Exactly how accurate are DNA tests for ancestry?

Written by Laura Heffernan
January 31, 2019

Gone are the days where everything you knew about DNA testing came from sci-fi films. Now everybody wants a closer look into their genetic makeup. Leading DNA testing kit provider AncestryDNA has a whopping 10 million people in its database, from users far and wide providing samples in order to find out about their ancestry, and maybe even locating a surprise family member or two.

Learning more about who we are and where we came from clearly appeals to the masses, but exactly how accurate are these tests? Here, we’ll take a look at some of the different purposes of DNA ancestry testing, and whether it’s worth ordering a home testing kit.

How do DNA tests for ancestry work?

Typically, companies will use a technique called microarray-based autosomal DNA testing to analyze your genes. Autosomal DNA—the 22 out of the 23 chromosomes that do not determine your biological sex—contains variants in your genetic code called single-nucleotide polymorphisms (SNPs). These determine traits like skin color, eye color, and height, and as certain variants tend to be most common in particular global populations, they make useful ancestry markers.

Some services conduct haplogroup testing — DNA tests you take depending on whether you’re male or female. Maternal haplogroup testing examines the mitochondrial DNA (mtDNA) inherited from your mother and can be taken by people of both biological sex. However, paternal haplogroup testing, which instead looks at the DNA inherited from your father (Y-DNA), can only be conducted on males. This is because the method focuses on the Y-chromosome passed down from father to son.

Short Tandem Repeats (STR) testing may also be used to establish familial relationships between people. This involves looking at the DNA sections that repeat themselves in a unique and identifiable pattern. Though these markers can identify individuals and their relationships with others, STR testing doesn’t analyze the genetic information itself, so can’t match DNA to a particular country or region.

How accurate are ancestry DNA tests in finding family?

DNA testing is very good at identifying close relations like parents, grandparents, and siblings. For example, a DNA paternity test could be carried out to discover whether a man is the biological father of a child. Comparing the DNA sequences allows scientists to determine whether a child has inherited similar patterns to their supposed parent.. Once the test is complete, the probability of parentage is 0% if the pair are not related, and 99.99% if they are.

While ancestry DNA tests cannot be used as legal proof of paternity, maternity, or any other familial relationship, online family finder tools like AncestryDNA’s “DNA Matches” or 23andMe’s “DNA Relatives” feature can be used to locate biological family members in their databases.

Unlike traditional DNA relationship tests, which look at Short Tandem Repeats, online family finder services look at the number of DNA segments shared between people in their database. The number of DNA segments shared between people can be used to estimate a familial relationship, which could be anywhere between sixth cousins and full siblings.

DNA ancestry testing is also useful in helping you discover long lost or distant family members. Platforms like AncestryDNA offers everyone the option to see their DNA matches after they have taken a test. The higher the person is on your list, the more closely you are related.

How accurate are DNA tests in finding ancestry from many years ago?

The accuracy of a DNA test entirely depends on the type of test you are taking. For example, maternal and paternal haplogroup testing can trace lineage further back (20-100 generations) than microarray-based autosomal DNA testing (around 10 generations). However, as maternal and paternal haplogroup testing is based on your parents’ genetics, all other ancestry outside those two direct lines of your family tree will be excluded. Although this can provide a link to more remote ancestors, the representation of such a small fraction of your ancestry means results would be less accurate if you were to rely on this method alone.

Another issue is the fact that the genetic information contained in a DNA testing reference database is based on living populations as opposed to those from thousands of years ago. This means that DNA tests generally assume that certain SNPs have remained the same, but this cannot be confirmed. Furthermore, SNP patterns may have changed geographically over time. Deborah Bolnick, an anthropological geneticist at the University of Texas at Austin, explained to ScienceNews that “As people move and the genes that they have move with them, it’s going to change what those geographic ancestries look like.” Therefore, if you learn that a stretch of DNA has come from Italy, it could trace back to Romania, Mongolia, or even Siberia if you go further back in time.

The best way to maximize the accuracy of a DNA test is to set realistic expectations. As Mark Thomas, professor of evolutionary genetics at University College London, noted in an interview: “By the time you go 10 generations back, there are ancestors from whom you inherit no DNA.” Tracing ancestry beyond 300 years is futile as you only directly inherit DNA from a small fraction of your ancestors – the others are called ‘pedigree ancestors.’ So, when taking a DNA test, focus on the information regarding more recent generations of ancestors and take anything beyond that with a pinch of salt.

How reliable is ancestry DNA testing for establishing ethnicity?

This is difficult to know for sure. According to Sheldon Krimsky, author of Genetic Justice and board chair of the Council for Responsible Genetics, companies that provide these services don’t share their data. Their methods have also not been validated by an independent group of scientists, which means there are no agreed-upon standards of accuracy.

Ancestry DNA tests determine ethnicity by using databases containing genetic analysis of these companies’ previous clients, where data is used as a basis of comparison for new clients. Ethnicities are determined by looking at specific SNPs in the client’s DNA against those most commonly associated with certain global populations in the reference database. However, this means that each company will be working with its own specific system, making it possible to receive different results from different services.

The degree of accuracy also depends on the ethnicity which is being tested. For example, the most accurate results are usually for Anglo-Saxon and European lineages as there is more data from people of these ethnicities in the respective databases to compare to. Comparatively, there is a much smaller genealogical pool available for people of Hispanic, East Asian, and South Asian descent.

There are also further limitations if you’re hoping to learn whether you have ancestry from specific countries. DNA test results will often claim that genes derive from a large region (e.g. Western Europe or Southeast Asia) rather than one country, as many databases do not contain enough information to distinguish SNPs according to a precise location.

DNA testing companies like 23andMe are working to improve their ability to provide detailed results for customers with non-European ancestry thanks to their Global Genetics Project initiative. By providing testing kits to people with four grandparents from one of over 50 underrepresented countries, this will help pave the way for more accurate findings.

In fact, encouraging more people around the world to undertake ancestry DNA tests can give the process a huge boost when it comes to accuracy. The more extensive the databases become, the more reliable test results will be in the future.