Genesis 46:20
And unto Joseph in the land of Egypt were born Manasseh and Ephraim, which Asenath the daughter of Poti-pherah priest of On bare unto him. And unto Joseph in the land of Egypt were born Manasseh and Ephraim, which Asenath the daughter of Poti-pherah priest of On bare unto him.
Genesis 46:20 stands in the detailed list of Jacob's family who went down to Egypt, recording that Manasseh and Ephraim were born to Joseph in Egypt. In that reckoning the men named are counted separately from their wives and daughters. The list in Genesis 46:8-27 names Jacob's sons and grandsons who went down, along with two women, and it does not include everyone who accompanied Jacob, such as his daughters, granddaughters, and some children of his grandsons.
The verse identifies their mother as Asenath, the daughter of Potipherah priest of On, and places the sons in Egypt. On was an Egyptian city also called Beth-shemesh, meaning house of the sun, and was a center of sun worship. The verse belongs to the account of Israel's journey to Egypt and reception by Joseph and Pharaoh, in the section that also records Jacob's blessing on his own sons and Jacob's death and burial.
The verse shows God's providential care in directing Israel's move to Egypt to preserve Jacob's family and the progenitors of the Seed from famine. That removal was to try His people, and to have them continually represent God's truth to the Egyptians. Joseph's history, including the birth of his sons in Egypt, shows that God guided him through a difficult and indirect course to the place intended for him.
Manasseh and Ephraim are the sons who receive Jacob's prophetic blessing. Their later adoption among Jacob's sons placed them among the twelve tribes of Israel. In the account of Israel's family, Manasseh's older age did not secure him the birthright or favor with God.