Twelve Tribes of Israel

A co-worker of mine brought this to my attention and I have since spent way too much time looking it over.

The question is this...Who exactly are the twelve tribes of Israel. Jacob (aka Israel) has 12 sons. Each son is listed as he is born in Genesis chapters 29, 30, 35. Also in chapter 35 we have a listing of the sons grouped according to their mother. The list is as follows:

Leah
Reuben, Simeon, Levi, Judah, Issachar, and Zebulun

Rachel
Joseph and Benjamin

Bilhah (Rachel's Maid)
Dan and Naphtali

Zilpah (Leah's Maid)
Gad and Asher

Now we have 12 sons listed which makes 12 tribes. Seems simple enough. However, Jacob (Israel) complicates things in Genesis 48:5 by claiming Joseph's two sons, Ephraim and Manasseh as his own sons. In Genesis 49:28 we get the statement, "All these are the twelve tribes of Israel". This after Jacob (Israel) has finished blessing each of the twelve sons listed in chapter 35.

Fast forward to the New Testament in the book of Revelation. In chapter 7, verses 4-8, the twelve tribes are listed as the following: Judah, Reuben, Gad, Asher, Naphtali, Manasseh, Simeon, Levi, Issachar, Zebulun, Joseph, and Benjamin. We are missing Dan and Manasseh has been added in. Recalling that Jacob (Israel) claimed Ephraim and Manasseh as his own we might expect them to both be listed, but instead Ephraim has been replaced by his father Joseph.

If we look in Numbers chapter 1 we see a census listing of the 'sons of Israel'. Broken into tribes Levi is left out because they were to be 'over the tabernacle of the covenant' and later would not be apportioned any specific area in the Promised Land. Manasseh is listed as a tribe and so is Ephraim, but curiously Ephraim is listed as 'of the people of Joseph, namely, of the people of Ephraim'. Since Ephraim was given the blessing of the first-born, this could explain why Joseph is listed in the listing of the tribes in Revelation instead of Ephraim. Since Ephraim's people were considered Joseph's people.

Now there are many other lists of the 'twelve' tribes of Israel throughout Scripture and strangely they are almost all different in content or order, but what I want to know is why Dan is missing in the Revelation list. Anybody...