Genèse 48:6

And thy children whom thou mayest beget after them shall be thine; by the name of their brethren shall they be called in their inheritance.

AND THE CHILDREN THAT WERE BORN TO YOU AFTER THEM. “If you beget any more children, they will not be numbered among my sons, but they will rather be included among the tribes of Ephraim and Menasheh. Nor shall they have a name amongst the tribes as far as inheritance in the Land of Israel is concerned.” Now even though the Land of Israel was divided according to the number of persons, as it is written, To the more thou shalt give the more inheritance, and each person received an equal share except for the first-born, who received a double share, nevertheless only these sons were designated as “tribes.” This is the language of Rashi.
Now this is not correct, for if so, then Jacob’s granting of the birthright to Joseph was just nominal, its only effect being that Joseph’s sons would be called “tribes,” whereas the verse states, in their inheritance. This indicates that the two sons of Joseph were to be given the extra right of the firstborn in the matter of inheritance. In the Gemara, the Sages have said: “I have likened Ephraim and Menasheh to Reuben and Simeon with respect to the matter of inheritance, but not with respect to other matters,” even though they did not constitute an actual majority of the entire population. In that case Ephraim and Menasheh are considered part of the tribe of Joseph. as is stated in Tractate Horayoth. Our Rabbis have mentioned in many places that Joseph was the first-born as far as inheritance was concerned, and that he received a double share in the Land, as is the rule of every first-born, but not that his being first-born consisted merely of his sons being called “tribes,” as the Rabbi [Rashi] would have it.
From this we further learn that the Land was not divided among all the tribes of Israel according to their populations for if so, what was the significance of this primogeniture with respect to inheritance? If we would say it meant that each and every individual offspring of Joseph was given double that of each person of all other tribes, this is not mentioned at all in Scripture, and we do not find Jacob giving the birthright to Joseph except by what he said here, As Reuben and Simeon shall they be to me,, and based upon this, Scripture states, His [Reuben’s] birthright was given to the sons of Joseph the son of Israel. If so, then Ephraim and Menasheh were fully considered as two tribes, and it was this which constituted Joseph’s birthright, and the words of …

ומולדתך, your children or grandchildren who will be born or have been born after my arrival in Egypt (during the last 17 years). על שם אחיהם יקראו, they will have the same status as their brothers, i.e. they will not each be considered as the founding father of one of the tribes, but בתי אבות, heads of family groupings within the tribe they belong to (Menashe or Ephrayim). All the seventy souls included in the count of people descending to Egypt with Yaakov, whether his sons or his grandsons, became the heads of such family groups. And this was of relevance when the land of Israel was distributed by Joshua. The Talmud Baba Batra 118 discusses this in detail, basing itself on Numbers 26,21 mentioning such families as Peretz and Chetzron, etc., as well as the members of the family of Elon and Ard and Naamon mentioned in verses 40 and 53-55 respectively in that same chapter. What applied in those situations would also apply to children of Joseph born after Yaakov’s arrival in Egypt.

ומולדתך אשר הולדת אחריהם, your grandchildren, who are truly called מולדתך, seeing that this expression includes children and grandchildren,לך יהיו, they will be known as part of בית יוסף, the house of Joseph, bearing your name. And their blessing will be part of your blessing, not separate blessings.על שם אחיהם יקראו בנחלתם, everyone of Menashe’s sons will also incorporate the name of his father Menashe, as will the sons of his brother, who will also incorporate the name of their father Ephrayim as part of their individual names. They were to inherit part of the land of Israel together with Yaakov’s other sons.

ומולדתך אשר הולדת, perhaps Joseph had sired other sons during the years that Yaakov had resided in Egypt even though the Torah did not see fit to mention this explicitly. The reason that Joseph did not bring them with him to his father’s bedside may have been that they were too young to travel.Even if you were to understand the word ומולדתך as does Onkelos, and other commentators, i.e. תוליד, as if Yaakov had meant: “whom you will sire,” we also do not find any mention of children born to Joseph after Menashe and Ephrayim.