In 2 Nephi 3 we find the blessing which Lehi gave to his son Joseph. Contained within that blessing are the prophecies made by Joseph of Egypt concerning "a choice seer unto the fruit of my loins" (2 Nephi 3:6). Some have incorrectly supposed that this "choice seer" was to be a direct descendant of Lehi's son Joseph. However, Nephi states that Joseph of Egypt "truly prophesied concerning all his seed" (2 Nephi 4:2). In 2 Nephi 3:23-25 we find an explanation concerning this "choice seer."
Wherefore, because of this covenant thou art blessed; for thy seed shall not be destroyed, for they shall hearken unto the words of the book. And there shall rise up one mighty among them, who shall do much good, both in word and in deed, being an instrument in the hands of God, with exceeding faith, to work mighty wonders, and do that thing which is great in the sight of God, unto the bringing to pass much restoration unto the house of Israel, and unto the seed of thy brethren.
Thus, first and foremost, this was a covenant promise, predicated on obedience to covenants. Secondly, we understand from the facts of the latter-day Restoration that Joseph Smith was the "one mighty among them" (or among the seed of Joseph of Egypt who would restore the gospel or "book" to the seed of Lehi's son Joseph). Thirdly, we realize that in spite of the final destruction of the Nephite nation, some of the seed of Joseph (although mixed and apostate to the covenant at the time of the destruction--see Moroni 9:24) would eventually be brought back to the knowledge and blessings of the covenant. [Alan C. Miner, Personal Notes]