system
Given the following post, respond as if you're commenting on Reddit:
human
How scientifically legitimate is commercial DNA testing to identify ethnicity? 'biology'

There have been many different sites over the past few years claiming that they can do anything from determining if you are related to this or that famous person (which to me personally seems absurdly improbable) to determining your ancestry more broadly in terms of ethnicity and a general background.

I'd like to hear from a few geneticists, biologists or any related field on how credible sites like 23andme are and their accuracy when it comes to identifying your ancestors' background, their ethnicity and possibly even their nationality.

Thank you in advance.
gpt
Most are actually pretty accurate.

There are a lot of bases in human DNA (around 3 billion), but most of those are invariant. That is to say, in every person whose DNA we've looked at, they have the same base at that position. After decades of examining the human genome, we have a pretty good idea of which bases are invariant, and which ones have variants.

The next part is to figure out if certain variant sites reflect human ancestry. Let's say most humans have a region that reads ' ATCG', but some have 'AACG'. That second base is the variant site we're interested in. If it turns out that all pacific islanders have the 'A', while everyone else has 'T'. Than this site can be associated with heritage from the pacific islands. 

We have now examined enough genomes from most ethnic backgrounds (excluding North American first nations) that we have a large number of variant sites associated with each ethnicity. 

A company like 23&me will determine what variants you have and then give you a breakdown of ancestry based on those. So being 20% Irish means that of the variant sites that they looked at in your genome, 20% of the variants that are associated with ancestry (instead of say disease) have strong associations with Irish peoples.