My understanding of this is fairly basic but here it is: All men have an X and a Y chromosome and all women have 2 X chromosomes. These are inherited from their parents. Therefore men will have the same Y chromosome as their father. What this means is that I will have the same Y chromosome as all my male ancestors as will any other man descended along the male line from a common male ancestor. Therefore if I find I have the same Y chromosome as someone else it means we have a common male ancestor. There is however a "minor" nicety to this system in that over time mutations occur; matches are therefore done on a set of "markers" (usually 12 or 25)and the level of matching can be used to estimate the probability of two people having a common ancestor and how long ago. In general anything more than one or two differences means no match. Studies using the Y chromosome can therefore be used to check for common ancestry, particularly where the spelling of the surname may have changed as well as migration patterns. For common ancestry such studies would need to be related to documentary evidence There is also mitochondrial DNA which everybody has but is only passed down from the female. This seems to be less used partly I think because it is not following the surname. A book on the subject I've found useful is DNA and family History by Chris Pomery published by the National Archives; I got my copy from the FRC shop Regards Peter Wharton