One thing that's good to know is that 23andMe and Ancestry don't currently do genetic testing. They do genotype testing instead. Genotype testing is more like a population comparison and it's not accurate, or only as accurate as the genotype database you're being compared against.
I don’t know where you get this idea because genotyping is (a form of) genetic testing. And as for accuracy, human reference genome databases are highly accurate. Sure, they’re not error-free — but then nothing is. Ancestry in particular is a tricky problem due to the highly variable representation of different ancestries in the reference set.
By contrast, 23andme delivers high accuracy for genetic variants. Meaning, for those variants in their reference set they can tell with very high accuracy whether a given sample has a given allele. Interpretability of this information unfortunately varies greatly because genetics is hard, and a lot of the connections between genotypes and disease predisposition are complex and still being fully discovered. It’s for this specific reason that the FDA told 23andme off for providing disease predispositions; not because the genetic test itself is inaccurate (it isn’t).
For simple (Mendelian) disease predispositions, 23andme is essentially as accurate as any other for of genetic testing (which would at any rate also use genotyping).
Ok - genotype testing is much less accurate as genome testing. A potential consequence is the common experience described in the article. Genotype testings's main advantage is cost.
On a side note, most of 23andme's reference set is limited to Europe, so it's even less accurate for a lot of people. Maybe things have changed?
I’m still not happy with this characterisation. By “genome testing” I suspect you mean whole-genome sequencing (WGS) [1]. First off, this is also a form of genotyping. And, depending on which definition of accuracy you’re using, it’s no more accurate. It just provides more signal (because it looks at much larger parts of the genome than microarray genotyping [2] does). The technology used by 23andme and DNA ancestry services (microarrays) will miss some information, compared to WGS (= lower recall [3]). But importantly its precision is no lower than WGS (in fact, until very recently it was higher).
The fact that DNA ancestry estimates are inaccurate for some people is largely unrelated to the technology (microarray vs sequencing), and rather to the fact that our reference set for ancestry is too sparse (especially outside of Europe).
