Ephraim and Manasseh

According to the Bible these two sons of the patriarch Joseph and his wife Asenath (daughter of the Egyptian priest Potiphera), were born during his period as governor of Egypt and before the seven years of famine (Genesis 41:50).

Genesis 48 records their adoption by their grandfather, Jacob/Israel, immediately before his death, when he gave his greater blessing or oracle to Ephraim, the younger son, in spite of Joseph's protestations. They would have been at least 24 years old at the time.

This separate blessing made each of them the ancestor of his own tribe, so Joseph was the progenitor of two tribes, whereas each of his brothers was father of only one. (Joseph himself was sold into slavery by his jealous older brothers, but he is omitted from this directory because it did not happen until he was 17 years old.)

References

Encyclopaedia Judaica, edited by Cecil Roth. (Jerusalem: Keter Publishing House, 1971)
International Standard Bible Encyclopedia, general editor Geoffrey W. Bromiley. (Grand Rapids: William B. Eerdmans Publishing Company, 1979-88)