By testing the DNA of males, direct mail line descendants of an ancestor can be determined. The Y chromosome is passed from father to son with almost no changes from generation to generation. So a man’s Y-DNA is almost exactly the same as that of his paternal grandfather, continuing back in the same line for generations. This is the male line that inherits the surname. Therefore, if two men have the exact same Y-DNA or almost exactly the same Y-DNA, they are descended from a common ancestor. There are mutations that occur, so that usually the more differences there are between DNA samples, the farther back in time the common ancestor is.

It is extremely likely that the DNA test results that were the same in both living descendants were ones that the common ancestor had also. If we test more than two descendants and they all share the same test results, that increases the confidence even more that the ancestor’s DNA had those values. So we don’t need to dig up and test our ancestors. We can discover their Y-DNA by testing the Y-DNA of their descendants.