Understanding DNA Ancestry Discrepancies

Have you ever stumbled upon an unexpected DNA ancestry result that doesn’t align with your family heritage? This was the case for someone who has a Japanese grandmother but received a DNA test result indicating only 18% Japanese ancestry, not the expected 25%. Let's break down the reasons behind such discrepancies and explore the possible answers.

The Science Behind DNA Ancestry

Your DNA is a complex mixture of genetic material from both parents, inherited randomly. Here are a few factors that can lead to discrepancies between expected and actual DNA ancestry:

Genetic Inheritance

Each parent contributes roughly half of your genetic material, but not necessarily in equal proportions. The specific segments of DNA you inherit from your grandmother and other ancestors can vary widely. If your grandmother is Japanese and her child (your parent) inherited less than 50% of her Japanese DNA, you would inherit even less. This variability in genetic inheritance can explain the 18% Japanese ancestry reported by the DNA test.

Ancestry Composition

Modern DNA tests estimate ancestry based on reference populations. The accuracy of these estimates can be affected if the reference population for Japanese ancestry does not perfectly represent your grandmother's specific region or genetic background. This can lead to discrepancies in the percentage reported. For example, if your grandmother comes from a less represented region in the reference database, the estimate might be lower than expected.

Testing Variation

Each DNA testing company uses different algorithms and reference panels, which can result in variations in ancestry estimates. Even with the same individual, different tests might provide slightly different ancestry percentages. This variability is inherent in DNA testing and contributes to the discrepancy you observed.

Mixed Ancestry

Your grandmother's own ancestry may not be purely Japanese. If she has mixed ancestry, such as from different ethnic backgrounds, there might be less direct Japanese DNA passed down to you. This can explain the lower percentage of Japanese ancestry reported.

Statistical Variation

DNA testing involves some level of statistical estimation and uncertainty. The percentages provided are estimates and can vary slightly due to the statistical margin of error. This is another reason why the DNA test might report a lower percentage of Japanese ancestry than you expected.

In summary, the 18% reported Japanese ancestry does not negate your grandmother’s heritage; it reflects the complex nature of genetic inheritance and the methodologies used in DNA analysis.

Possible Answers to the Ancestry Discrepancy

There are two likely explanations for why your DNA test reported 18% Japanese ancestry:

1. An Unknown Japanese Ancestor

The most likely but also the most unfortunate answer is the presence of an unknown Japanese ancestor. This could be due to your female relatives being in the Far East during World War II, where anything that happened might have been covered up. RESEARCH LINES INTO YOUR PARENTS' AND GRANDPARENTS' LIVES, LOOKING FOR ANY CONNECTIONS TO THE FAR EAST OR JOBS THAT MIGHT HAVE BRING THEM INTO CONTACT WITH JAPANESE PEOPLE.

2. An Error in Test Reporting

Another possible answer is a glitch in the test reporting. DNA companies base ancestry estimates on similarity matches within their own databases, which are often incomplete. In such cases, it's not uncommon to receive a report that doesn’t match your known family tree. For example, my own DNA test reported 8% Swedish ancestry, which is highly unlikely given my full family tree dating back four generations. If someone had any Swedish ancestors further back, the chances of them contributing that much DNA are extremely small.

Checking for 'Distant Cousins'

While 18% is higher than a typical glitch, it still might be worth checking. Look for any Japanese matches in the 'distant cousin' section of the DNA site. If there are matches, it’s likely that your Japanese DNA is genuine. Conversely, if there are no Japanese matches, a glitch might be more likely.

Performing a thorough investigation into your family history and checking your DNA results for any distant cousins can help you determine whether your Japanese ancestry is real or the result of a testing error.