Heredity is so hot right now. In 2017, the number of people who who’ve had their DNA analyzed for the purposes of tracing their genealogy doubled to more than 15 million. The largest of these direct-to-consumer companies, Utah-based Ancestry, tested two million people in the last four months of 2017 alone. But that’s still just a tiny fraction of the world’s population. Which means there are a lot more people out there who could be tested, and the company that woos them can use their valuable data to do other kinds of research and product development. There’s a lot more than just spit at stake.

Genealogy products—tests that tell people where their ancestors hailed from and match them with distant family members—have captured the consumer imagination: People are much more likely to send in their saliva if it will tell them where they came from than if it will tell them their risk for Parkinson’s disease. The methods for finding these long-lost cousins aren’t that complicated. But it just got a whole lot more complicated for Ancestry to keep using using them.

Earlier this month, Ancestry’s chief competitor in the genetic genealogy game, 23andMe, filed a lawsuit in California federal district court, alleging that Ancestry infringed on its patented method for identifying relatives from tidbits of DNA. 23andMe also accused its rival of false advertising, and asked the court to nullify the trademarked “Ancestry” name, arguing that the word has become a generic term used by other companies in the field (including 23andMe).

“23andMe is a company built on innovation and we have brought this action to defend our intellectual property rights,” 23andMe said in a statement. An Ancestry spokesperson told WIRED the company is aware of the suit and that “we intend to vigorously defend ourselves against these claims.”

The patent in dispute describes a method of analyzing regions of the genome shared by family members—dubbed “identical by descent,” or IBDs. The further back in time you go, the shorter and more spread out the long idententical stretches become. And that happens in a mathematically predictable way; IBD levels decay exponentially with each generation. Which means you can pretty easily determine how related two people are just by calculating the sum of the lengths of every IBD they share and the percentage of shared DNA in each one.

Now, it’s a little more complicated than that, but not much. In fact, the method is so widely used by population geneticists and hobbyist genealogists that many were surprised to learn that 23andMe had managed to patent it.

“This is a super ubiquitous technique that’s been around for a long time,” says Alicia R. Martin, a genetics researcher at the Broad Institute. She just published a study using IBDs to reconstruct how population size and disease rates in Finland changed over generations. “In fact, as more large-scale databases of genotypes and phenotypes are becoming available, it’s having a bit of a resurgence.”

The theory behind IBD actually dates back to the 1940s, to a French mathematician named Gustave Malécot who first described the probability that any two alleles might be identical copies from an ancestor. Of course he didn’t have any real DNA to work with. But after the Human Genome Project came along and sequenced Homo sapiens it didn’t take long for scientists to put those principles to work.

In 2005, hundreds of scientists from around the world used the IDB method to catalogue how much DNA people from faraway places actually have in common. Called the HapMap, the first phase of this international project was the first published application of the IBD method. A few years later, the research community released the first computational framework for analyzing IBD as an open source software program called GERMLINE. It was published online October 29, 2008. Two months and two days later 23andMe filed a provisional application for a patent entitled “Finding relatives in a database.”