(We've moved to: http://christinesleeter.org/dna-testing/)
DNA testing has become an increasingly popular, and gradually more affordable, way to obtain information about one's genealogy. Several years ago, I completed a DNA test to attempt to verify whether I had Native American ancestry, as I had grown up hearing was the case.
It is very easy to locate online companies that do DNA testing. The one I used was Ancestry by DNA, but there several are others as well, and I don't have any particular recommendation. Generally the procedure involves ordering a testing kit that will come in the mail, then swabbing the inside of your cheek (or doing something similar such as spitting), and sending the sample back in. The cost generally runs from about $100 on up.
The main idea behind DNA testing is that, as humans migrated out of Africa and across the globe, a few genetic markers gradually changed among large gene pools, and that by identifying your specific genetic sequences for these markers (based on comparing your genetic sequences with genetic research on contemporary population pools), the gene pools yours most likely came from can be identified. The most basic DNA test (autosomal testing) estimates the percentage of your ancestry from four major population groupings: sub-Saharan African, Indo-European, East-Asian, and Native American. More specific tests can trace your direct maternal line (mitochondrial DNA) and paternal line (Y chromosome DNA) to estimate more specific ancestry pools, such as which part of Africa, Europe, or Asia your ancestry is from, or likelihood of Jewish ancestry.
If you decide to have your DNA tested, there are several important things to keep in mind. First, the results will be estimates of likelihood rather than fixed determination of ancestry. Anything under 4% is too little to interpret. Further, humans have always moved about, and the groupings themselves do not have distinct boundaries, so while results suggest where your ancestors most likely came from, they do not tell you definitively where they came from. In my case, the results came back 94% Indo-European and 6% sub-Saharan African. These results strongly suggest that any possible Native American lineage would be so long ago that it has been diluted; or, more likely, I have no Native American lineage. However, it is likely that I do have African lineage.
Second, while you should not expect to use DNA testing alone to confirm your relationship to specific families, used in conjunction with what else you know about your genealogy, it can open up interesting questions. For example, in my case, I became curious about why the family story would have shifted from African to Native American ancestry, which led me to look into the history of "passing," and the historic use of "Cherokee ancestry" as a cover for African ancestry during the rein of Jim Crow laws. (For an insightful discussion of the socially-constructed nature of the colorline, I highly recommend Who is Black? by F. James Davis, Penn State University Press, 1991/2001). And while I do not have enough African ancestry to probe what part of Africa ancestors may have come from, I could undergo further testing to find out proportions of my ancestry from various parts of Europe. Since I have fairly good information already, however, from other sources such as the census, I don't see the point in spending more money on further DNA testing.
Finally, while biology and identity are related, they are not the same thing and should not be taken as the same. You may encounter surprises in your DNA, including surprises that unsettle your conception of who you are. For me, the discovery of African ancestry was a surprise, but having been raised white and identified all my life as white, it didn't change my interpretation of who I am. (Prior to 1955, however, it's possible that I could have been treated as Black under Jim Crow laws, had African ancestry been documented while the 1 drop rule was in effect.) Identity is formed on the basis of relationships with people, experiences within those relationships, who others think you are and treat you as, as well as your own interpretation of your experiences in various contexts. DNA gives you your genotype; it doesn't determine your relationships, experiences, and interpretations of your life. So if you decide to try DNA testing, be clear with yourself about what you hope to gain by doing this, what surprises you may discover, and whether you actually want potential surprises.