Manasseh was the firstborn son of Joseph and his wife Asenath, daughter of Potipherah. He was born in Egypt in the days of the famine prior to his father's reunion with Israel and his other sons. He was adopted as a son of his grandfather at the time of the patriarchal blessings.
Since Joseph was Jacob's favorite son, the old man gave the double blessing traditionally reserved for the the first born to his two grandsons. When he crossed his arms, placing the right hand upon Ephraim, Manasseh younger brother over Joseph's objections. As a result, the future of Manasseh's descendants would take a secondary role in the affairs of what would become the nation of Israel.
The descendants of Manasseh became the largest of the tribes, being split into two half-tribes when the Machirites took possession of Gilead, dispossessing the Amorites who had dwelt there before them. Along with the tribes of Reuben and Gad this half-tribe inherited land outside of the "promised" land. The other half of the tribe joined their cousins to become a large contingency of the "northern" kingdom of Israel.