Jewish History: Biblical History: Ephrayim and Menasheh:
Why did Jacob favor Ephrayim, the younger son, over Menasheh, the younger son, when he blessed Joseph's children?
Rashi (on Gen. 48:19) explains that Jacob knew that Joshua, Moses' successor,
would be descendend from Ephrayim. It should also be pointed out that there
were many precedents in the family for favoring the younger son over the older:
Isaac over Ishmael, Jacob over Esau, Joseph over Reuben. Indeed, the Jews
were chosen over other nations because they were the smallest (Deut.7:7). This
explanation of Jacob's choice is given in the commentary Keli Yakar by
R.Shelomoh Ephrayim Luntchitz on Gen.48:17-19.
Ask a follow-up question