Jerome : (Chapter 48, Verse 1) And it came to pass after these things, that one told Joseph, Behold, thy father is sick: and he took with him his two sons, Manasseh and Ephraim. And from this, what we said above is demonstrated, that Joseph had only two sons in that place, Ephraim and Manasseh. For if many years later, when Jacob his father died, he led only two sons for the blessing, surely at that time, when his sons were unable to beget children, being young and nursing infants, he could not have grandchildren from them at the entrance of his father and brothers. — Hebrew Questions on Genesis