Sons of Reuben. Chapter 5 deals with the tribes that settled east of the Jordan: Reuben, Gad, and the half tribe of Manasseh. Verses 3 to 10 give the genealogy of Reuben. Reuben was the first-born son of Leah, who was also the mother of Judah and Simeon (Gen. 35:23), whose genealogies have already been given.
Sons of Joseph. As the eldest son of Jacob, Reuben would have been entitled to the rights of the first-born. The inheritance of a double portion (Deut. 21:15-17), which Reuben had forfeited by his sin (Gen. 35:22; 49:4), was given to the sons of Joseph (Gen. 48:21,22).
After the birthright. As the first-born, Reuben should have appeared first in the genealogical list. But that place went to Judah.