Traditionally, a person born to a Jewish mother OR converted to Judaism according to Jewish law is 100% Jewish forever.
Certain actions may prevent one from being treated as a Jew, but the essential Jewishness remains, and, if the person in question is female, her children are fully Jewish. Should such a person return to Jewish observance, no ‘conversion’ is necessary.
If Sammy Davis and Liz Taylor were converted according to Jewish law, they are 100% Jewish despite having no Jewish DNA.
I am not aware of anyone being denied or granted the privileges of being a Kohen based on DNA.
There is frequently confusion between popular usage or phraseology and the actual facts. Even the term “Jew” itself is not official; the proper term is “ben/bat Yisrael”.
Thank you for clarifying. I always wondered about Sammy. But liz was born Jewish, right?