Updated July 13, 2018: NPE is a common enough term for those of us who follow DNA. I prefer the less commonly used explanation of “Not the Parent(s) Expected instead of the more commonly used “Non-Paternal Event” or Non-Paternity Event” since it’s not always a case of “Who’s the Daddy” and can be “Who’s the Mommy” or “Who are the Parents.” I like the new term mentioned in the below blog: Surname or DNA Switch (SDS). It is more accurate in many ways as a Surname Switch happens for any number of reasons and Maurice offers plenty of examples in the link below. Also, there may be no way of knowing if it’s a DNA switch or a Surname switch, or a combination of the two. Nothing like having a DNA switch at some point in the line that was followed or preceded by a Surname switch. It gets real messy when you have multiple instances of both in a line. I have an ancestor where the father is unknown (there is a Bastardy Bond, and while he paid the bond, he denied being the father) and the daughter was given the mother’s maiden name as her last name instead of the father’s last name. To complicate matters, the alleged father married a cousin of my ancestor so any DNA from their descendants wouldn’t necessarily rule him in or out as the father. Another less commonly used term used is MPE (or Misattributed Parentage Event).
DNA and Family Tree Research: Goodbye NPE, Hello SDS – some causes of Surname or…: The term NPE stands for Non-Paternity Event, or Non-Parental Event, or alternatively Not the Parent Expected – a much more descriptive and ..
Maurice gives a number of examples of SDS in the above link. Some of my favorites are the cartoon ones.