The popularization of genealogical DNA testing has a dangerous downside. First of all, before anyone gets overly agitated about this issue, genealogical DNA testing has its place in carefully supported research. However, there is a huge, ongoing controversy as to when, if ever, the genealogically pertinent DNA testing can or should be used in diagnostic medical testing. I do not intend to get into a long discussion on the details of DNA testing either for genealogy or for medical purposes, but I do think that the genealogical community ought to be informed of the controversial nature of the procedures.
One quite extensive discussion of this very issue is an article published online on the Genomemag.com website entitled, "Why is this $99 Home DNA Kit Causing Such an Uproar?" This articles discusses the recent issues between the 23AndMe.com company and its relationship with the Food and Drug Administration. If you do not see this as an issue, perhaps you should read the recent FamilySearch.org post entitled, "Researching the Past to Ensure and Healthy Future."
General DNA testing has been the subject of a post-apocalyptic dystopian novel trilogy called the Divergent Trilogy. See Roth, Veronica, Veronica Roth, Veronica Roth, and Veronica Roth. 2014. Divergent trilogy. Although this is a fictional account of a world where genetic testing has gone rampant, we have our own historical issues with race relations and other major issues that are partially based on the differences between people because of their inherited characteristics.
Although there are really good examples of how DNA testing has solved otherwise intractable genealogical mysteries and relationships, it is not a cure-all for other issues. I had a long and interesting conversation with a patron at the Brigham Young University Family History Library over the issue of a potential relative's demand that they become involved in an expensive DNA test to prove a relationship that was almost certainly non-existent back about 10 or 12 generations. I pointed out that her FamilySearch.org Family Tree data was insufficient to establish the relationship with the remote ancestor and that taking a DNA test would not likely produce supportable results unless the relationship to the remote ancestor was more definite.
On the website maintained by the International Society of Genetic Genealogy Wiki called Genealogical DNA testing myths, it says:
Myth #4: Do my DNA results reveal any medical conditions?
The section of the Y-chromosome used for genealogical DNA testing is non-coding DNA, in that it does not recombine (mix) or have any known uses other than to fill the spaces in between your genes. However, because this DNA does not mix, and it changes very slowly (mutates) it's beneficial for use in genealogical applications.
With mitochondrial DNA testing, the low-resolution HVR1 and HVR2 tests sequence a portion of the mtDNA genome known as the "Hypervariable Region" (HVR) which, like the Y-chromosome DNA, does not mix and it changes very slowly. Both HVR1 and HVR2 tests which are used for genealogical and deep ancestry purposes, do not reveal any medical conditions. However, a full mitochondrial sequence test may reveal medical conditions, but this information is not analyzed by your testing facility. You would need to seek further analysis from a specialist (for example, the custom mtDNA reports from Dr Ann Turner). The full sequence test results are not public information, nor made available to your DNA Project Administrator.
The autosomal DNA test from 23andMe includes health and trait reports. Autosomal DNA tests for genealogical purposes are also offered by Ancestry DNA and Family Tree DNA (the Family Finder test), but neither company provides any medical information. However, it is sometimes possible to extract some health and trait information by analyzing the data with one of the autosomal DNA tools such as Promethease.