Understanding DNA Tests
April 21, 2026 2026-04-22 1:11Understanding DNA Tests
Understanding DNA Tests
If you are new to genetic genealogy, this is the place to start. The three test types answer three very different questions — and the company you test with determines which questions you can ask.
I. The three families of test
One molecule, three very different stories. Every commercial DNA test you can buy falls into one of three families. They use different chromosomes, they are inherited differently, and they each have their own strengths and limits. Picking the right one is almost always a matter of matching the test to the question you are actually trying to answer.
Y-DNA
Father to son, unchanged. Only men can test.
Traces the direct paternal line back thousands of years. Because it travels with the surname (when surnames descend father-to-son), it is the natural tool for clan research.
Mitochondrial (mtDNA)
Mother to child, unchanged. Everyone has it; only women pass it on.
Traces the direct maternal line — your mother's mother's mother, back tens of thousands of years. It does not follow a surname, so it is less useful for clan research, but essential for the other half of your heritage.
Autosomal
A shuffled mix from all recent ancestors.
Looks at 22 of your chromosome pairs — the ones that reshuffle every generation. This is the test behind ethnicity pie-charts and "DNA cousin" matching. Powerful for the last five to eight generations; fades into noise beyond that.
II. Which test answers which question?
| The question you're asking | The test you need |
|---|---|
| "Am I descended from Fiachra?" | Y-DNA (male line only) |
| "Where did my direct maternal line come from?" | mtDNA |
| "What percentage Irish am I?" | Autosomal |
| "Who are my living second or third cousins?" | Autosomal |
| "Are two Dowd families with the same tradition actually related?" | Y-DNA for the male lines; autosomal for the wider cousinship |
| "What health traits did I inherit?" | Autosomal — but only at companies offering health reports |
III. The companies, and what they actually sell
Not every company sells every test. This is the detail most new testers trip over.
- AncestryDNA, 23andMe, MyHeritage — autosomal only. They give you ethnicity estimates and cousin matches. They do not do serious Y-DNA work. 23andMe does report a basic Y-haplogroup label for men, but nothing detailed enough for clan research.
- FamilyTreeDNA — the one laboratory still doing serious Y-DNA and mtDNA work at consumer prices. Their flagship Big Y-700 test is what surname projects (including the O'Dowd project) rely on. They also sell autosomal, but it is not their strength.
- LivingDNA — a smaller UK firm with a distinctive strength: the most detailed sub-regional breakdown of Ireland and Britain on the market. Useful as a supplement.
The upshot: if you want the dynastic answer, you will test at FamilyTreeDNA — there is no alternative. Autosomal matching and ethnicity can be done at any of the big three (and often transferred for free between them).
IV. A word on haplogroups
When a Y-DNA test is done, the result is a haplogroup — a label like R-M222 or R-FGC23742. Think of a haplogroup as a branch of the world's paternal family tree, defined by a single inherited mutation. Every man alive today belongs to one branch, and every branch has a branch it split off from, back and back to a single common ancestor in deep Africa.
For our clan, the branches that matter most are the Irish ones — specifically R-M222 (the "Northwest Irish" cluster shared by the Uí Néill, Uí Briúin, and Uí Fiachrach) and its refined sub-branches. The branch we believe is specific to our kindred is R-FGC23742. More on that in The Y-DNA Project.
V. What DNA cannot do
It is as well to be honest about the limits. DNA cannot give you a specific ancestor's name. It cannot, by itself, tell you which townland your great-grandfather came from. Ethnicity percentages are estimates — useful for orientation, not for legal purposes. And a test taken today cannot prove a particular historical figure is your direct ancestor; at best, it can show that you fit within the same descendant group. These are tools to be used alongside the paper record, not in place of it.
Continue Your Journey
A Note from the Clan
This page is written and maintained by volunteer members of the O'Dubhda clan. We are not a laboratory; we are kin, working to understand our shared inheritance. If you find an error, or if you would like to contribute your own DNA results to the clan project, please get in touch.
The companies, technologies, and scientific findings described here change regularly — we update when we can.
