Can a DNA test be wrong? Usually not, and very rarely yes. Here we bust 3 common misconceptions about why you may think your Ancestry, 23andMe, MyHeritage, Family Tree DNA or Living DNA results are wrong. And, if you have unexpected results, what to do next.

Faced with unexpected results, DNA testing clients sometimes wonder, “Can my DNA test be wrong?” Here’s the answer: usually not, and very rarely yes. Most of the time, when you see a result you do not expect, it is your DNA trying to tell you something about your family relationships. More on that later. First, let’s bust the three most common myths about why your DNA test results might be wrong.

Myth #1: The lab mixed-up my sample.

Myth #2 I have a biological condition that limits DNA test accuracy.

Myth #3 The company interpreted my DNA incorrectly.

Understanding the implications of these three scenarios can help you have greater confidence in your ability to personally interpret your DNA test results. And greater confidence will be your greatest tool of all if you suddenly find yourself looking at a DNA report that DOES shake up your world a bit.

#1: Lab mix-up

SNRE Lab: Molecular DNA by University of Michigan School for Environment and Sustainability. Flickr image, CC BY 2.0, no changes made.

While there are some hearsay instances of lab error producing unexpected results for clients, it has been over a decade since there was a documented mishandling of samples by a commercial genetic-genealogy lab.

A tightly controlled “chain of custody” follows your DNA sample from the moment you spit or swab to when you first login to receive your results. In the course of your DNA being analyzed in the lab, your sample will change tubes several times, and go through several phases of electronic analysis.

State-of-the-art labs handle each of these steps through automation, so there is little room for human misstep. Robots are pretty good at what they do, so it’s usually humans that introduce workflow error.

One of the key events that may set up a DNA mix-up is if you are doing your own DNA test with another person at the same time, and the two of you inadvertently switch vials between registering your kit and collecting your sample. Either only open one kit at a time, or go to different areas of the room with your own sample kit materials so this doesn’t happen!

Here’s a bit of insight into how the process works (in case you are interested): DNA samples are processed in 8×12 trays that hold 96 samples at a time. In 2010, 23andMe traced mis-reported results for an entire tray that was placed on a machine by a technician 180-degrees off from its correct orientation.

Industry-standard safeguards that have now long been in place include quality-control checks at each stage of analysis that flag for contamination or mix-ups like this, including checking that gender data matches the reported biological gender for all DNA samples. None of the genetic genealogy companies report a sample mix-up rate, but this should be considered to be an exceedingly rare occurrence that would be caught by laboratory safeguards and handled internally by re-running samples before the results would ever be reported to clients.

Can a laboratory mix-up be the cause of your “wrong” DNA test results? It is highly unlikely

#2 Biological conditions

For most people, your unique DNA fingerprint is found in only one person–YOU! However, there are cases where your DNA is not uniquely your own (like when you have a twin), or you have two different copies of DNA roaming around in your body (the very, very rare chimera). But this doesn’t make your DNA test “wrong,” as it is just a reflection of your biology. These rare cases will only effect you if you have had a stem cell or bone marrow transplant, are a twin, or a chimera. Read on for more information about each of these situations. Or, if none of those cases apply to you, skip to #3.

DNA testing and transplant recipients

For certain cancers and other illnesses, patients undergo an extended procedure that wipes out their own immune cells which are then replaced by cells from a donor. The white blood cells in a patient that receives a successful bone marrow or stem cell transplant will be entirely replaced by donor white blood cells containing the donor’s DNA. Saliva can contain white blood cells, so a person that has received this type of transplant may show two different genomes in their DNA test: their own original genome, and also the genome of their donor. At times, individuals can respond to bone marrow and stem cell transplants in different ways and other body systems may take up the donor genome as well. Companies recommend that bone marrow and stem cell recipients do not take their current DNA tests, and some even offer refunds for those that previously did. (Read more from 23andMe and Ancestry.)

People who have received standard blood transfusions in the past would not be expected to carry their donor’s genome for long, and are still candidates for genetic genealogy tests. Also, it is not expected that patients who receive solid organ transplants would have saliva containing the donor genome.

DNA tests, identical twins and chimerism

A well-known scenario where different people carry the same DNA is in the case of identical twins. When descendants of identical twins take genetic tests, they may find relationships that are different than they expected: the children of identical twins would be reported as half-siblings rather than first cousins. This kind of relationship misattribution could continue into subsequent generations as well.

Other, more rare conditions could cause an individual to carry more than one kind of DNA. Chimerism is usually initiated in a person when cells and genetic material from an unborn fraternal twin is taken up by the healthy fetus and incorporated into their body. This can result in a healthy person who is unaware of their unborn fraternal twin, and who harbors two distinct genomes: their own and their sibling’s.

A well-documented case of this involved a man whose 23andMe test showed that he shared only 25% of his DNA with his son, suggesting an uncle-nephew relationship rather than the full 50% that would be typical of a father and son. Genetic consultants working on this case confirmed that the client was in fact the father of his son, but he had a mixed population of sperm cells with some carrying his genome and some with the genome of his unborn fraternal-twin brother. Incidents of chimerism like this are exceedingly rare, and are not readily investigated with standard at-home DNA tests.

Can your own biology cause your DNA test results to be “wrong?” The results aren’t “wrong” in that they will accurately reflect your biology. So if you find yourself in one of these rare situations, it is best to investigate carefully.

#3 DNA test interpretation

You get two kinds of results from your DNA test: Ethnicity percentages and DNA matches. Both can yield unexpected results that you may be temped to say are “wrong” based on what you know about your own family history.

Unexpected ethnicity results

Unexpected ethnicity results can be due to several factors.

Unexpected ethnicity results can occur when your population of origin is not well-represented in the testing company’s database. Twenty years ago, in the very early days of ethnicity prediction, I worked with the sample of a co-worker who was a multi-generational Italian, but who was consistently grouped with samples in our database from Pacific Islanders, which was bafflingly unexpected to us. Since then, it has been determined that he carries a Y-chromosome haplotype that is typical of East Asia (which is very atypical for an Italian, but it is what it is). He clustered with Pacific Islanders in our preliminary tests because at that time, East Asia was severely under-represented in our database. So the algorithm clustered him with the next closest group it recognized.

Will DNA ethnicity estimates ever get any better?

Unexpected ethnicity predictions may also occur when genetic tests are not yet sensitive enough to distinguish between the mixed origins of some cosmopolitan populations. (They still do better recognizing historical groups that were more genetically isolated or distinct.) That said, increasingly specific and accurate Genetic Groups (at MyHeritage) and Genetic Communities (at Ancestry) are now able to identify genetic associations with many little historical communities or migratory groups.

What about small ethnicity percentages for populations that are surprising to you? Just take those with a grain of salt if they look unfamiliar. These are estimates, not irrefutable truths.

Which is the best DNA test to identify your origins? Most DNA testing companies identify many western European origins fairly well. Here are some other populations we’ve written about:

Best DNA test for Asian Origins
Best DNA test for African Origins
Best DNA test for Jewish Origins

Unexpected relationships

In your DNA match list the testing company is taking a genetic relationship (measured in those cMs you see listed) and trying to assign a genealogical relationship. This is tricky because family members share a predictable amount of DNA depending on the degree of their relationship. However, it is not an exact amount, but rather a range of shared DNA that is expected due to the random nature of inheritance. To further complicate matters, there are many different types of relationships where the expected range of shared DNA is the same. The table below shows relationship groupings based on the degree of relatedness:

As shown, half-siblings have the same range of shared DNA as a grandparent and grandchild (group 3 in the table). Second cousins expect the same range of shared DNA as a pair of first cousins twice removed (group 5). Notice as well that the ranges of shared DNA between adjacent groups abut one another. These ranges are just estimates, and in real life there are certainly cases that cause these ranges to overlap.

A well-known case involves a woman and her 1st cousin who, to their surprise, received 23andMe reports that predicted them as half siblings. This implication rattled them as it suggested that for one of them the person they had always known as an uncle was actually their father. Naturally they asked, “Can this be wrong?”

For these 1st cousins purported as half siblings, they learned that they shared upwards of 1300 cM of DNA. Looking in Table 1, this is at the very high end of the range for first cousins to share and at the very low end of the range for half siblings. The 23andMe algorithm that predicts relationships states it in a way that makes it seem that a specific relationship is authoritatively confirmed by science, rather than bringing to light the uncertainty that is inherent in all of these predictions.

In reality, this is a borderline case where the amount of shared DNA doesn’t make a clear statement on whether these two people are half siblings or first cousins. That was great news for these cousins to hear! They probably didn’t need to reconsider their family tree because they are probably just 1st cousins who share an atypically high amount of DNA for their degree of relationship.

Other scenarios that can confound relationship prediction algorithms occur when relatives share inflated levels of DNA. This can happen when they descend from endogamous populations (families who intermarried a lot over many generations) or from family trees that experience pedigree collapse in recent generations.

Your DNA test isn’t wrong

There are certainly instances where a DNA test will bring to light actual elements of your family history that were previously unknown. Meaning, surprise birth relatives or a lack of genetic connection to one of your relatives. Barring the very rare events discussed above (twins, transplants, and chimerism), this will often indicate a family history curveball.

If you find yourself wondering if you’ve been thrown a family history curveball and thinking, “Can this test be wrong?” your first job is to breathe. Next, your job is to define your suspicion specifically:

• Are you wondering if there has been a lab mix-up and these aren’t your DNA results at all? (This is extremely rare)

• Do you suspect a biological origin for the reason you’re not seeing the results you expect? (Also extremely rare)

• Do you see ethnicities that don’t make any sense to you? (Read more about the limits of ethnicity prediction, and see if it makes sense in your case to take the results with a grain of salt)

• Are there people in your match list who have different predicted relationships than how you’ve previously understood? (Consider the inherent uncertainty in DNA-based relationship predictions, and perhaps schedule a confidential Mentoring session with us if you’d like some help to sort out the specifics of your results)

• Do you know your extended family well, but have people in your match list shown as close relatives who are completely unfamiliar to you? Or are there people who you expect to be there who aren’t? Is your ethnicity report wildly different than you had previously understood your heritage? This DNA test may have revealed a previously unknown element of your family history. Again, you may want to schedule a Mentoring session or at least learn more about working with your DNA matches on your own.

Wondering what to do next?

Here at Your DNA Guide, we help you answer your DNA testing questions. So, what would you like to learn next?

How I’m related to my matches
How to Explore Birth Roots