Genesis 48 is an adoption scene with legal force. Jacob, dying, tells Joseph that his two Egyptian born sons are now Jacob's own, as much as Reuben and Simeon are, and that any children born to Joseph afterwards will be reckoned under their brothers' names. In one sentence Joseph disappears from the tribal list and is replaced by two tribes, which is how twelve tribes are still counted after Levi is set apart for the priesthood and given no territory.
Then the blessing. Joseph places Manasseh, the elder, at Jacob's right hand and Ephraim at his left, and Jacob crosses his arms so that the right hand falls on the younger. Joseph takes his father's hand to move it, saying he has made a mistake. Jacob refuses. He knows, he says, and the younger will be greater, and his descendants will become a multitude of nations.
The scene is the last in a series that runs through the whole of Genesis. Isaac is preferred over Ishmael, Jacob over Esau, Judah and Joseph over their elder brothers, Perez over Zerah at the moment of birth. Primogeniture is the assumed rule of the world the book describes, and the book overturns it in almost every generation it narrates.
Jacob adds a detail that outlives the scene. He tells Joseph that Israel will use the two boys as the formula of blessing: God make you as Ephraim and as Manasseh. That sentence is still said over sons in Jewish households on the eve of the Sabbath.