It’s worth bearing in mind that when you’re presented with the population groups that have contributed to your DNA, some of the groups revealed may be very general (e.g. Western European) and the report may not tell you when or for how long each group was located in the region that it’s named after. The specificity of the population groups depends on the reference populations used by the company you test with (discussed later). Therefore, if a detailed ethnic breakdown is important to you, look for example reports from the company you’re considering, or get in touch with them to ask for a list of the reference populations that they use.
Of course, most DNA used by law enforcement in the U.S. does not come from direct-to-consumer DNA tests. The federal government and many states collect DNA samples from suspects of violent crimes after arrest or due to probable cause. These samples are added to the Combined DNA Index System, or CODIS, which is a national database for forensic information.

Then comes the section about serious genetic variants. So far as “counselling” goes, previously, I’d waved away concern for my psychological welfare from the Observer’s science editor (“I’m a former goth,” I said. “My default setting is ‘doomed’”), but it turns out to be quite daunting. It doesn’t help that I initially mistake the full list of potential conditions for my own results, hence (thankfully briefly) thinking that I have higher risk factors for everything going. It makes me wonder – how many other people are going to do that?

Consult a doctor on any health data: Cancer. Leukemia. Heart disease. Alzheimer's. There are a lot of scary afflictions out there, and your DNA testing may well indicate which ones to which you are genetically predispositioned. But the data from DNA testing exists in isolation. You should consult your doctor to explore the data from any of these tests. They'll help you determine how to implement any lifestyle changes or followup testing as a result, if it's worth doing so.
In testing, we found that many tests have much more specific and detailed results for European ancestry than anywhere else. This is due more to the diversity of the database than size. For example, AncestryDNA has the largest database with over 10 million samples yet results for Asian ancestry are markedly less specific than results from several companies with much smaller databases, including 23andMe and Living DNA.
A few of the DNA tests we tested, including the National Geographic Geno 2.0, use genetic sequencing instead of genotyping. Sequencing is newer in the mainstream direct-to-consumer DNA testing market, as it used to cost more and take much longer to sequence a person’s DNA. Sequencing identifies the exact makeup of a certain piece of DNA – be it a short segment or the whole genome. The Helix tests sequence the Exome, which are the parts of the genome responsible for protein production, plus several other regions of interest. 

mtDNA Tests: These type of tests look at the DNA found in our cell’s mitochondria. Mitochondria is located in the cytoplasm of the cell (surrounding and separate from the nucleus where most DNA is found) and is therefore only inherited from your mother. This means that a mtDNA test can help you understand the ancestry of your maternal line only. The nature of this test option also limits it to the understanding of deep ancestry (not so helpful for genealogists)–although the more detailed the test, the more accurate and specific your results become. The most commonly used tests for this purpose can be found through Family Tree DNA (FTDNA) and they have a huge amount of helpful information on better understanding the science and genealogical significance of this method of genetic testing.