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.