According to myth, the seventh son of a seventh son is said to be destined for great magical or occult power, due to the fortunate mystical influence of those double sevens. He will be persecuted by those who hate and distrust magic but will attract followers hoping to ride the coattails of a powerful wizard.
There are no myths about the seventh daughter of a seventh daughter, but there should be, because she's likely to be even more powerful, right?