Take for example this sentence.
"The natives, or natural-born citizens, are those born in the country, of parents who are citizens."
This is plural natives, plural parents. If it is applied to one native does it mean both parents must be citizens or would it say parents' plural possessive if it meant that?
.....are born in the country, to (not of) parents, one of whom must be a citizen.
.....are born in the country, to parents who are/must both be citizens.