Lehi explains that his family’s presence in the promised land is the fulfillment of a promise to their ancestor Joseph: “Joseph is a fruitful bough, even a fruitful bough by a well; whose branches run over the wall” (Gen. 49:22). Joseph Fielding Smith elaborates:
There is strong presumptive evidence in the blessings given by Israel to his son Joseph, and his grandsons Ephraim and Manasseh, as recorded in Genesis, that they were to inherit a land far from Jerusalem and become a multitude of nations. Joseph was promised that his inheritance should be to the “utmost bounds of the everlasting hills” (Gen. 49:26), that he was “a fruitful bough by a well whose branches run over the wall” (Gen. 49:22). Moreover, he was to receive a greater inheritance than his progenitors, who were given the land of Palestine.
The Book of Mormon is the record of the descendants of Joseph who were led across the “great waters” to inherit this western land, which is designated as being choice above all other lands. Surely these blessings could not be realized in Palestine. Joseph and his sons did not become a multitude of nations there; the tribes of Ephraim and Manasseh did not receive a more wonderful inheritance in Palestine than any other of the tribes of Israel. There the chief honors were conferred first on Benjamin and then on Judah. Here in America all these promises were fulfilled when the descendants of Joseph possessed the land given as their inheritance.
Joseph’s lineage would produce a righteous people but would not be ancestors of the Messiah. This declaration is meant to clarify the accompanying prophecy Lehi mentions in the next verse about the seer coming from Joseph. There is a prophecy that someone important would come from the lineage of Joseph. Before clarifying who it was, Lehi makes sure we are not confused by who it was not.
The image of the branch “broken off” positions his sons as the bearers of this tradition and blessing but emphasizes that they left Jerusalem (the “breaking off”) by divine command, a subtle reminder for Laman and Lemuel. Although Lehi does not expect his descendants to be uniformly righteous, he promises that the Messiah will bring them to the light. Such imagery predicts a falling away into darkness, requiring their redemption.