You have done the test. The results have arrived. And now you are staring at a list of hundreds — possibly thousands — of DNA matches, most of them people you have never heard of, with percentages and centimorgans and predicted relationships that may or may not match what you expected. If your reaction was somewhere between bewildered and overwhelmed, you are in excellent company.
DNA testing is truly one of the most powerful tools available to genealogists today. It can confirm relationships that documents cannot prove, break through brick walls that have stood for decades, and occasionally reveal family stories that nobody knew — or that some people actively worked to conceal. But like any powerful tool, it works best when you understand what it is actually doing.
This guide starts from the very beginning. No prior knowledge of genetics is assumed, and no science degree is required. By the end, you will understand what autosomal DNA is, why it is useful for genealogy, what its limits are, and how to approach your matches in a way that is methodical rather than overwhelming.
What DNA testing actually measures
Every cell in your body contains a complete copy of your DNA — the biological instruction manual that makes you you. This DNA is organised into 23 pairs of chromosomes. You inherited one copy of each chromosome from your mother and one from your father, who in turn inherited theirs from their own parents, and so on back through your family tree.
When you take a genealogical DNA test, the testing company analyses specific locations along your chromosomes where human DNA commonly varies from person to person. By comparing these variations between two people, the company can calculate how much DNA they share — and from that, estimate how closely related they are.
The type of DNA used for most genealogical testing is called autosomal DNA. The word autosomal simply means it comes from the 22 pairs of non-sex chromosomes — the chromosomes that both men and women have, inherited from both sides of the family. This is what makes autosomal DNA so useful for genealogy: it traces relatives on every branch of your family tree, not just one line.
You may also encounter two other types of DNA testing: Y-DNA, which traces the direct paternal line (father's father's father, and so on) and is only available to biological males, and mitochondrial DNA, which traces the direct maternal line (mother's mother's mother, and so on) for both sexes. These are specialised tools with specific uses. For most genealogical purposes — and certainly for beginners — autosomal DNA is the place to start.
How DNA is shared between relatives
Here is the core principle that everything else builds on: the more closely related two people are, the more DNA they share. The amount of shared DNA is measured in units called centimorgans, often abbreviated as cM. You do not need to understand the technical definition of a centimorgan — what matters is that a higher centimorgan figure means a closer relationship.
To give you a sense of the scale: you share approximately 3,400–3,900 cM with a parent or full sibling. You share roughly 1,700–2,300 cM with a grandparent, aunt, uncle, or half-sibling. First cousins typically share somewhere between 550 and 1,200 cM. Second cousins share roughly 40–360 cM. Beyond that, the amounts become small enough that individual variation is significant.
These figures are ranges rather than fixed values because DNA inheritance is not perfectly even. Each time DNA is passed from parent to child, the chromosomes shuffle and recombine in a process called recombination. This means that two full siblings do not inherit identical DNA from their parents — they each get a different random selection from the same pool. Over generations, this randomness compounds, which is why distant cousins can share surprisingly different amounts of DNA.
This also explains one of the most common surprises in DNA results: two people who share a predicted relationship — say, second cousins — may share significantly more or less DNA than the average for that relationship. The prediction is a statistical estimate, not a guarantee.
What the ethnicity estimate is — and isn't
Most testing companies display an ethnicity breakdown alongside your matches — a map or percentage chart showing your apparent ancestry by region. This is often the most visually striking part of the results, and it tends to generate the most excitement and, sometimes, the most confusion.
It is important to understand what this estimate actually represents. The testing company compares your DNA to reference populations — panels of people whose ancestry from a particular region is well documented — and calculates which reference populations your DNA most closely resembles. The result is a probability estimate, not a definitive statement of where your ancestors came from.
This means several things in practice. The regions used are defined by the testing company, not by historical or political boundaries, and different companies use different reference populations — which is why the same person can get noticeably different ethnicity breakdowns from different testing services. The estimates also tend to be more reliable for some regions than others, depending on the size and quality of the reference panel available.
For genealogical research, the ethnicity estimate is most useful as a broad orientation — a starting hypothesis about which parts of the world your ancestry may lie in — rather than a precise record. The DNA matches themselves are where the genealogical value really lives.
Making sense of your match list
Your match list is a list of other people who have tested with the same company and share enough DNA with you to suggest a family relationship. Depending on the platform and how many people have tested, this list may contain dozens or thousands of names.
The first thing to do is not to try to work through all of them. That way lies madness. Instead, start at the top — with your closest matches — and work your way down methodically.
Your closest matches are your most valuable starting point for two reasons. First, the relationship predictions are more reliable at close range — the centimorgan figures are large enough that the likely relationship is fairly clear. Second, close matches who have family trees attached can help you quickly confirm which branch of your family they connect to, which in turn helps you sort and understand the more distant matches below them.
As you review each close match, ask yourself these questions: Do I recognise this person? Have they attached a family tree? If so, do I recognise any of the surnames or locations in it? Which side of my family might they connect to — maternal or paternal?
Many matches will have no tree attached, or a very sparse one. This is frustrating but normal. Some people test out of curiosity and never engage further. Others may have private trees, or be adoptees searching for biological family who are understandably cautious. Do not be discouraged by this — the matches with trees will still give you plenty to work with.
The most useful tool you may not be using: shared matches
Almost every testing platform offers a feature called shared matches, in-common-with, or something similar. This shows you which of your DNA matches also share DNA with a specific other match. It is one of the most powerful features available to genealogists using DNA, and it is often underused by beginners.
Here is why it matters. Suppose you have a match — call her Match A — and you cannot immediately work out how you are related. If you look at Match A's shared matches with you, you may see other people you recognise — a known cousin, perhaps, or someone whose tree you've already examined. If Match A shares DNA with your known cousin on your mother's side, that tells you Match A almost certainly connects to you through your mother's family too. You have just narrowed down an unknown match to half your family tree.
This technique — using shared matches to sort unknown matches into family groups — is called clustering, and it is the foundation of most serious DNA genealogy work. You do not need specialised software to start doing it. Simply begin noting which of your matches appear together in each other's shared match lists. Patterns will emerge.
What autosomal DNA cannot do
It is just as important to understand the limits of autosomal DNA as it is to understand its strengths. Going in with realistic expectations will save you considerable frustration.
It cannot reliably trace ancestry beyond five or six generations. Because of the randomness of DNA inheritance, you may share no detectable DNA at all with some ancestors beyond your fourth or fifth great- grandparents — even though they are unquestionably your ancestors. Their DNA simply may not have made it through to you in a detectable amount. This means that DNA testing cannot replace documentary research for deep ancestry — the two approaches work best together.
It cannot tell you which specific ancestor a segment of DNA came from. A testing company can tell you that a particular segment of your chromosome 7 matches another person's chromosome 7, but it cannot tell you whether that segment came from your paternal grandmother's mother's side or any other specific branch. Working out the origin of a DNA segment requires triangulating it against multiple known relatives — which is the work of advanced DNA analysis rather than beginner exploration.
It cannot confirm a relationship on its own — it can only suggest one. Two people who share 1,800 cM could be a grandparent and grandchild, an aunt and niece, a half-sibling pair, or a great-aunt and great-niece, among other possibilities. The DNA figure alone does not distinguish between these. Documentary evidence — dates, locations, other family members — is what confirms which relationship is correct.
It may reveal things you were not expecting. DNA results occasionally surface family secrets — a biological parent who is not who you believed, a half-sibling you did not know existed, an adoption that was never discussed. These discoveries are more common than many people anticipate, and they can be emotionally significant for everyone involved. If you are testing primarily to break through a genealogical brick wall, it is worth being psychologically prepared for the possibility of unexpected findings, and thinking in advance about how you would want to handle them.
A calm, practical starting point
If you have just received your results and are not sure where to begin, here is a simple sequence that will give you a solid foundation without overwhelming you.
Start by noting your five to ten closest matches and writing down what you know about each one — their name, the centimorgan figure, whether they have a tree, and any surnames or locations you recognise. This is your anchor list.
Next, for any close matches with trees, spend some time in those trees looking for surnames and places that appear in your own research. You are looking for the point of connection — the common ancestor you share. When you find it, note it down. Each confirmed connection helps you understand where the matches below it in your list are likely to come from.
Then try the shared matches feature for your closest unknown match. See who else shares DNA with that person. Do any of those shared matches connect to a branch of your family you already recognise? If so, you have begun clustering without even realising it.
Finally, be patient with yourself. DNA genealogy has a learning curve. The researchers who use it most effectively have usually spent months or years developing their understanding — and they still encounter results that puzzle them. The goal at the beginning is not to solve everything at once but to understand the tool well enough to use it purposefully alongside your documentary research.
If your DNA results relate to a specific brick wall — an ancestor who vanishes before a certain date, an unknown biological parent, or a family that seems to have arrived from nowhere — the Geneablocks matcher can help you identify which type of brick wall you are facing and suggest practical strategies for that specific problem. DNA is often one piece of a larger puzzle, and knowing which puzzle you are working on makes all the difference.
