Categories: Isaac and Rebekah, Jacob and Esau, Jacob, Old Testament
In the Bible we often hear about the descendents of Jacob's sons referred to by the son's name--Gad, Zebulun, Issachar, etc.--but Joseph's descendents are referred to as the names of Joseph's two sons.
An example of this would be in II Chronicles 30.
1) And Hezekiah sent to all Israel and Judah, and wrote letters also to Ephraim and Manasseh, that they should come to the house of the Lord at Jerusalem, to keep the passover unto the Lord God of Israel.
10) So the posts passed from city to city through the country of Ephraim and Manasseh even unto Zebulun: but they laughed them to scorn, and mocked them. Nevertheless divers of Asher and Manasseh and of Zebulun humbled themselves, and came to Jerusalem.
18) For a multitude of the people, even many of Ephraim, and Manasseh, Issachar, and Zebulun, had not cleansed themselves, yet did they eat the passover otherwise than it was written. But Hezekiah prayed for them, saying, The good Lord pardon every one.
Why were they divided between the two sons, rather than just being called Joseph?
Perhaps the main point in Jacob's blessing of Joseph's two sons was to formally adopt them. He essentially put them on par with his two eldest sons, Reuben and Simeon. Such adoptions are well attested in the ancient Near East. We must remember that these grandsons were the product of Joseph and an Egyptian wife. It was necessary for Jacob to bring them fully into his family.
We don't really know why the later tribes became known as Manasseh and Ephraim, but let's speculate for a moment. Joseph had attained a great position in Egypt. Nowhere is it stated that he gave up his position when his family moved to Egypt. Undoubtedly, he remained at his post in the palace. Furthermore, by this time, Joseph is known by his Egyptian name, and his wife is the daughter of an Egyptian priest. Perhaps Joseph was "preoccupied" with greater things. By formally adopting his two sons, Jacob ensured that the line of Joseph would continue as part of the people of Israel. (The adoption process only applied to these two sons; any future sons would remain with Joseph - though there is no record of his having other sons.)
Also by the time these texts have been written and finalized, the authors know that the twelve tribes included Ephraim and Manasseh, but not Levi and Joseph. There is nothing in Jacob's deathbed blessing to indicate that Levi's descendants would be set aside for the sacred temple duties, thereby forfeiting their portion of land. (And, as a matter of fact, considering what Levi and Simeon did in relation to Dinah and the city of Shechem, we might be wondering why they were set aside at all. However, that comes 400+ years later. A lot can happen in 400 years.) In order to keep the notation of "twelve tribes", there had to be some substitutions. Joseph's descendants were known by the names of his children.
Some scholars think that during Jacob's final moments, he essentially elevated Joseph to being his firstborn (He was the firstborn of Rachel, Jacob's beloved wife). In so doing, one could make the argument that Joseph was entitled to a double portion of Jacob's estate, which gets translated into an equal portion for both of his sons, i.e. his line gets twice as much. The land really isn't apportioned with this criterion in mind, but it's another possibility. More than likely, however, it had to do with the fact that Joseph was thoroughly engrained in Egyptian life, so his sons were adopted by Jacob and raised as Israelites.