Ancestry and Genealogy with DNA: The Complete Guide

A DNA test does two things for ancestry. It estimates the regions your DNA most resembles, and it identifies other people in the same database who share enough DNA with you to be related. Both of these are usually misunderstood at signup. The ethnicity estimate is a probabilistic comparison against a reference panel that the company updates every few years, which is why your “27% Scottish” can become “31% Irish” the next time the panel is revised. The matching is real and useful, but only inside the database you tested in.

This pillar is our complete guide to using DNA for genealogy. How matches work, how to read centimorgan amounts, how to find a biological parent or half-sibling, how to integrate DNA into a traditional family tree, and where Y-DNA and mitochondrial testing fit alongside the autosomal tests most people start with.

A few things worth knowing before you dig in.

Ethnicity estimates work better for some regions than others. British Isles and Western European reference panels are large and detailed; African, Asian, and many Indigenous American reference panels are coarser, and the breakdown reflects that limitation rather than your actual ancestry being fuzzy.

Centimorgans, abbreviated cM, are the actual currency of relationship prediction. Shared cM does not give a single answer to how two people are related; it usually narrows the options to a small set of possibilities. Our relationship predictor tool returns that set rather than a single guess, because pretending there is a single guess is how you end up confidently wrong about a relationship.

Autosomal tests like AncestryDNA and 23andMe sample DNA inherited from both parents and tell you about the past few hundred years. Y-DNA tests trace the strictly paternal line and reach much further back, but only along that one line; FamilyTreeDNA is the consumer company that still offers them. Mitochondrial (mtDNA) tests do the same for the strictly maternal line.

Matching only works within one company’s database. If your target relative tested at AncestryDNA and you tested at 23andMe, you will not see each other in either system. Uploading your raw data to MyHeritage, FamilyTreeDNA, or GEDmatch is the standard workaround.

A note on unexpected results. A meaningful share of people doing DNA tests for genealogy discover something they were not looking for: a previously unknown half-sibling, a non-paternity event, a closer biological relative than the family tree implied. We do not treat that as a curiosity. The articles below include practical guidance and pointers to support communities when a test surfaces something the family was not prepared for.

Start with the explainers below or jump straight to the article that matches your question.