Read Genesis 48
When the tribes of Israel are counted there is no tribe of Joseph. I found that interesting, there are actually two tribes from Joseph's sons. The traditions held that the first born son would receive a double portion as inheritance. Jacob's first was Reuben, who had a falling out with dad when he slept with his concubine. We know that Jacob favored Joseph over all others so perhaps by adopting his sons, Jacob was able to give the double portion to Joseph.