Understanding Your
DNA Results
A practical guide from a professional investigator
Most people who take a DNA test know what they received. Far fewer know what to do with it. This guide explains how consumer DNA tests work for genealogy, what the results can and cannot tell you, and when an expert investigation makes a real difference.
How consumer DNA tests actually work
Consumer autosomal DNA tests analyse specific locations across your genome and compare the patterns they find with other people in the testing company's database. When two people share enough matching segments at the same positions, the system flags them as a potential match and estimates the likely relationship.
The key word is "autosomal." Autosomal DNA is inherited from both parents across all family lines. This is the type of test sold by AncestryDNA, 23andMe, MyHeritage, and most general-purpose testing services. It is useful for identifying relatives across roughly five to six generations. Beyond that, the shared segments become too small and too fragmented to interpret with confidence.
There are two other test types which serve more specific purposes. Y-DNA tests analyse the Y chromosome passed from father to son in an unbroken male line. Mitochondrial DNA (mtDNA) tests analyse the DNA passed from mothers to all their children, tracing a direct female line. These are more specialised and are offered primarily through FamilyTreeDNA. I will advise whether either is relevant to your specific research question.
Autosomal DNA
Inherited from both parents. Covers all family lines. Useful for relatives up to roughly five or six generations. The standard test type sold by most major platforms.
Y-DNA
Passed from father to son unchanged across generations. Traces the direct patrilineal line only. Useful for surname studies and proving or disproving a specific male-line connection.
Mitochondrial DNA
Passed from mother to all her children. Traces the direct matrilineal line only. Highly stable across long time periods, but less useful for identifying living relatives within recent generations.
Centimorgans and what they tell you
The centimorgan (cM) is the unit used to measure shared DNA. It is the single most important number in any DNA investigation. Understanding what a cM value means, and crucially what it does not mean, is the foundation of interpreting your results correctly.
The centimorgan figure on a match does not tell you exactly how two people are related. It tells you roughly how much DNA they share. Because the same cM range can apply to more than one type of relationship, a DNA match requires documentary corroboration before a specific relationship can be confirmed. The table below gives the standard expected ranges.
| Relationship | Typical cM Range | Relationship Type | Notes |
|---|---|---|---|
| Identical twin | ~3,587 cM | First degree | Full genome match |
| Parent / child | 3,330 – 3,720 cM | First degree | Consistent and reliable |
| Full sibling | 2,300 – 3,900 cM | First degree | Wide range; overlaps with half-sibling |
| Half-sibling | 1,160 – 2,650 cM | First degree | Can resemble aunt/uncle or grandparent |
| Grandparent / grandchild | 1,156 – 2,311 cM | Second degree | Overlaps with half-sibling and aunt/uncle |
| Aunt or uncle / niece or nephew | 1,201 – 2,284 cM | Second degree | Closely overlaps with grandparent range |
| First cousin | 553 – 1,225 cM | Third degree | One of the most common match types |
| Half first cousin | 215 – 650 cM | Third degree | Overlaps with first cousin once removed |
| First cousin once removed | 173 – 531 cM | Fourth degree | Requires documentary work to confirm |
| Second cousin | 41 – 592 cM | Fourth degree | Wide range; probability tools helpful here |
| Third cousin | 0 – 173 cM | Fifth degree | Many third cousins share no detectable DNA |
Ethnicity estimates: what they are and what they are not
Ethnicity estimates attract more attention than almost any other part of a DNA test result. They also cause more confusion and disappointment. Understanding how they are generated matters.
When you test with AncestryDNA or any comparable service, your DNA is compared against a reference panel: a collection of samples from people whose ancestry is known and documented over several generations. The algorithm assigns probabilities to different regions based on how closely your DNA resembles those reference populations.
The practical limitations are significant. Reference panels vary by company, which is why two siblings can receive noticeably different estimates from the same platform. The panels are updated over time, which is why your percentages may change without you doing anything. Regions in the historical record that are poorly represented in modern populations, including large parts of Ireland before the famine, tend to produce less reliable estimates.
Ethnicity estimates are best understood as a broad guide to deep ancestral origins rather than a precise breakdown of your family tree. They are not evidence of where specific ancestors came from, and they cannot tell you which branch of your family carried a particular heritage.
Choosing the right testing platform
The platform you test on determines the match pool available to you. This is often the single most consequential decision in a DNA investigation. The right choice depends on what you are trying to find out.
The most widely used consumer DNA platform in the world and currently the most useful for most genealogical purposes. The match pool is significantly larger than any competitor, which gives more opportunities to identify relevant connections.
A large and growing database with particular strength in Europe and among the diaspora from Central and Eastern Europe. Useful as a secondary test, and the free raw data upload from other platforms means additional matches can be found at no extra cost.
The only major platform offering Y-DNA and mitochondrial DNA tests alongside autosomal. If you need to investigate a specific patrilineal or matrilineal line, FTDNA is the only option. Their autosomal database is smaller than Ancestry or MyHeritage, but raw data uploads from other platforms are accepted free of charge.
GEDmatch is a third-party tool rather than a testing company. By uploading your raw DNA data from any major platform, you can compare your results against people who tested elsewhere. This is particularly useful when a key match has tested on a different platform from you. The basic tools are free; some advanced features require a paid subscription.
If you have not yet tested, I recommend starting with AncestryDNA in most cases. If you have already tested, uploading your raw data to GEDmatch and MyHeritage at no extra cost will expand your match pool considerably. If you are investigating a specific family question, I will advise you on the best approach before you spend anything. A free consultation call takes thirty minutes.
Book a free callWhat DNA evidence can and cannot do
Being clear about the limits of DNA evidence is not pessimism. It is the only way to interpret results honestly and avoid building a family history on shaky foundations. DNA and documentary records each do things the other cannot. A professional investigation uses both.
- Confirm or contradict a suspected biological relationship
- Identify previously unknown relatives who have tested
- Narrow down which branch of a family an unknown parent or ancestor came from
- Provide strong evidence for a non-paternity event when combined with documentary records
- Give a broad indication of deep ancestral geographic origins
- Break through brick walls by finding cousins with documented trees
- Identify a biological parent if that parent has not tested and has no close relatives in any database
- Distinguish between relationships with overlapping cM ranges without documentary evidence
- Pinpoint where in Ireland, Scotland, or Wales a specific ancestor came from
- Confirm a relationship to an ancestor who died before the DNA testing era without a known living relative to test
- Provide a precise family tree: DNA shows connections, not names and dates
- Serve as a legal document: a genealogical report is not a court-admissible DNA test
The most powerful investigations combine both methods. DNA evidence identifies which family the unknown person came from. Documentary research identifies the individual. Neither method alone gives the complete answer.
When a professional investigation makes a difference
Most people begin by exploring their DNA results independently. Many reach a point where the investigation stalls, the evidence becomes contradictory, or the question at stake feels too important to leave unresolved. This is when a professional investigation is worth commissioning.
An unexpected close match
A match sharing 1,500 cM or more points to a first-degree relationship. Identifying exactly who this person is and how they connect to your known family requires systematic investigation. This is one of the most common reasons people contact me.
Unknown biological parent
Whether through adoption, donor conception, or a family secret, the process of identifying a biological parent from DNA evidence is methodical and achievable in most cases where sufficient matches exist in the databases.
A brick wall in documentary research
When records run out in the early nineteenth century or earlier, DNA can sometimes provide a route around the wall. Cousins with documented trees may carry records that fill the gap. I combine both approaches in a single investigation.
Book a free call. Bring whatever results you have, and I will tell you honestly what they suggest, what would be needed to go further, and whether the evidence is likely to lead anywhere. There is no obligation to proceed.
Common questions answered
Questions asked regularly by people at the beginning of a DNA investigation.
No. If you have already tested with AncestryDNA, 23andMe, MyHeritage, FTDNA, or LivingDNA, I work directly with your existing results. If you have not yet tested, I will advise you on which platform gives the best match pool for your specific question before you spend anything.
An unexpected close match sharing 1,700 cM or more almost always indicates a first-degree relationship: parent, sibling, or child. The investigation begins by establishing exactly how that person connects to your known family tree, and whether documentary records explain or confirm the connection. This is one of the most common reasons people commission a DNA investigation.
AncestryDNA currently has the largest match database, which makes it the most useful starting point for most people. However, the best platform depends on your specific research question. If you are investigating Irish roots, MyHeritage has a strong European database. If you need Y-DNA or mitochondrial DNA analysis, FamilyTreeDNA is the only major platform offering those tests. Uploading your raw data to GEDmatch gives access to matches tested across multiple platforms, at no additional cost.
The investigation I provide is a genealogical and evidential service, not a legal one. The report I produce documents the genetic and documentary evidence clearly and with full source citations, but it is not a legal document. If you need DNA evidence for a legal purpose, I can discuss what the investigation can establish and advise on additional steps.
Most investigations take four to ten weeks, depending on the complexity of the match pool, the availability of documentary records, and the depth of corroboration required. I keep you updated at agreed milestones throughout, so you are never left wondering what is happening.
DNA investigations occasionally reveal unexpected family information: a previously unknown sibling, a non-paternity event, or a family secret that was deliberately kept. I handle every discovery with the same care I would want for my own family. Whatever the evidence shows, I will tell you clearly, honestly, and without judgement. Nothing in twenty years of research has changed the way I approach this. The truth belongs to you.
Yes. DNA investigations are conducted entirely remotely. I access your results through your chosen platform and work with documentary archives online. I work with clients across the UK and internationally. Where you are located has no bearing on the investigation.
Ethnicity estimates are useful as a broad guide but should never be treated as precise. The reference populations used by each testing company differ, and the algorithms change over time. Two siblings tested on the same platform will often receive slightly different ethnicity estimates. For genealogical purposes, documented records and centimorgan analysis with named matches are far more reliable than percentage breakdowns.
Ready to take the investigation further?
If this guide has raised questions you would like to resolve, these are the services that will take them forward.
DNA Investigation
Full centimorgan analysis, shared match clustering, and documentary corroboration. A written report documents every finding with full source citations.
From £449
Full detailsUnknown Parent Investigation
Designed specifically for cases involving adoption, donor conception, or an undisclosed biological parent. Systematic cluster analysis combined with full documentary research.
From £599
Full detailsDNA and Full Roots
A complete DNA investigation combined with full genealogical research across all family lines. The most comprehensive way to understand your family's story.
From £999
Full detailsYour results are a starting point,
not the final word
Bring whatever you have. A match list you cannot interpret. An unexpected result you do not understand. A question you have been sitting with for years. A free thirty-minute call will tell you what the evidence suggests and what it would take to go further.