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Identical Twins Get Different DNA Test Results: What’s Going On?

How is it possible that identical twins have different DNA results? It is not only surprising but shocking as well. If you do not live close to identical twins, it can be challenging to identify them. It follows that identical twins have the same DNA.

The best way to learn about your DNA is to get a test.

Nowadays, DNA tests for families are increasingly popular, and different companies offer services. These companies rely on large databases consisting of reference samples from previous customers upon which you can find matches.

A DNA test could help you to find family links, trace your ancestry as well as find biological parents if you were adopted. For instance, Ancestry DNA is useful for finding family matches, and it offers other genealogical tools such as family trees.

However, in a somewhat surprising incident, identical twins got different results from their DNA tests. If the tests were 100 percent accurate, then the chances of identical results were also high.

 DNA Test Results For The Identical Twins

Charlsie and Carly Agro are identical twin sisters, and they bought different home DNA kits from the following five brands:

  1. 23andMe
  2. AncestryDNA
  3. MyHeritage
  4. FamilyTreeDNA
  5. Living DNA

After collecting the samples of their DNA, they sent them back for testing to the companies.

Since the Agro sisters are identical twins, one would also expect identical results from their DNA tests. That is not the case, and the results have some differences. According to the Living DNA results, Charlsie has DNA from Scotland while Carly’s ancestry is from England.

Charlie is 10 percent more “broadly European” than Charlsie according to 23andMe.  On the other hand, MyHeritage gives the twins Balkan heritage 60.6 % & 60.7% while FamilyTreeDNA gives them Middle East DNA of 13 to 14 percent. AncestryDNA gives them Eastern European culture of 38 to 39 percent. There is no reason why results from these five DNA tests could not be identical if they were 100 percent accurate.

According to Mark Gerstein, the twins should get identical results since they would be sending the same raw data to the company. Gerstein is a Yale University, a computational biologist whose research proved that raw data for the twins was statistically identical. In case of the differences in results from the five tests, there are different explanations.


Reasons For Differences In DNA

The first reason for the different results of these twins is probably because the DNA databases consist of different reference panels. These are DNA samples of previous clients, and they vary from company to company. The reference panels also vary across borders, and each company has a different reference panel.

As a result, these companies are likely to provide different ancestry results to the same customer. It follows that the accuracy of DNA results mainly depends on a large number of people getting DNA tests from the same company. The other issue is that your results can also be accurate if other members from your lineage exist in the reference panel.

Therefore, the results for people who come from underrepresented groups in the reference panels are likely to be less accurate. Before getting a DNA test, you must make an effort to investigate the size of its database. The larger it is, the higher the chances of getting accurate results.

Different Algorithms

The other issue is that a DNA sample consists of about three billion parts. However, ancestry DNA companies tend to focus on about 700, 000 parts only to identify the genetic differences. The companies then use an algorithm which compares these 700,000 DNA parts to those found in the reference panel.

The algorithm used tries to find the closest match from the reference panel. It means that the level of accuracy depends on the size and quality of data in the company’s reference panel.  The aspect of the diversity of data in the reference panel can also affect the accuracy of your results.

If the reference panel consists of a sample of people who are predominantly from a particular region, accuracy might not be reliable. The reference panel should include data from diverse areas to increase the chances of the accuracy of the results. The sample should be more representative and cover large areas.

You should also not quickly take DNA results as conclusive since many companies assign different definitions for the region. Many DNA companies mainly cover areas consisting of European ancestry as reference panels. Some companies are however making inroads into other areas.

You need to be careful about companies that claim to trace your ancestry to your home town or country. It may not be surprising that such company’s reference panel does not have many samples from your country. However, they will still try to find some close matches even though they are not accurate.

The other factor is that no governing board ensures the validity of DNA results. All companies keep the data to ensure privacy, so it is difficult to test its efficacy by independent players. The tests are not subject to a standard for testing or diagnosis like in the medical field.

identical twins DNA testing

Limitations of Ancestry DNA Tests

The ancestry DNA results are subject to change if there are additional reference samples. In most cases, these results are based on estimates; hence you should treat them with caution.  The other thing that you should note is that many reference groups consist of people self-reporting their ancestry. If you are a minority and do not belong to their group, you may get inaccurate results.

You also need to remember that Ancestry DNA tests do not tell you the location where your family members lived. The experiments show you about the DNA you inherited from other members within your family lineage.

However, Ancestry DNA companies tend to track down DNA to European countries. The challenge is that other regions with diverse ethnicities like Africa are underrepresented in their reference groups. That is the reason why siblings at times can get different results from these Ancestry companies.

About the Author Charles McKnight

I'm just another amateur genealogist investigating my American-Scots-Irish lineage. I built MyFamilyDNATest.com after buying all of the leading DNA tests to discover everything I could about my family history. Hopefully, this site will save you time and demystify the emerging science of DNA-based genealogy, for your family project.

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