If you have taken a consumer DNA test in the past decade — at Ancestry, 23andMe, MyHeritage, or any of the other major companies — you have almost certainly taken an autosomal DNA (atDNA) test, whether or not you knew it by that name. It is the most widely used form of DNA testing in genealogy today, and for good reason: it traces all of your family lines simultaneously, works for both men and women, and can identify relatives across a range of relationships from close family to distant cousins. Understanding what atDNA is, how it works, and what it can realistically do for your research is the foundation of effective genetic genealogy.
What Is Autosomal DNA?
The human body contains 23 pairs of chromosomes in the nucleus of most cells. Twenty-two of those pairs are called autosomes — the non-sex chromosomes, numbered 1 through 22. The 23rd pair consists of the sex chromosomes (X and Y), which determine biological sex and follow their own distinct inheritance rules. Autosomal DNA refers to the genetic material contained in those first 22 pairs.
Every person inherits one chromosome in each pair from their mother and one from their father, which means roughly half of your autosomal DNA comes from each parent. Before passing a chromosome to a child, the body shuffles together the two chromosomes in each pair — a process called recombination (or crossover) — creating a new, unique combination. This is why full siblings share an average of 50% of their autosomal DNA but not the exact same 50%: each sibling received a different shuffled combination from the same two parents.
With each successive generation, recombination breaks shared segments into smaller and smaller pieces, and the percentage of DNA inherited from any given distant ancestor diminishes. By roughly the fifth to seventh generation back, it becomes increasingly possible — not guaranteed, but possible — to share no detectable autosomal DNA with a specific ancestor, even though that person is genuinely in your family tree. This is the fundamental constraint of autosomal DNA testing for genealogy, and understanding it from the outset will help you interpret your results realistically.
How Autosomal DNA Matching Works
When you take an atDNA test, your sample is analyzed at hundreds of thousands of specific locations across your 22 autosomal chromosome pairs. At each location, called a single nucleotide polymorphism (SNP, pronounced “snip”), the lab records which of the possible genetic variants you carry. The result is a long string of data representing your unique genetic profile across the genome.
Testing companies then compare your profile against everyone else in their database, looking for long stretches of chromosome where your SNPs match another person’s SNPs. These stretches of matching DNA — called segments — are evidence that you and that person inherited the same piece of DNA from a common ancestor. The longer and more numerous the matching segments, the more recently that common ancestor likely lived.
Segment sizes are measured in centimorgans (cM), a unit of genetic distance that reflects how frequently a stretch of DNA is broken up by recombination across generations. A larger cM value indicates a longer shared segment and, generally, a more recent common ancestor. A parent and child share approximately 3,400–3,500 cM of DNA. First cousins share an average of around 850 cM. Fourth cousins share an average of only around 35 cM — a small enough amount that some fourth cousins share no detectable DNA at all.
The relationship between shared cM and genealogical relationship is not one-to-one. Because recombination is random, two people who are second cousins may share more or less DNA than the average for that relationship. The Shared cM Project[1] — a community research initiative that has compiled shared cM data from thousands of known relationships — provides a widely used reference tool for interpreting what a given cM amount likely means in terms of relationship. It is available interactively at DNAPainter (dnapainter.com).
🧬 What Makes Autosomal DNA Unique Among DNA Tests
Of the four types of DNA used in genealogy — autosomal, Y-DNA, mitochondrial (mtDNA), and X-DNA — autosomal DNA is the only one that traces all of your family lines at once. Y-DNA follows only the direct paternal line; mtDNA follows only the direct maternal line; X-DNA follows a specific subset of lines. Autosomal DNA, by contrast, reflects contributions from all of your ancestors, on every branch of your family tree, going back roughly five to seven generations.
This breadth is autosomal DNA’s greatest strength: a single test can reveal relatives on your mother’s side, your father’s side, and every branch in between — all from the same sample. It is also available to everyone, regardless of sex. Women who cannot take a Y-DNA test can still use autosomal DNA to research their paternal surname lines, by either testing directly or working with the results of male relatives.
The tradeoff for that breadth is that autosomal DNA becomes less useful as relationships become more distant. Beyond fourth or fifth cousins, shared segments become small and intermittent — harder to detect and harder to interpret. For brick walls that lie more than six or seven generations back, Y-DNA and mtDNA testing (which leave a traceable signal across many more generations on their respective lines) may be more productive.
What Autosomal DNA Testing Is Good For
Finding relatives: The most immediate use of atDNA testing is discovering other people who share DNA with you. These matches may be known relatives whose relationship to you the test confirms, or they may be previously unknown cousins who can help you fill in branches of your family tree you haven’t been able to document. A single close match — a second cousin you didn’t know existed — can sometimes break open an entire line of research.
Confirming relationships: Documentary evidence can be wrong, incomplete, or ambiguous. A DNA test that shows two people share the expected amount of DNA for a claimed relationship provides biological corroboration of that relationship. Conversely, an unexpected result — two people who should be first cousins sharing much less DNA than expected — can signal that the documented relationship may not be accurate.
Ethnicity estimates: All major testing companies provide an estimate of your ancestral ethnic and geographic origins based on your DNA. These estimates are genuinely interesting and can provide useful broad-strokes context for your research. They should be understood as population-level probabilities, not precise measurements — the same raw data will produce somewhat different estimates at different companies, and estimates shift as companies update their reference populations. They are a starting point for research, not a conclusion.
Breaking through brick walls: When documentary records run out, DNA matches can sometimes point the way forward. A cluster of matches who all share a common set of surnames and geographic origins may indicate which line a brick-wall ancestor belongs to, even when records are silent.
Confirming or identifying parentage: In cases of unknown parentage, adoption, or family secrets, autosomal DNA is the primary tool for biological identification, given its ability to identify relationships across all family lines.
The Major Testing Companies
Five companies currently offer autosomal DNA testing for genealogical purposes. Each has distinct strengths, and serious researchers often test at more than one.
AncestryDNA — The largest consumer DNA database in the world, with approximately 27 million testers[2] as of early 2026. AncestryDNA’s primary strength is the size of its matching pool combined with its deep integration with Ancestry’s documentary record collections. The ThruLines feature uses members’ family trees to generate hypotheses about how you may be related to your matches — connecting genetic evidence to documentary research in a way no other platform currently replicates at the same scale. The test costs $99. AncestryDNA does not provide a chromosome browser, which limits segment-level analysis. An Ancestry.com subscription (separate from the DNA kit cost) is needed to access historical records. Raw DNA data can be downloaded and uploaded to other platforms.
23andMe — The second-largest database, with a strong focus on health reporting alongside ancestry. 23andMe provides a chromosome browser and includes mtDNA and Y-DNA haplogroup assignments as part of its standard ancestry test. The DNA Relatives matching feature requires opting in. At $99 for the ancestry-only test, it is competitively priced. 23andMe’s genealogy tools are less developed than Ancestry’s, but its health reporting is unmatched among major testing companies and has Food and Drug Administration (FDA) approval[3] for multiple health risk categories.
MyHeritage DNA — Strong international representation, particularly for European, Scandinavian, Jewish, and Middle Eastern ancestry. MyHeritage provides a chromosome browser, an AutoClusters tool, and the Theory of Family Relativity — a feature similar to AncestryDNA’s ThruLines that connects DNA matches to hypothetical common ancestors using family trees. In late 2025, MyHeritage introduced Whole Genome Sequencing[4] (WGS) for new kits, making it the first major consumer genealogy company to offer this level of coverage. The test costs $89. Note that MyHeritage discontinued accepting raw DNA uploads[5] from other companies in August 2025; access to the MyHeritage matching database now requires purchasing a MyHeritage kit.
FamilyTreeDNA (FTDNA) — The only major company offering dedicated Y-DNA and mtDNA testing alongside its autosomal Family Finder test ($79). FTDNA provides a chromosome browser and accepts raw DNA uploads from other companies for a one-time $19 fee to unlock advanced tools. Its database is smaller than Ancestry’s or 23andMe’s, but it skews toward serious genealogical researchers, and the platform’s project system — surname, haplogroup, and geographic projects — makes it uniquely useful for organized group research. The Dillman Family Association’s surname project at FTDNA is an active example of this kind of collaborative research.
Living DNA — Smaller database but notable for its highly detailed regional breakdown of British and Irish ancestry, and for providing mtDNA and Y-DNA haplogroup assignments as part of its standard kit. Accepts raw DNA uploads from other companies. Best suited as a supplementary test for researchers with British, Irish, or broadly European ancestry. As covered in our earlier Living DNA article, the platform has some unique features that complement testing at larger companies.
The Dillman Connection
For members of the Dillman Family Association (DFA), autosomal DNA testing is a natural complement to documentary research. The DFA’s Dillman DNA Project coordinates genetic genealogy research across Dillman lines, and atDNA results from multiple family members — even distant cousins — can be combined to extend the reach of the research and confirm or challenge relationships suggested by documentary evidence. If you have tested at any major company, consider connecting with the DFA’s DNA project and sharing your results with fellow Dillman researchers.
Getting Started
If you haven’t yet taken an autosomal DNA test, AncestryDNA is the most common recommendation for a first test, given the size of its matching database. If you have already tested at one company, uploading your raw data to FTDNA and GEDmatch is an easy and inexpensive way to expand your reach significantly.
Our companion article — Autosomal DNA: A Deeper Dive — goes further into how different companies test different parts of the genome, strategies for combining results across platforms, advanced third-party tools, and the honest pitfalls of working with atDNA results. Look for it later this week on the DFA blog.
Autosomal DNA tests are available from AncestryDNA ($99), 23andMe ($99 ancestry / $199 health + ancestry), MyHeritage DNA ($89), FamilyTreeDNA Family Finder ($79), and Living DNA (pricing varies). Raw DNA uploads are currently accepted at FamilyTreeDNA, GEDmatch, and Living DNA. MyHeritage discontinued raw DNA uploads in August 2025. Sale pricing is offered periodically at all companies.
Sources
- Blaine Bettinger, “The Shared cM Project 4.0,” March 2020, thegeneticgenealogist.com (accessed May 2026).
- Ancestry, “AncestryDNA — Company Facts,” ancestry.com/corporate/about-ancestry/company-facts (accessed May 2026).
- 23andMe, “FDA-Authorized Reports,” 23andme.com/en-us/health/ (accessed May 2026).
- MyHeritage, “MyHeritage DNA — Whole Genome Sequencing,” myheritage.com/dna (accessed May 2026).
- MyHeritage, DNA upload discontinuation notice, August 2025, myheritage.com/dna (accessed May 2026).
AI Disclosure: This article was researched and written with the assistance of Claude, an AI assistant developed by Anthropic. All facts have been reviewed for accuracy, but readers are encouraged to verify information independently, particularly for time-sensitive details such as pricing and software versions.
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