What Your DNA Test Is Actually Telling You
You've swabbed your cheek, mailed the kit, and waited weeks. Now your results are in — and you're staring at percentages, haplogroups, and a long list of DNA matches wondering what it all means. You're not alone. DNA results are genuinely powerful tools for genealogy, but they require some context to be useful. This guide breaks it all down in plain language.
The Three Main Types of DNA Tests
Not all DNA tests work the same way. Understanding which test you've taken (or should take) matters enormously for genealogy purposes.
- Autosomal DNA (atDNA): The most common type, offered by AncestryDNA, 23andMe, MyHeritage, and FamilyTreeDNA. It tests DNA inherited from both parents and can identify relatives within roughly 5–7 generations. This is your starting point.
- Y-DNA: Passed directly from father to son. Useful for tracing a direct patrilineal line (your father's father's father, etc.). Only males can take this test directly, though females can test a male relative.
- Mitochondrial DNA (mtDNA): Passed from mothers to all children. Traces the direct matrilineal line far back in time, though it changes very slowly and is less useful for recent genealogy.
Ethnicity Estimates: Useful but Not Exact
The colourful pie chart showing you're "34% Scottish" or "18% West African" is often the first thing people look at — and the most misunderstood. Here's what to know:
- These are estimates, not precise measurements. Companies compare your DNA to reference populations, and those reference populations differ between companies.
- The same person can get noticeably different ethnicity results from different testing companies. Neither is definitively "wrong."
- Estimates are most reliable at the broad regional level and less reliable at specific country level.
- Ethnicity estimates update over time as companies refine their reference panels — your percentages may shift.
Use ethnicity estimates as a starting hypothesis, not a final answer. They're best used to point you toward geographic areas worth researching in traditional records.
DNA Matches: Where the Real Genealogy Happens
Your list of DNA matches — people who share segments of DNA with you — is far more genealogically useful than the ethnicity estimate. Here's how to approach them:
Understanding Shared Centimorgans (cM)
DNA matches are measured in centimorgans (cM). The more cM you share, the more closely related you are. As a rough guide:
| Shared cM (approx.) | Likely Relationship |
|---|---|
| 3,300–3,900 cM | Parent / Child |
| 2,200–3,900 cM | Full Sibling |
| 1,700–2,300 cM | Grandparent / Half Sibling |
| 575–1,330 cM | First Cousin |
| 41–592 cM | Second Cousin |
| Less than 20 cM | Distant Cousin (use caution) |
Note: These ranges overlap significantly. Use the Shared cM Project tool (available free online) for probability breakdowns of any given cM amount.
Grouping and Clustering Your Matches
When you have hundreds or thousands of matches, the Leeds Method is a popular technique for sorting them. By grouping matches who also match each other, you can identify which cluster likely corresponds to each of your four grandparents' family lines. This helps you focus your research systematically.
Using DNA to Break Through Brick Walls
DNA is particularly powerful when documentary records fail — common with immigrant ancestors, enslaved ancestors, or records destroyed by war or disaster. Strategies include:
- Identify a close DNA match (second cousin or nearer) and compare their known tree to yours
- Look for shared surnames or locations across multiple matches in the same cluster
- Use chromosome browsers (available on FamilyTreeDNA and MyHeritage) to confirm relationships
- Consider Y-DNA or mtDNA testing for direct-line questions where autosomal DNA falls short
Final Thoughts
DNA testing is a remarkable tool, but it works best when combined with traditional documentary research. Think of your DNA results as a compass that points you in the right direction — the paper trail fills in the details. The two approaches together are far more powerful than either one alone.