ייִדישער מחשבֿה אויף בראשית 25:6

We see, then, that the hegemony was apportioned to these two twins, each to inherit it at different times. And because of this strong bond between them they were both called "the generations of Isaac," for which reason the parshah opens (Genesis 25:19): "And these are the generations of Isaac…" That is, these two, Jacob and Esau, the subjects of this Parshah, are the generations of Isaac. For though Esau was an evildoer, he is still reckoned as Isaac's son — unlike Ishmael (Genesis [21:10]: "…for the son of this bondwoman will not inherit with my son, Isaac"), who is not accorded the honor of being referred to as "the generations of Abraham," as Isaac is, for which reason it is added "Abraham begot Isaac." The reason for this is that Isaac and Ishmael were not equally beloved of Abraham; but he made Isaac the apple of his eye and bequeathed to him the glory of his wealth, whereas to the sons of the concubines he gave gifts and sent them away form Isaac. After that they had no relationship with Isaac whatsoever and deserved to be forgotten. This was not the case, however, with Jacob and Esau vis-à-vis Isaac — to the point that it was almost Esau who was his favorite son! For both of them were the sons of the noble mistress [Rivkah], attached to each other from birth and conception, and conjoined in kingdom almost forever, as I have explained. For this reason Esau's name cannot be forgotten, though he be an evil-doer, for our fates are so inextricably intertwined. For this reason both of these children are referred to as "the generations of Isaac," and the parshah goes on to describe how their later history is foreshadowed.