Manasseh was the son of Joseph and Asnat (Pharoah's daughter) and brother of Ephraim. Jacob adopts both Manasseh and his brother Ephraim as part of the tribe of Simeon and Reuben. Although Manasseh was technically the eldest son, he does not receive the greater blessing. Ephraim does, as Jacob foresaw that his descendants were more worthy of the blessing than Manasseh's (Genesis 48:13-20).
Sources: Bridger, David. Ed. The New Jewish Encyclopedia, NY: Behrman House, 1976; Navigating the Bible II