This table illustrates the importance of y-dna testing, helping to eliminate a lot of blind searching. You'll notice that proven related individuals are grouped together. Most Thompsons are in Haplogroup R, with the next most common group being I. As different haplogroups are refined into individual subclades, the better the information available to us. I happen to fall into Haplogroup I, and can in no way be related by blood to anyone in R, E, J, C, or whatever, except by marriage or adoption, allowing me to eliminate them in the search process.
The numbers in parentheses are the SNP Single Nucleotide Polymorphism) markers that define distinct subclades, refining the classification process. Mine happens to be positive for L22 and P109, placing me more precisely into group I1d1. Therefore, I know I'm directly related by blood to anyone with the P109 SNP markers even if the connection is 10 generations or more removed to a common ancestor.
If you haven't done y-dna testing on a male descendant, it will save you a lot of wasted time and effort regardless of where you decide to have it done, and may even disprove some stuff you believe to be fact.
It's best if you can spring for at least a 67 marker test plus a SNP test. A 12 marker test will do nothing more than show you're in a specific haplogroup with miilions of others regardless of surname--a waste in my opinion.
Most dna testing companies will allow you to upgrade from a simpler test to a more complex test at a discount, since they will have your sample on hand. I've done this, and was able to prove a group of us were unique by ordering the SNP test a year later.http://www.familytreedna.com/public/thompson/default.aspx?se...