Can a tube of saliva really tell you where you come from? Partly. The adverts promise a story of who you are. What arrives is two very different things: a colourful map, and a long list of strangers. One of them interesting to look at. The other is the most powerful genealogical tool to appear in my seventeen years of doing this work.
The two things a test actually gives you
Every major testing company hands you the same pair of results.
The first is the ethnicity estimate: the pie chart telling you that you are 75 percent English, 21 percent Irish and 4 percent Scandinavian. This is the part people share at family parties, and it is the least reliable part of the whole exercise. The percentages are a statistical comparison against the company’s reference populations, and they shift every time the company updates its data. I have seen a client’s “Scandinavian” portion vanish overnight in one such update. Treat the map as a conversation starter, not a finding.
The second is the match list: every other tester who shares identifiable stretches of DNA with you, ranked by how much you share. That shared amount is measured in centimorgans, and the number matters. A parent or child shares roughly 3,400 centimorgans with you. A first cousin shares around 850. A fourth cousin might share 35. The list is not a curiosity. It is evidence, and evidence is what builds a family tree that holds.
What DNA can do
Used alongside the paper records, a match list can do things no archive can.
It can confirm a line you have already built. If your tree says your great-grandfather came from a particular Preston family, and you match a dozen descendants of that family at the right strength, the paper and the biology agree. That is as close to certainty as family history gets.
It can expose an error. Sometimes the records are correct and the family was not: an informal adoption, a child registered under a stepfather’s name, a parentage nobody wrote down. The records cannot see this. DNA can.
And it can solve the mysteries the records were built to hide. Unknown fathers, concealed births, adoptions from the era before open files. In one current case I am managing DNA results across two generations of the same family to identify a man whose name was deliberately left off every document. The method is slow and logical: isolate the cluster of matches that belongs to the mystery line, build the trees of those matches back until they intersect, then trace forward again to find the men who were in the right place at the right time. There is no shortcut, but there is a path, which is more than the documents offered.
What DNA cannot do
A test will not name a single ancestor. There is no gene for “your great-grandmother was Margaret Ainsworth of Blackburn”. Every name still has to come from records, connected by research. The DNA tells you the connection is real; it never tells you who the connection is.
It also fades with distance. You inherit half your DNA from each parent, but beyond that the halving is uneven, and by five or six generations back some ancestors have left you no measurable DNA at all. A test can say nothing about most of your tree in the 1700s. For those generations, parish registers do the work, not test tubes.
And it cannot replace judgement. A match list without research skills is just a list of names. The companies’ automated “common ancestor” hints are guesses built on other users’ trees, and other users’ trees are wrong more often than anyone likes to admit.
One practical rule: test the oldest generation
If you take one thing from this post, take this. DNA halves with every generation, so a test taken by your mother sees twice as far as a test taken by you, and a test taken by her mother sees twice as far again. If you have a parent, aunt, uncle or grandparent willing to spit in the tube, their test is worth more than yours. Take yours as well, by all means: comparing two generations side by side is exactly how I separate one family line from another.
A word of care before you swab
DNA does not respect the stories families tell. A small but real number of testers discover a parent or grandparent was not who they believed. If there is any possibility your family holds a secret of that kind, decide before you test whether you want the answer.
Where I come in
I do not sell DNA tests and I have no tie to any company. What I offer is the part the kit cannot do: managing your match list, building the trees behind your matches, and combining the genetic evidence with the records to give you an answer you can trust. If your family has a question the documents refuse to answer, DNA may be the way through, and my Bespoke service exists for exactly this kind of puzzle.
Is there an unknown ancestor in your tree? Get in touch for a free consultation and I will tell you honestly whether DNA can help.