There are many scientific limitations to the home DNA test. “These companies aren’t actually testing your ancestry at all,” says Mark Thomas, professor of evolutionary genetics at University College London. “They’re problematic in their claims to be able to infer an individual’s ancestry.”
There are a few reasons for this. First, the genetic information these DNA testing companies hold is based on living populations. When you send your spit off in a little tube, it is specific snippets, or markers, in your genome (the total collection of DNA that resides in your cells) that are being analysed, and then compared to the markers of others who are good representatives for distinct regions or ethnicities around the world. But as Thomas notes, the companies are only looking at very recent samples, from a relatively small group, in one specific database. “They are just saying: ‘If I wanted to make your genome, I could pull bits of your DNA from people all over the world who are around today. And this is just one way I could do it,’” he says.
The databases are skewed towards different parts of the world, too. “23andMe has more American customers, and AncestryDNA has more British and Australian,” Thomas explains. “And none of these companies asks: ‘What do we know about the genetics of the past, and which of those past inferred genetic clusters do we get our ancestry from?’ They are giving us what the market wants, not what the genetics tells us.”
There’s also the question of just how much information is passed down through a person’s DNA. Thomas explains that we probably inherit very few genes from our ancestors; DNA is inherited in “chunks” that break up the further back in time you go. “You start with two parents, then four grandparents, then eight great-grandparents, it goes to 16, 32 and so on. And by the time you go 10 generations back, there are ancestors from whom you inherit no DNA.”
