Genesis 48:19

And his father refused and said, I know it, my son, I know it, he was well aware of the fact that Manasseh, and not Ephraim, was the firstborn: he (Manasseh) also shall become a people, and he also shall be great; but truly his younger brother shall be greater than he, and his seed shall become a multitude of nations. It was not merely an old man's whim or caprice, but Jacob was acting with prophetic insight and wisdom and transmitting the blessing of the Lord. As a matter of fact, the tribe of Ephraim did pass the tribe of Manasseh in numbers and power, finally assuming the leadership of the northern tribes.