I got interested in DNA testing and its implications in genealogy research in the "olden days" before genetic genealogy was widely used. I knew my fourth great-grandfather on my father's side was a man named Jacob Arnett, who appeared in what became East Tennessee in 1783. I'd exhaustively investigated every man in colonial America whose name was anything close to that and was pretty sure that my Jacob was from a family that settled in Virginia. That family had a Jacob that disappeared from Loudoun County, Virginia records about the time of the Revolutionary War. As luck would have it, I was exchanging information with several Arnett researchers, including Wayne Charles Arnett, a Phoenix area attorney who is descended from that Loudoun County family. (Curiously, I'm an attorney as well. Maybe we have a gene for lawyering.) One of the things I shared with Wayne was a wedding photo of my Arnett grandparents. He was astonished by the resemblance between my grandfather and his great-grandfather.. So -- long story short -- Wayne took a Y-DNA test and so did my brother. And guess what! Though the common ancestor was 7 generations back, Wayne and Norm were a 100% match. Bingo! My hypothesis was confirmed.
Since then, I've roped my youngest son and one of my mother's male cousins into taking y-DNA tests so that info would be available. Neither my son nor the cousin have particularly close matches -- the common ancestor would be back thousands of years. I've got several close matches, but haven't explored connections. I've taken the mt-DNA test, which looks at the contribution I got from my mother's mother's mother back through time. As mentioned above, via my matriarchal inheritance, I'm a descendent of one of the seven daughters of Eve -- the one named Ursula. Unfortunately, I haven't been matched with any close cousins yet so I haven't been able to break through the brick wall about 7 generations back.