Read Genesis 48-50 at Bible Gateway
Genesis 48
Joseph visits Jacob as he is about to die. Jacob blesses both Manasseh and Ephraim but declares that Ephraim, the younger will be mightier than Manasseh. Jacob also gives Joseph two portions of the inheritance but adopting the sons of Joseph. It was customary for the first-born to get two portions and Jacob seems to be giving the birth right to Joseph even though he was the eleventh out of his twelve sons. There other instances in the Bible where the birth right did not always belong to the first-born.