In human DNA only males carry the Y-Chromosome so NORMALLY the yDNA follows a particular surname. My GG-grandfather Muncy gave his y-chromosome to my great-grandfather Muncy, who gave that same y-chromosome to my grandfather, who in turn gave it to my father and then to me. Mutations of specific markers happen along the way, but mutations don’t happen frequently. Mutations become key to identifying one branch from another. A mutation in the y-chromosome of my 4G-grandfather will also show up in my y-chromsome. There might be additional mutations during that time, but his mutations will be evident in my y-chromosome. If someone who believes he is my cousin also descended from my 4G-grandfather has the same mutation(s) then we can assume that connection. But if he doesn’t share the same mutations as my 4G-grandfather and me, then we don’t share that descent.
Non-Paternal Events (NPEs)
Non-Paternity Events (NPEs). I really hate that term because it conjures up virgin birth or something similar. Every child born is the result of a paternal event, but I didn’t create the term and I guess we have to live with it. Think of Non-Paternity Events as situations in which the parent you expected is not the parent you thought was the parent.
You will note that in a paragraph above I said the y-chromsome NORMALLY follows the surname, but there are exceptions. The so-called Non-Paternity Events (NPE) such as adoptions or out-of-wedlock births can result in a person’s surname not matching that expected from the y-chromsome. For those who must deal with these issue, y-chromsome testing and comparisons offer some hope of finding their true ancestry.