September 18, 2019

The Limits of DNA Tests

“Identical twins have virtually identical DNA. So you’d think if a set of twins both sent in a DNA sample for genetic ancestry testing, they’d get the exact same results, right?

Not necessarily, according to a recent investigation by the Canadian Broadcasting Corporation. In fact, the journalists demonstrated that twins don’t often get the same results from a single company. And across the industry, estimates of where an individual’s ancestors lived can differ significantly from company to company.

In one instance, the consumer genetics company 23andMe told one twin she was 13 percent “Broadly European.” The other twin’s test, meanwhile, showed she had just 3 percent European ancestry. What’s more, when the twins had their DNA tested by five companies, each one gave them different results.

One computational biologist told the CBC that the differences in the results were “mystifying.”

So what accounts for these differences? Overall, discrepancies in ancestry testing don’t mean that genetic science is a fraud, and that the companies are just making up these numbers. They have more to do with the limitations of the science and some key assumptions companies make when analyzing DNA for ancestry.

The companies that provide ancestry testing, like Ancestry.com and 23andMe, deliver the precise ancestral breakdown of their customers’ DNA. They might say, for example, someone’s ancestry is 25 percent Italian, 74 percent East Asian, and 0.1 percent Sardinian. They also market their product in a way that suggests their test reveals something deeply meaningful about you.”

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