According to Hugh Nibley this is a very interesting note on race in 3 Nephi 5:20, "I am Mormon, and a pure descendant of Lehi." Well, I thought everybody was a descendant of Lehi. Oh no, not by any means. It is something to boast about to be a pure descendant of Lehi. The blood of dozens of stocks is all mixed up by now. So when he says he's a pure descendant of Lehi, there's a reason for saying that. It's a kind of boast; he is proud of that. [Hugh W. Nibley, Teachings of the Book of Mormon, Semester 3, p. 304]
Note* Mormon is of royal birth. His lineage makes him heir to the Nephite records (the record of Joseph). [Alan C. Miner, Personal Notes] [See the commentary on Mosiah 9:1]