vary across ancestral groups. Rather than sorting through billions of letters of code, they’re honing in on a few million. That still sounds like a lot, and it’s far from negligible — but it’s by no means a picture of your whole genome.

Once companies have isolated the parts of your sequence they want to look at, they’ll compare it to a reference library of other people. Here’s where some of the trickiness comes into play. The reference libraries that many companies use are proprietary, meaning we don’t know exactly what’s in them, and they can differ from company to company. So for example, when they say they found Scandinavian DNA in your sequence, what are they using as the baseline Scandinavian DNA? Beyond that, the ancestry given can only be delivered with current geopolitical lines. If a company says you have Italian sequences in your DNA, does that mean modern-day Italy? Or does that extend back to the Roman empire?

In all likelihood, the comparisons being made are between your DNA markers and markers of other current residents of these places, with the exception of tests that show what proportion of your genome is shared with Neanderthal . . . but that’s a blog post for another day.  

Again, given the proprietary nature of these tests, it’s impossible to ever have a complete understanding of what they show. Besides, there are natural limits to what we can learn about ancestry from DNA anyway.

What it means

Once we have genetic results on ancestry in hand, it’s important to know what we can be sure of and what we can’t. For starters, your genome, while enormously large, isn’t large enough to store your family’s whole genetic history. If your family has long talked of an ancestor in Germany, don’t write them off just because a DNA test finds no “German DNA.”

Because of the nuance of the conversation, I brought in one of our genetic counselors at HudsonAlpha, Whitley Kelley, to discuss a little bit about how medical professionals view and talk about ancestral data in DNA. She noted, “You can have distant genealogical relatives with no genetic relationship to you.”

It’s a great point, if a little puzzling at first glance. Let’s unpack that further.

You receive half of your genome from each of your parents. And they received their genetic code from your grandparents  who in turn received it from your great-grandparents (and so on). Without going into too much detail, there’s a mixing process called meiotic recombination that takes place during the formation of eggs and sperm. The DNA blending of recombination can minimize the genetic contribution of more distant relatives, making it harder to identify all our ancestral information.  

Kelley summarized it like this, “As far as direct-to-consumer DNA tests are concerned, you can’t rely on them as the final authority when it comes to your genealogical heritage. You might have whole branches of your distant family tree that just don’t significantly appear in your genome any longer. It doesn’t mean these individuals are not your relatives.”

On the flip side, you also need to know that ancestry tests can reveal unexpected findings —  genetic contributions that might not be part of our pre-existing narratives. That can include evidence of nonpaternity or relatives from unexpected corners of the world. This can upend family relationships and cause significant emotional stress, especially if you’re not prepared for the possibility. Too often we don’t stop to consider the full implications before sending off our DNA samples. That can also apply to what we don’t find in our DNA.

You also need to know that there are some types of heritage that are just less likely to show up or be identified. The DNA of some ancestral populations, such as Native American or African, can be harder to recognize when trying to identify particular segments of these populations, since these groups have historically been sequenced less frequently and thoroughly. Also, people are often looking for specific information — like a specific Native American tribe — for which no reference DNA even exists. For that reason, while it may be possible to affirm Native American or African heritage, the ability to be specific about tribes or regions or origins may still be extremely limited.

Why it matters

None of this is to say that knowing and understanding what you can about your genealogical ancestry isn’t interesting or even important. You just need to know the context for the information you receive.

Kelley noted in our conversation that in the clinical setting they often ask about genetic heritage. She noted, “There are some groups that have been well studied, like those of Ashkenazi Jewish ancestry. If you have Ashkenazi heritage, we might consider some genetic disorders as possibilities that we otherwise wouldn’t look to first.”

“It doesn’t mean that people of Ashkenazi heritage have more genetic disorders than other groups. It just means we happen to know more about some of the links between genetic variation and health problems in that population than we might for a person of another heritage.”

All that said, Kelley emphasized that they’re usually only looking back so far for clinical purposes. She said, “A standard pedigree is three generations — we’re looking back to grandparents. Once you go further back than that, the combination of those relatives being more distant and a frequent lack of accurate records available for them means the information isn’t often relevant in the same way.”

It’s also important to acknowledge how much of heritage and culture are handed down through tradition, not genetic code. In the clinical setting, geneticists and genetic counselors often ask patients their ethnicity, because their self-reporting gives more than just the genetic context, it gives perspective on the patient’s life.

Genetic tests for ancestry can be an informative and interesting window into our past. They allow us to paint a watercolor of sorts, showing the global roots of our genetic makeup. However, these tests have real and meaningful limits that need to be understood when we talk about their results. The diversity of human life is written into our genetic code, but it should stand alongside — rather than replace — other ways of expressing our heritage, culture and personal experience.

To schedule a media interview with Dr. Neil Lamb or to invite him to speak at an event or conference, please contact Margetta Thomas by email at mthomas@hudsonalpha.org or by phone: Office (256) 327-0425 | Cell (256) 937-8210