To be fair, such concept does exist.
There are example like people in India who consider themselves "pashtuns", even though they arent more than 1% pashtun ancestrially. It would sound ridicolous to call them pashtuns, but since the "real ones" have their pashtun ancestry from a paternal ancestor, who passed the titel to future male descendant through generations, and each descendant considered himself "pashtun", then the 36th generation can still consider himself/herself pashtun, even if 99,9% indian, since we pashtuns have this tribal structure that father's heritage and identity matters the most. I think there are more examples out there IRL of different ethnicities, though i cant come up with any who would strongly identify themselves as those ethnicities, except descendants of jews and pashtuns?
I think its similiar case with jews?