Manasseh was the first born son of Joseph and his Egyptian wife Asenath, daughter of Potiphera, priest of On. (Gen 41:51) His brother was Ephraim. The story is told in the Bible in the book of Genesis.
The descendents of both brothers became part of the Twelve tribes of Israel. When Jacob gave his blessing on the sons of Joseph, he put his right hand on the younger, Ephraim, instead of on Manasseh. Joseph tried to correct this, but Jacob told him that while both brothers would be great, Ephraim would be greater than his brother Manasseh. (Gen 48)