If two 12 marker Y-DNA results match for two participants with the same surname and the genealogy research shows a common ancestor in 1835, the DNA test has validated the research and proven that the two descendants are related. In this example, you have two items of evidence to support that the individuals tested are related…a documented paper trail and the Y-DNA results. In addition, the research provided a precise time frame for the common ancestor.
Without the genealogy research, and where 2 participants with the same surname match on the 12 marker Y-DNA test, then the scientific answer to the degree of relatedness is that 50% of the time the common ancestor would have occurred within 7 generations, or within approximately 150 years. The range of generations for the common ancestor extends to 76.9 generations or almost 2000 years for those cases where there is not a surname in common. Therefore the importance of a surname link is paramount to provide a comfortable conclusion of relatedness. Most of the time random matches with people with different surnames do not stand the test for extended DNA testing.