Would testing Elizabeth Warren’s DNA resolve the fight over her heritage?

Gut Check looks at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The claim:

Testing Senator Elizabeth Warren’s DNA will prove once and for all whether she is part Native American, former Massachusetts senator Scott Brown said this week.

Tell me more:

After Warren emerged as Donald Trump’s Twitter Adversary in Chief, he revived accusations that she lied about having a Native American — Cherokee or Delaware — ancestor, as her family lore said, and began calling her Pocohontas. When the issue first arose, during Warren’s successful 2012 race against Brown, documents (later called into doubt) suggested a great-great-great grandmother was Cherokee. “She can take a DNA test,” Brown told reporters in a conference call arranged by the Republican National Committee.

The two giant commercial DNA-testing companies, AncestryDNA (which Ancestry.com started in 2012) and 23andMe, both sell tests that tell customers what “percent” they are of more than two dozen geographic and ethnic groups, from “European Jewish” and “Finland/Northwest Russia” to “Benin/Togo” and “Asia East” — and Native American. They comb DNA, from spit samples that customers mail in, for hundreds of thousands of genetic variants across the 23 human chromosomes, looking for telltale signatures that are associated with one or more of those groups.

Applying proprietary algorithms to the test results — 23andMe explains its method here and Ancestry’s white paper is here — yields a report telling a customer that she is some percent Sardinian, for instance, some percent Balkan, some percent East African, and so on. “It’s the percent of your DNA that’s associated with that group,” said Jake Byrnes, manager of population genomics at AncestryDNA. “We don’t try to give you the precise number of ancestors.” So “10 percent Korean” doesn’t mean three of your 32 great-great-great grandparents were Korean.

The reasons for that also explain the limits of DNA for revealing ancestry.

Really?

Both AncestryDNA and 23andMe have improved their ancestor analysis since 2010, when the American Society of Human Genetics pointed out the many ways the results can be wrong. But there are still challenges.

One is practical. To determine which genetic variations go along with which ethnic or geographic groups, both companies (as well as academic researchers) rely on existing databases. AncestryDNA’s leans on one containing about 3,000 individuals from across the world, Byrnes said, complete with their DNA information and their family tree. But only 131 are Native Americans, from both North and South America.

That small number means only a handful are, say, full Cherokees or Navajos or Delaware, which may be too few to fully capture all the DNA variants that a specific tribe, or Native Americans generally, have. If someone has Native American DNA that happens not to have been captured in the database, the testing companies can’t identify it.

As the genetics society warned, “even the [best] databases … reflect a woefully incomplete sampling of human genetic diversity, and this has important consequences for the accuracy of ancestry” testing.

The small number also means the companies do not tell customers which tribe their ancestors belonged to, and their ancestry tests are uncertain enough that they “cannot be used as a substitute for legal documentation,” Ancestry warns.

The biggest problem is that our DNA comes from only a fraction of our ancestors. Every time an egg and sperm get together, roughly half the DNA of each makes it into the child-to-be. “If you go enough generations back there is a chance you don’t have any DNA from a particular ancestor,” said population geneticist Kasia Bryc of 23andMe. That chance rises the farther back you go, as population biologist Graham Coop of the University of California, Davis, has calculated.

The chance of an ancestor’s DNA not making it to a descendant is greater when a family tree has extensive “admixture,” meaning ancestors from multiple population groups — as Warren has said hers does.

The verdict:

“If Warren did have testing and it found Native American ancestry, we could be confident” she had a tribal ancestor, said 23andMe’s Bryc. But if it weren’t found, it could be because DNA from a single Native American ancestor happened not to survive five generations of DNA scrambling. “Just because DNA [associated with Native Americans or any other population group] isn’t there doesn’t mean you should give up your family stories,” said AncestryDNA’s Byrnes.