What DNA Can and Cannot Tell You

What DNA Can and Cannot Tell You.

Welcome to Stuck in the Branches. Most people arrive here having already spat in a tube, looked at a pie chart, and wondered what they were supposed to do next. This guide is about what the test actually proves, what it cannot prove, and how to use the part that matters.

Three Different Tests.

There are three kinds of test and they answer different questions. Autosomal covers all your branches but fades after about five or six generations. Y DNA follows the direct male line and the surname, and only men carry it. Mitochondrial follows the direct female line and goes back a very long way.

Which Company Does What.

Ancestry and twenty three and me sell autosomal only, with the largest databases. MyHeritage is autosomal with strong European coverage. Family Tree DNA is the one that still sells Y and mitochondrial tests, which makes it the place to go for a surname line or a deep maternal question.

The full site index, with costs and links

Test the Oldest Person You Can.

The most valuable thing you can do with DNA is test your oldest living relatives, now. Each generation you go up doubles the amount of ancestral DNA captured. A grandparent's test is worth more than yours and your cousin's combined, and it is the one that becomes impossible later.

The Ethnicity Estimate Is an Estimate.

The percentages are not a record. They are a statistical comparison of your DNA against a reference panel of modern people, and both the panel and the maths get updated. When your numbers change overnight, your ancestry did not. The company improved its model.

It Will Not Find Your Village.

An ethnicity result can tell you a region. It cannot tell you a town, a surname or a great grandfather. If you want the village, that comes from a naturalisation file, a manifest or a parish register, not from a pie chart. Judge an estimate by the continent, treat the sub regions as a hint.

Small Percentages Mean Very Little.

A result of two or three percent of somewhere unexpected is within the noise of the method, and is frequently gone after the next update. Do not rebuild a family story around a small slice. Do take a large unexplained block seriously, because that usually means a real and recent ancestor.

The Matches Are the Actual Product.

Here is the sentence that matters. The ethnicity estimate is the marketing, and your match list is the research tool. Those are real living relatives, each sharing a measured amount of DNA with you, and the whole value of the test is in working out how each one connects.

What Centimorgans Tell You.

Matches are measured in centimorgans, which is the amount of shared DNA. A big number means a close relative, a small number means distant and uncertain. A given amount is consistent with several different relationships at once, so the number narrows the possibilities, it does not name the cousin.

Shared Matches Are the Technique.

The real method is grouping. Take a match you can identify, then look at who else matches both of you. That cluster usually belongs to one branch of your tree. Label the groups by branch and the unknown matches sort themselves into which side of your family they belong to.

A Test With No Tree Is Nearly Useless.

Link a tree to your test, even a small one with four grandparents in it. Most people do not, which is why so many match lists are a wall of usernames. Your tree is what lets a match work out the connection, and theirs is what lets you work out yours.

Write to Your Matches, Briefly.

Short, warm, specific. These are your surnames and places, here is what we appear to share, do any of those look familiar. No wall of text, no demands, no attachments. Most people will not reply, and the ones who do are often the single best source you will find on that branch.

GEDmatch Puts Them All Together.

Because the companies will not talk to each other, you can upload your raw data to a third party site to compare across databases, which is what GEDmatch is for. It is free to use with paid extras. Read what you are agreeing to before you upload, because this is your genetic data.

The full site index, with costs and links

DNA Will Not Build Your Tree.

The test does not tell you a single name, date or place. It tells you that two people are related by some amount. Every conclusion you draw from it still has to be built out of documents, which is why this is Part 9 and not Part 1.

Use It to Confirm or to Break a Tie.

DNA is at its best doing two jobs: confirming a paper line you already built, and deciding between two candidate families when the documents will not. Used that way it is powerful. Used as a starting point with no research behind it, it mostly produces confident errors.

An Absence of Matches Proves Little.

Not matching somebody you expected to match is weak evidence on its own, especially past second cousins, because autosomal DNA is inherited in random amounts. Real second cousins sometimes share almost nothing. Do not demolish a documented line because of one missing match.

Be Ready for a Surprise.

Say this part plainly. These tests reveal unknown siblings, undisclosed adoptions and parents who are not who the family said. It happens often enough that you should decide beforehand how you would handle it, and remember that a result like that is somebody else's private life too.

Think About Privacy Before You Upload.

Your DNA is also partly your relatives' DNA, and they did not consent. Read each site's settings for matching, research sharing and law enforcement access, and set them deliberately rather than leaving the defaults. You can be in the databases you want and out of the ones you do not.

Start with the matches, not the pie chart.

Go back to your test, link a tree to it, and group your top twenty matches by which side of the family they sit on. That one afternoon will teach you more than the ethnicity page ever will. We are here when you get stuck.