The seventh son of a seventh son was a figure who, according to folklore, possessed certain psychic abilities. Angus MacRanald was such a figure and supposedly had a second sight. (TV: Terror of the Zygons)
The Obverse Eighth Doctor was the seventh son of a seventh son. (PROSE: The Blue Angel)
Billy Coote was "more or less" the seventh son of a seventh son. (PROSE: Nightshade)