system
Given the following post, respond as if you're commenting on Reddit:
human
How accurate are the DNA/ancestry composition tests by companies like 23andMe?

I, like many of you, have seen dozens of commercials for companies like 23andMe. This companies claim that with a DNA sample, typically saliva, they can tell you about your ancestry.

So, how accurate are these tests? Are there several peer-review articles that confirm what these companies do?
gpt
The company determines if you have mutations at certain locations in your genome, known as genotyping, and compares them to those seen in ancestral populations to determine your "ancestry". The cheaper tests will use SNP microarray to do this and the more expensive services will do exome, or possibly whole genome sequencing and have the potential to identify inherited diseases or traits.  
One thing to keep in mind with the ancestry is that inheritance is random, so your inherited genetic ancestry and your genealogical ancestry is extremely unlikely to be the same. For example, 7 out of your 8 great grandparents could be 100% Italian and the last one is 100% Irish. By random chance, the chromosomes from the Irish ancestor (and therefore the ancestral mutations) could be completely passed on to you. In this event, you would be genetically 50:50 Italian:Irish but genealogically 7:1 Italian:Irish. It's an extreme example, but the important thing is that genetics can only give an approximate answer to who your ancestors were.