5 Mistakes to Avoid in DNA Testing

Taking a commercial DNA test is one of the most exciting milestones in modern family history research. With a simple cheek swab or saliva sample, you can unlock centuries of hidden ancestral lines, connect with genetic cousins worldwide, and discover the geographic origins of your deep lineage. However, because genetic genealogy is a complex science that blends molecular biology, history, and statistical probability, it is incredibly easy for beginners to make critical errors. Without a clear understanding of how DNA is inherited, analyzed, and matched, researchers often make costly missteps that lead to false conclusions, unverified lineages, and immense frustration.

To help you navigate this complex field with confidence and scientific rigor, we have compiled a practical guide to the most common pitfalls in genetic genealogy. Below, we break down the 5 mistakes to avoid in DNA testing, explain why these errors occur, and provide actionable strategies to help ensure your family history research remains accurate, verifiable, and secure.

Mistake 1: Choosing the Wrong DNA Test Type (Autosomal vs. Y-DNA vs. mtDNA)

One of the first and most frequent mistakes to avoid in DNA testing is purchasing the wrong type of test for your specific research question. Commercial DNA companies offer three primary classes of genetic tests, each with a different inheritance pattern and genealogical purpose.

• Autosomal DNA Tests: These tests analyze your 22 pairs of autosomes. Because DNA is reshuffled through genetic recombination each generation, autosomal DNA is an important tool for finding cousin matches across multiple ancestral lines. Its usefulness for identifying specific relationships generally decreases with increasing genealogical distance. Learn more in our guide to understanding autosomal DNA testing.
• Y-DNA Testing: Traces the direct paternal line through the Y chromosome, which is generally passed from father to son. It is useful for surname projects and investigating direct paternal ancestry. A woman can arrange testing for an appropriate male relative. Explore our Y-DNA testing guide.
• mtDNA Testing: Traces the direct maternal line through mitochondrial DNA, inherited from mothers by children. It is particularly useful for deep maternal ancestry and haplogroup research, although a close mtDNA match alone may not identify a recent common ancestor. Read our mtDNA testing guide.

The Fix: Define your research goal before purchasing a kit. If you want to identify cousins, investigate an adoption, or verify a family tree, begin with autosomal DNA. If your question concerns a direct paternal line, consider Y-DNA. For direct maternal ancestry, consider mtDNA. For complex cases, explore combining multiple DNA tests.

Mistake 2: Treating Ethnicity Estimates as Absolute Historical Fact

It is common for beginners to open their testing dashboard, see an ethnicity estimate, and treat it as a precise, unalterable record of their ancestry. If a test reports “12% Scottish,” for example, a researcher might immediately start searching for a Scottish great-grandparent. This is a misunderstanding of how admixture models work.

Ethnicity estimates are statistical approximations based on comparisons with reference panels representing present-day populations. Because populations have migrated and mixed over thousands of years, neighboring regions can share substantial genetic similarities. Estimates may also change when companies update their algorithms, reference panels, and databases. Your DNA does not change, but the interpretation of it can.

The Fix: Treat ethnicity estimates as broad historical clues rather than proof of a particular ancestor. Investigate your DNA cousin matches, shared matches, documented family trees, and historical records. For more guidance, read how to interpret DNA test results.

Mistake 3: Relying Blindly on Predicted Relationships

Commercial DNA databases often display a predicted relationship, such as “3rd Cousin” or “2nd–3rd Cousin,” beside each match. A common mistake is to treat this prediction as certain. Because genetic recombination is random, the amount of shared DNA, measured in centimorgans (cM), can overlap across multiple relationship categories.

For example, a match sharing 120 cM may fit several possible relationships, depending on the family structure and generation differences. Consult the Shared cM Project to explore the statistical ranges. Assuming that every match is a standard third cousin can send your research toward the wrong generation and waste valuable time.

The Fix: Never rely solely on a database’s default prediction. Enter the shared cM value into our DNA Relationship Probability Calculator to explore possible relationships. Then compare those possibilities with documented ages, known family connections, and shared matches. Read more about how DNA matching works.

Mistake 4: Fishing in Only One Database (The Silo Pitfall)

When you take a DNA test, your results are generally matched against people in that service’s database. If you test with AncestryDNA and a biological cousin tests exclusively with MyHeritage, you may not see that cousin in your original match list.

Relying on a single database can limit your research, particularly if you are investigating an adoption, searching for biological relatives, or exploring international ancestry. Different services attract different users and geographic communities, so additional databases may reveal useful matches that you would otherwise miss.

The Fix: Consider a multi-database strategy. Research the current upload and download options offered by AncestryDNA, MyHeritage, FamilyTreeDNA, and GEDmatch. Where raw-data uploads are supported, follow the service’s instructions and privacy requirements. If a platform requires a separate test, weigh its database coverage against the cost. For adoption research, read our guide to DNA testing for adoptees.

Mistake 5: Copying Matches’ Public Trees Without Verification

When you find a DNA match with a familiar surname, it can be tempting to copy branches from their public family tree directly into your own. Automated features such as Ancestry’s ThruLines and MyHeritage’s Theory of Family Relativity can help generate research hypotheses, but their suggestions depend in part on the accuracy and completeness of available family trees and records.

If you copy an unverified line, you risk attaching an incorrect pedigree to your own research. Such a mistaken lineage can be difficult to detect later and may cause you to misinterpret future DNA matches.

The Fix: Treat matches’ trees as hypotheses, not proof. Verify parent–child relationships with primary or authoritative historical records, such as birth, marriage, and death registers, census records, parish books, and wills. Cross-check dates, places, and identities before adding a connection to your own tree. Read our guide to traditional genealogy research methods. You can also explore downloaded GEDCOM files with our GEDCOM Tree Viewer.

Frequently Asked Questions About DNA Mistakes

Can my DNA test results be wrong?

Commercial genotyping platforms are designed to measure selected genetic markers with high technical accuracy, but performance depends on the assay, sample quality, and analytical process. Ethnicity estimates and predicted relationships are interpretations rather than direct readings of a complete family tree. DNA matches provide evidence of shared genetic material, but they do not automatically establish a precise genealogical relationship.

What is the minimum segment size for a reliable DNA match?

There is no universal segment-size threshold that makes every match reliable or unreliable. Small segments, especially those under approximately 7–10 centimorgans (cM), can be more difficult to interpret and may reflect population-level sharing rather than a recent common ancestor. Larger segments are often more informative, but total shared DNA, segment distribution, shared matches, population history, and documentary evidence all matter. A 15 cM segment can be a useful clue, not an absolute guarantee.

How do I verify if a shared segment is maternally or paternally inherited?

One useful approach is to compare your match with known maternal and paternal relatives using the service’s shared-match tools. If a match also shares DNA with a known maternal cousin, that may support a maternal connection, although shared-match evidence alone does not always establish the exact path. When close relatives are unavailable, you can organize matches with the color-coded Leeds Method and a spreadsheet from our free genealogy templates directory.