The first decision that you should make is atDNA vs. YDNA. These are very different tests that work very differently. You can always later order the second type of test but then you would then be testing two different ways which increases your overall budget. Also, with both atDNA and YDNA, few people only order one test. Many people order several atDNA tests for the parents, aunts, uncles, cousins and siblings. By doing so, you can figure out which inherited segments belong to which ancestor. With YDNA, there are three useful resolutions YSTR tests that you can order. Many start out with 37 markers which is useful but later they may upgrade to 67 markers or 111 markers. The higher the resolution the better the results. Then there are YSNP testing which has many testing options.
atDNA testing covers all ancestors in you chart but some ancestors start to be undetectable around 1800 to 1850. Some get matches back in the late 1700s. So it is a broad test where matches from any of your ancestors will show up. If you are recently adopted or their is a recent adoption, etc. in the first three or four generations AND this line is your primary interest, atDNA tests are great for adoptee scenarios. Each atDNA is just under $100.
YDNA has many more varied tests and overall is more expensive but there are no time line limitations like atDNA testing. But you start over for every ancestor - so this approach only works if you are only interested in two or three lines. Eventually, YDNA will replace atDNA testing as the YDNA database grows and the YDNA technology becomes more robust and less costly, you will be able to chart how all testers are related. Eventually (with enough tests), we will be able to assign several mutations to each ancestor on our pedigree charts and this is the best approach to break through brick walls in the 1700s.
As an admin of the second largest Irish haplogroup (R-L226). I am able to chart exactly how 80 % of 556 testers at 67 markers are related with fairly high reliability. This number will grow to 90 % by the year end. Four years ago, all we had was L226 itself which is around 1,500 years old when it started to become prolific in offspring. L226 is actually around 2,500 to 3,000 years old, but less than 2 or 3 % survived during the first 1,000 to 1,500 years. We now have 52 branches under L226 and 25 % of these branches are genealogical in nature (have a common ancestor with the same surname - but this ancestor could be 1,000 years ago when surnames were first used). So advances are being made at a fast pace and lot of genealogists with brick walls are now YDNA testing. But this is more of an investment in the future as well. I was the first "Next Generation Sequencing" tester for L226 which cost $1,350. FTDNA now has a NGS on sale for $395 - the lowest price to date. NGS tests reveal unique mutations to your particular line. When tested by cousins, it is now routine to find new branches under 500 years in age for haplogroups like L226.
L226 includes a direct descendant of King Brian Boru - the first king to unite / conquer all of Ireland. Sir Conor O'Brien is the official title holder of the O'Brien surname and this title has been passed to male offspring for the last 40 generations when King Brian Boru created the O'Brien surname. If you test positive for the YSNP mutation, Y5610, you are a direct descendant of King Brian Boru. We now have six YSNP branches that cover different lines under King Brian Boru. YDNA is finally "starting" to deliver impressive results - but these results vary dramatically. L226 is doing so well since his descendants enjoyed wealth and power at the cost of each neighbor that they conquered. Many lines barely survived to the present. In fact, less than ten percent of the males living in Ireland in 1,000 AD (when Irish men started using surnames), have any living male descendants. M222 is the largest haplogroup of Irish origin and has around four times as many descendants. So you could belong to line that have only 20 living males or descend from line that has 500,000 living descendants. With atDNA you just do not get matches for lines with few living descendants but get a lot of matches for lines that are more prolific in offspring (in the last 200 years vs. 1,000 years).
Being Irish or Scottish results in your lines being tested several times more than French or German lines and twice as much as English lines. This is because clan names were given to many fewer men and were given a 100 or 200 years earlier. My English line, Brooks, every fourth new YDNA test reveals new genetic lines. Living by a brook (or being a Blacksmith) does lead to connections like we thought. Genetics is pretty revealing about surname creation and how people are related. Also, over 40 generations, around only 50 % of descendants will have the same surname due to adoptions, etc. So genetic genealogy will really change the way you approach genealogy.
