Why are both Joseph and Manasseh listed as separate tribes in Revelation chapter seven?

The Patriarch Joseph had two sons Manasseh, and Ephraim: >Genesis 46:20 KJV And unto Joseph in the land of Egypt were born Manasseh and Ephraim, which Asenath the daughter of Potipherah priest of On bare unto him. Israel gave Joseph an extra share among his brothers in: >Genesis 48:22 KJV Moreover I have given to thee one portion above thy brethren, which I took out of the hand of the Amorite with my sword and with my bow. Both Manasseh and Ephraim were given equal shares of the promised land, when it was divided among the tribes. Manasseh also was given an area outside the promised land. >Joshua 14:4 KJV For the children of Joseph were two tribes, Manasseh and Ephraim: therefore they gave no part unto the Levites in the land, save cities to dwell in, with their suburbs for their cattle and for their substance. >Joshua 22:7 KJV Now to the one half of the tribe of Manasseh Moses had given possession in Bashan: but unto the other half thereof gave Joshua among their brethren on this side Jordan westward. And when Joshua sent them away also unto their tents, then he blessed them, In the Revelation chapter 7 Manasseh and Joseph are listed as tribes, but Ephraim is not. Rev 7:4 through 8 KJV >And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel. Of the tribe of ***Juda*** were sealed twelve thousand. Of the tribe of ***Reuben*** were sealed twelve thousand. Of the tribe of ***Gad*** were sealed twelve thousand. Of the tribe of ***Aser***were sealed twelve thousand. Of the tribe of ***Nepthalim*** were sealed twelve thousand. Of the tribe of ***Manasses ***were sealed twelve thousand. Of the tribe of ***Simeon*** were sealed twelve thousand. Of the tribe of ***Levi*** were sealed twelve thousand. Of the tribe of ***Issachar*** were sealed twelve thousand. Of the tribe of ***Zabulon*** were sealed twelve thousand. Of the tribe of ***Joseph*** were sealed twelve thousand. Of the tribe of ***Benjamin*** were sealed twelve thousand. Is the Tribe of Joseph supposed to be Ephraim?