Book of Mormon 121-122 Student Manual, p. 41; Omni 1:15; refer in this text to Alma 10:3
“It is true that Lehi and his family were descendants of Joseph through the lineage of Manasseh (Alma 10:3), and Ishmael was a descendant of Ephraim, according to the statement of the Prophet Joseph Smith. That the Nephites were descendants of Joseph is in fulfillment of the blessings given to Joseph by his father Israel. The Nephites were of the Jews, not so much by descent as by citizenship, although in the long descent from Jacob, it could be possible of some mixing of the tribes by intermarriage.” (Joseph Fielding Smith, Answers to Gospel Questions, 1:142-143)