That's my understanding, that the cheap tests really aren't very accurate. Are you able to explain a little for a layperson?
This might sound paranoid, but I also don't necessarily like the idea that some of the companies administering the tests may keep your results indefinitely, without necessarily defining who they will or won't share the results with. I don't know if that's still an issue, but I remember it being a potential issue a few years ago.
Most of your DNA is just blueprints for "how to make a cell". These sequences don't really tell you anything since they're shared with most organisms on the planet. Instead of looking at the whole genetic code (2 sets of DNA, each being 3 billion letters long) they look at specific genomic landmarks called "haplotypes". These are areas of DNA that vary between populations.
A haplotype is like a genetic fingerprint. If a haplotype is good, it's preserved, and all the sequences around it are preserved as well. Over time, small benign mutations will accumulate around this haplotype. The mutations will be unique to each lineage. A scientist can work backwards and figure out the history of this haplotype over time.
So if a haplotype originated 200,000 years ago (that means it's present in all living humans), I can look at it in an African, an Asian, a European, an Australian, and a Native American and they'll all have a unique "finger print" surrounding that haplotype based on the different mutations they've accumulated during that time. Maybe I have a haplotype that's only present in Native Americans - that would mean I have to have an ancestor of Native American heritage somewhere in my past.
Some haplotypes are so specific they can be identified down to the geographical location they originated on. There are some very specific haplotypes that only exist in Sardinia, for example, because the population there is very inbred and isolated.
In an academic lab, they look at *all* the DNA, which is much more data intensive. Medical labs look at specific disease haplotypes, but much deeper - they look at large haplotype clusters, because one haplotype can affect the behavior of another haplotype. Turning on a disease pathway can cause problems in a lot of other areas in your DNA, so they have to examine many more haplotypes to get a full picture.
It basically comes down to how many haplotype markers you want to look at. The less markers, the cheaper and quicker it is. These ancestry tests usually run for like $80-$120 I think? Full genome sequencing costs about $1000 today. Ten years ago it was in the millions.
If you're doing sequencing for a new species it's a lot more expensive, because you have to do "assembly", which is building the genome from scratch and figuring out what sequences go where. Then there's "annotation", where you discover each individual haplotype manually. Luckily the human genome is already assembled and heavily annotated, so DNA tests can be done quick and easy.
