Gen 48:17-19
17 When Joseph saw that his father had placed his right hand on Ephraim's head, he thought he had made a mistake, so he took hold of his father's hand to move it from Ephraim's head to Manasseh's, 18 saying, "That's the wrong head, Father; the other one is the firstborn; place your right hand on his head."
19 But his father wouldn't do it. He said, "I know, my son; but I know what I'm doing. He also will develop into a people, and he also will be great. But his younger brother will be even greater and his descendants will enrich nations."

This is yet another example of God’s plan as opposed to man’s plan.