Why Test for DNA?
I don't know about your family, but my family's records aren't perfect - and neither were our ancestors. Just in researching my own Galbraith line, which dates back to Arthur Galbraith d. 1818 Hawkins County, Tennessee, where was he before he popped up in 1758 Pennsylvania for submitting a bill for a horse to the Pennsylvania Assembly? If I knew that, maybe I could find his parents and where he was born. Another brick wall for me is my 3rd great grandmother Betsy Knight nee Ozment who lived a relatively short life and died about 1857. There is only one census record, one marriage record, and her tombstone available to tell me anything about her and unfortunately, none of those tell me who her parents were.
Those frustrations about the lack of records recorded, wrong information on actual records, a plethora of bad information on the internet, constant migration, and changed names create our genealogy research hurdles. Did you have an interest then or think to ask good questions of your older relatives before they left us? Yeah, me neither. That is when we can use the only evidence our ancestors left behind - segments of their DNA that we and our cousins have inherited.
DNA evidence can corroborate the written evidence and family stories that have been passed down over the generations. With my 4th great grandfather Arthur Galbraith, it actually disproved a relationship to James Galbraith of DNA Group 1 that had been assumed and written into history books 100 years ago. Testing can also give us the opportunity to knock down 'brick walls' by using DNA evidence to fill in gaps in the records, or to point us in the direction of records where we wouldn't otherwise have thought to look. With autosomal DNA testing, I now know with my Ozment autosomal matches who were the great grandparents (and probably grandparents) of my Betsy Knight nee Ozment. I still don't have her parent's names, but I have now just a handful of possibilities.
This is why DNA evidence simply cannot be ignored!
Types of DNA Testing and Which Test Should You Consider
For a little background at a high level, there are three different types of DNA testing available today: YDNA, autosomal, and mtDNA. Only males can test for YDNA, but both men and women can do autosomal and mtDNA testing. Here is a simple little chart that may help visualize where each type of testing can be used to help your family tree research.
The blue squares represent the line that is tested for YDNA. YDNA is the kind of DNA that only men have and that's passed from father to son to son so it tells us about our ancestry in our father's father's father's line. It typically is your family's surname's line in American culture. YDNA changes very little over time which makes it easier to determine which Smith (as a surname example) you are coming from as there may be thousands of different lines out there. But YDNA only gives you part of the tree. It is also great at identifying whether a non-paternal event occurred in the paternal surname line; i.e. you really aren't the Smith you thought you were, but another surname entirely. I have been doing DNA related research for a long time and I can't tell you how often this occurs! In my own Galbraith line, approximately 50% of the YDNA matches are Kings, so somewhere (we believe back in Ireland 300 years ago), someone was raised as a Galbraith that was a King or vice versa.
In the above diagram, the pink circles represent the mitochondrial DNA line (mtDNA for short). MTDNA is the kind of DNA that we all have but that's passed down from a mother to all of her children, and then passed down to the next generation only by the daughters. So it tells us about our ancestry in our mother's mother's mother's line. This test is valid for finding matches in this line for thousands of years back, similar to YDNA. MtDNA changes the least of all types of DNA but it is also the hardest one to find matches for unfortunately. In 7 years, I have only found 1 close match with my mtDNA and I still don't know how we are related, other than our ancestors' location is the same 300 years ago. It is also more expensive than autosomal DNA testing.
All of the shapes (blue, green, pink and yellow) in the tree example above can show up as matches through an autosomal test. That test is the kind of DNA we all inherit from both parents and that helps us find cousins to work with on our genealogical research. It is the cheapest test to do and will give you the most matches. However, it is important to note that this DNA changes rapidly. Autosomal DNA is inherited equally from both parents. The amount of autosomal DNA inherited from more distant ancestors is randomly shuffled up in a process called recombination and the percentage of autosomal DNA coming from each ancestor is diluted with each new generation. (Ex. because of recombination, my sister got a piece of DNA from our French ancestors that I didn't get. Luckily, I had her tested or I wouldn't have connected to these relatives in Alsace.) It is thought to be reliable up to 5 generations back for matches, although in some instances it can go back further.
See the chart below - the red numbers represent the % of DNA shared over time.
If testing other people in your family other than yourself for autosomal DNA, it is always wise to get the oldest living person on your mother or father's side to test because of the phenomena of diminishing DNA shown above (like your grandparents if they are still living.)
The three most predominant companies with autosomal testing are FamilyTreeDNA (Family Finder product), 23andme, and Ancestry. Here are my pluses and minuses by testing company:
FamilyTreeDNA Family Finder
FTDNA accepts transfers from Ancestry and 23andme for a nominal fee. It is always wise to be in as many databases or testers as one can afford. Ancestry and 23andme do not accept transfers so if you want your information in multiple databases, start with Ancestry (cheapest) or 23andme and then transfer to FTDNA.
No matter which company you choose for autosomal testing, PLEASE upload your autosomal results to the free site Gedmatch.com. This site allows you to compare your DNA to testers of all companies, provided they have also uploaded their results there. It also alleviates many of the deltas of Ancestry particularly by providing quantifiable chromosomal information and email addresses. Instructions are provided on the Gedmatch.com site for uploading your results.
Here are some more helpful links from people that can explain it better than I can: