Why your home DNA test is mostly an educated guess

Spitting into a plastic tube feels like unlocking a vault of pure, unalterable historical truth. You send off your saliva, wait four weeks, and receive a pie chart telling you that you are 42 percent British, 18 percent Scandinavian, and 4 percent Indigenous Americas. It feels precise down to the single digit. But if you spit into a second tube and mail it to a competing company, your Scandinavian heritage might suddenly vanish, replaced by a slice of French or Eastern European. You have not changed your ancestors in the mail, but the software reading your spit has changed its mind.

Home DNA companies do not possess a magical ledger of every human who lived five hundred years ago. They cannot read your sequence and spot a tiny tag that says 'Made in Scotland.' What they actually measure are single nucleotide polymorphisms, or SNPs—tiny points of variation along your genome. Out of your three billion genetic letters, they scan around 700,000 specific spots where humans frequently differ. Once they gather your specific pattern of variations, they do not look backward in time. They look sideways, comparing your pattern against a database of modern people whose grandparents all lived in the same place.

These databases are called reference panels, and they are inherently biased. If a company has sequenced ten thousand people from modern Ireland but only two hundred people from Nigeria, their statistical confidence for West African lineage is wildly lower than for Western European lineage. To make matters worse, human populations have never stayed put. People have been migrating, invading, trading, and intermarrying across continents for tens of thousands of years. There is no such thing as pure historic German DNA sitting neatly inside a modern boundary line; there are only broad, overlapping gradients of genetic traits spread across map borders.