Near the end of his life, Jacob reshapes the future structure of the nation of Israel by using his divine authority to adopt his grandsons, elevating them to the status of his own sons. This action effectively transfers the right of the firstborn to Joseph, granting him a double portion. Several motivations drive this extraordinary gift. Because Jacob was unable to bury his beloved Rachel with proper honor in the Cave of Machpelah, he chooses to honor her memory by granting a double portion to her firstborn, Joseph [תולדות יצחק, חזקוני, מנחת עני]. Furthermore, Joseph earned this privilege by sustaining Jacob and the entire family during the years of famine [חזקוני]. There is also a divine dimension to this decision: God had previously promised Jacob at Bethel that he would be fruitful and multiply. Since Jacob had no additional children after that promise, he retroactively understood that God intended for Joseph’s two sons to fulfill this blessing and be considered his own [אור החיים, מלבי״ם]. Additionally, despite being raised in the foreign environment of Egypt, Ephraim and Manasseh remained entirely faithful to their grandfather's ideals, proving themselves worthy of being embraced as his actual sons [חומש קה״ת].
Jacob carefully defines the parameters of this adoption. He exclusively elevates the sons born to Joseph during the years of their separation, prior to his own arrival in Egypt and before the onset of the famine [רש״י, רשב״ם, מזרחי, גור אריה, שפתי חכמים, ביאור יש״ר]. This new status is strictly limited to these two; any future children born to Joseph will not attain the rank of an independent tribe, but will instead be absorbed into the territories of Ephraim and Manasseh [רד״ק, קונטרס חיבה יתירה]. By claiming the boys as his own, Jacob ensures they are counted equally among his other sons [רש״י, ביאור יש״ר], a move that mirrors the Jewish legal principle that grandchildren are considered just like children [תורה תמימה].
By placing his grandsons on the exact same level as his eldest sons, Reuben and Simeon, Jacob formally transfers the firstborn rights from Reuben to Joseph. This grants Ephraim and Manasseh inheritance rights in the Land of Israel equal to those of their oldest uncles [רשב״ם, בכור שור, ביאור שטיינזלץ, מלבי״ם]. This comparison is a tremendous honor, positioning the grandsons alongside the most senior brothers of the family [נחל קדומים]. This elevation also carries a subtle legal consequence. Had they simply inherited from Joseph, Manasseh, as the firstborn, would have received twice as much of his father's estate as Ephraim. By becoming direct sons of Jacob, their status is equalized, allowing both to inherit an equal share directly from Jacob's estate [הכתב והקבלה].
The practical impact of becoming independent tribes is a subject of discussion among commentators. One perspective suggests that because the Land of Israel was eventually divided based on individual headcount, designating them as separate tribes was primarily a matter of honor and lineage rather than an increase in actual land [טור הארוך, רבנו בחיי בשם רש״י]. A contrasting view firmly argues that the Land was divided into twelve equal portions regardless of population size. Therefore, establishing Joseph's sons as two distinct tribes literally granted them a double portion of the Land, properly fulfilling the firstborn right [טור הארוך, רבנו בחיי בשם הרמב״ן]. A middle approach explains that even if the land distribution depended on population size, becoming two separate tribes provided a significant advantage. Instead of being confined to a single shared border, each new tribe received its own distinct geographic territory, complete with diverse landscapes like mountains and valleys [מזרחי, גור אריה, דברי דוד].
Finally, Jacob deliberately places the younger Ephraim before the older Manasseh. Guided by divine inspiration, Jacob knew that the younger brother was destined to achieve greater things than the firstborn, and he prioritized him accordingly [העמק דבר, שד״ל]. Joseph did not understand this at the time, mistakenly assuming that Jacob prioritized Ephraim in order to humble Reuben, who was also a displaced firstborn [העמק דבר]. Ultimately, Jacob's decision to grant a double portion stemmed from his intense love for Joseph, and some suggest that this very act established the future Torah law that guarantees a double inheritance for the firstborn [שד״ל].