biblical? you've been reading too many fantasy stories. The 7th son of a 7th son is supposed to be born a wizard. (the 7th son of a 7th son of a 7th son is a wizard squared, according to Pratchett :-)
Errrr except that Pratchett was taking the mick out of the 7th son of a 7th son as he used 8th of 8 for his book. It is used in the bible apparently but predates that. And the 7th of 7 can do all sorts of old rubbish varying from precognition to healing ability and is prsent in loads of cultures.
In Ireland, the 7th son of a 7th son is alleged to have healing powers. By laying his hands on a wound or a sick person, he can cure them of any ailments. The most recent (of which I have heard) is a guy called Finbar ? from Longford (a small county in the middle of Ireland) which was in the late 1980's or thereabouts.