Manasseh was also the first son of
Joseph, the second being
Ephraim.
When Jacob, or Israel, as he was also known, gathered his children to bless them before he died, he gave the two brothers the blessing that "may you be blessed like Ephraim and Manasseh". This aludes to the fact that, while they grew up in a foreign culture in the upper class (their father was an advisor to the Pharoah) with little influence from the teachings of their fathers (that of monotheism), they still maintained a strong belief in these teachings and remained loyal to them and their family. This blessing is said over sons by parents every Friday night before the Sabbath evening meal.
When Jacob blessed the two brothers, he declared them to be like his sons, saying that he considered them direct descendants of him and therefore worthy of being tribes. However, when Jacob blessed them, he placed his right hand on Ephraim instead of upon Manasseh. When Joseph asked why, Jacob told him that, while Manasseh was the first born, Ephraim would be greater than Manasseh and would receive a double portion in the inheritance. By giving Ephraim the blessing of the first born, Jacob also forsook Reuven and continued a division that began with Jacob's favoritism of Joseph.