Or HaChaim on Genesis 48:6/אור החיים על בראשית מ״ח:ו׳

1 ומולדתך אשר הולדת אחריהם, "and any sons you beget after them, etc." This means that Joseph and his sons Menashe and Ephrayim would not be counted as three tribes but only as two. Accordingly, whenever the tribes are enumerated we do not find a tribe referred to as "the tribe of Joseph," but as the tribes Ephrayim and Menashe, respectively.

2The fact that Jacob excluded mention of Joseph whenever the tribes would be mentioned or enumerated deserves our closer attention. Why would it bother Jacob that including Levi there would be 14 tribes if Joseph were to be mentioned separately? After all it appears from G'd's promise in 35,11 that Jacob was to produce two more tribes, not just one more tribe? Where do we have an indication that the name Joseph as a tribe would be eliminated or superseded? Besides, who told Jacob that the two additional children G'd promised him in 35,11 were to be the two sons of Joseph rather than two sons of another one of his children? Was it not more likely for Jacob to assume that the two additional children G'd had in mind were Peretz and Zerach, the children of the union of Yehudah and Tamar, truly superior children as we know from the fact that dynasties were descended from them?

3Perhaps Jacob thought G'd that must have referred to Joseph's children as He had already alluded to the role of Yehudah's children when He had told him that kings would be descended from him in the same verse i.e. ומלכים מחלציך יצאו. However, why do we have to assume that these words referred to the sons of Yehudah rather than that the גוי וקהל גוים themselves would be the kings? Besides, there was always the possibility that G'd had referred to the sons of another one of Jacob's sons.

4The plain meaning of the prophecy in 35,11 that "nations and a community of nations" would emanate from Jacob surely must have been that they would emanate from Jacob directly, not from his sons. The Zohar section 1 item 180 claims that the נפש of Jacob and Joseph were closely intertwined and that we have proof of this in Psalms 77,16 where the sons of Jacob and Joseph are almost equated by the use of a conjunctive letter ו by the psalmist to show how close these two were to one another. Kabbalists describe that letter ו as מלוי הוי׳ו, i.e. that the מלוי is equivalent to the word itself. [In other words, Joseph equals Jacob because the psalmist joined his name to Jacob by means of this letter ו. Ed.] In this way Joseph became a "founding father" (patriarch) of tribes. Once we accept this, it was easy for Jacob to realise that of all his sons only his son Joseph was endowed with the ability to be a founding father of the tribes. When G'd had told Jacob that he would beget still more sons, this promise could therefore just as easily have been meant to apply to Joseph having these sons. As a result, Jacob had no reason at all to consider that prophecy as applicable to any of his other sons.

5This is why Jacob was quite precise when he said: שני בניך…לי הם, "your two sons…are really mine, just as Reuben and Shimon are mine." He wanted Joseph to understand that he did not look at his sons as his grandsons but as his real sons. It followed that Joseph could no more be part of the tribes than Jacob himself could be part of the tribes.