Unto Joseph - were born Manasseh and Ephraim - There is a remarkable addition here in the Septuagint, which must be noticed: Εγενοντο δε υἱοι Μανασση, οὑς ετεκεν αυτῳ ἡ παλλακη ἡ Συρα, τον Μαχιρ· Μαχιρ δε εγεννησε τον Γαλααδ. Υἱοι δε Εφραιμ αδελφου Μανασσῃ, Σουταλααμ και Τααμ. Υἱοι δε Σουταλααμ, Εδεμ· These were the sons of Manasseh whom his Syrian concubine bore unto him: Machir; and Machir begat Galaad. The sons of Ephraim, Manasseh's brother, were Sutalaam and Taam; and the sons of Sutalaam, Edem. These add five persons to the list, and make out the number given by Stephen, Acts 7:14, which it seems he had taken from the text of the Septuagint, unless we could suppose that the text of Stephen had been altered to make it correspond to the Septuagint, of which there is not the slightest evidence from ancient MSS. or versions. The addition in the Septuagint is not found in either the Hebrew or the Samaritan at present; and some suppose that it was taken either from Numbers 26:29, Numbers 26:35, or 1-Chronicles 7:14-20, but in none of these places does the addition appear as it stands in the Septuagint, thought some of the names are found interspersed. Various means have been proposed to find the seventy persons in the text, and to reconcile the Hebrew with the Septuagint and the New Testament. A table given by Scheuchzer, extracted from the Memoires de Trevoux, gives the following general view:
The Twelve Sons of Jacob with Their Children and Grandchildren. Reuben and his four sons 5 Simeon and his six sons 7 Levi and his three sons 4 Judah and his seven sons and grandsons 8 Issachar and his four sons 5 Zebulun and his three sons 4 Total sons of Jacob and Leah 33 Gad and his seven sons 8 Asher and his seven sons and grandsons 8 Total sons of Jacob and Zilpah 16 Joseph and his two sons 3 Benjamin and his ten sons 11 Total sons of Jacob and Rachel 14 Daniel and his son 2 Naphtali and his four sons 5 Total sons of Jacob and Bilhah 7 Total sons of Jacob and his four wives 70
"To harmonize this with the Septuagint and St. Stephen, Acts 7:14, to the number sixty-six (all the souls that came out of Jacob's loins, Genesis 46:26) add nine of the patriarchs' wives, Judah's wife being already dead in Canaan, (Genesis 38:12), Benjamin being supposed to be as yet unmarried, and the wife of Joseph being already in Egypt, and therefore out of the case: the number will amount to seventy-five, which is that found in the Acts." - Universal History.
Dr. Hales' method is more simple, and I think more satisfactory: "Moses states that all the souls that came with Jacob into Egypt which issued from his loins, (except his sons wives), were sixty-six souls, Genesis 46:26; and this number is thus collected:
Jacob's Family Jacob's children, eleven sons and one daughter 12 Reuben's sons 4 Simeon's sons 6 Levi's sons 3 Judah's three sons and two grandsons 5 Issachar's sons 4 Zebulun's sons 3 Gad's sons 7 Asher's four sons, one daughter, and two grandsons 7 Daniel's son 1 Naphtali's sons 4 Benjamin's sons 10 Total 66
"If to these sixty-six children, and grandchildren, and great grandchildren, we add Jacob himself, Joseph and his two sons, the amount is seventy, the whole amount of Jacob's family which settled in Egypt.
"In this statement the wives of Jacob's sons, who formed part of the household, are omitted; but they amounted to nine, for of the twelve wives of the twelve sons of Jacob, Judah's wife was dead, Genesis 38:12, and Simeon's also, as we may collect from his youngest son Shaul by a Canannitess, Genesis 46:10, and Joseph's wife was already in Egypt. These nine wives, therefore, added to the sixty-six, give seventy-five souls the whole amount of Jacob's household that went down with him to Egypt; critically corresponding with the statement in the New Testament, that 'Joseph sent for his father Jacob and all his kindred, amounting to seventy-five souls.' The expression all his kindred, including the wives which were Joseph's kindred, not only by affinity, but also by consanguinity, being probably of the families of Esau, Ishmael, or Keturah. Thus does the New Testament furnish an admirable comment on the Old." - Analysis, vol. ii., p. 159.
It is necessary to observe that this statement, which appears on the whole the most consistent, supposes that Judah was married when about fourteen years of age, his son Er at the same age, Pharez at the same, Asher and his fourth son Beriah under twenty, Benjamin about fifteen, and Joseph's sons and grandsons about twenty. But this is not improbable, as the children of Israel must all have married at a very early age, to have produced in about two hundred and fifteen years no less than six hundred thousand persons above twenty years old, besides women and children.

And unto Joseph in the land of Egypt were born Manasseh and Ephraim,.... And therefore not to be reckoned with those that went down with Jacob thither; for which reason the clause, "in the land of Egypt", is inserted, see Genesis 41:50,
which Asenath the daughter of Potipherah, priest of On, bare unto him; here again the Targum of Jonathan makes Asenath to be the daughter of Dinah, who it says was educated in the house of Potipherah prince of Tanis; See Gill on Genesis 41:50.