This week’s Parsha begins with the Pasuk .ואלה תולדות יצחק בן אברהם אברהם הוליד את יצחק The word in this Pasuk that gives Parshanim the most difficulty is the word תולדות. The traditional explanation of תולדות is that it comes from ולד, offspring. However, based on this explanation, the Pasuk would appear to be repeating itself, since it subsequently says that אברהם gave birth to יצחק.
Rashi attempts to explain the Pasuk based on the Midrash. He says that the first half of the Pasuk refers to בני יצחק. The second half of the Pasuk then comes to validate Yitzchak as Avraham’s son, due to Avraham’s advanced age at the time of Yitzchak’s birth.
The Abarbanel challenges Rashi’s approach with the fact that if the Torah wanted to validate Yitzchak’s conception, it should have come right after Yitzchak’s birth, not forty years later. Therefore, the Abarbanel explains that all of Perek 26 discusses the children and grandchildren of Avraham. It began with Bnai Ketura, to be followed by Bnai Yishmael. Then, the Torah is up to Yitzchak. The second half of the Pasuk serves to explain how Yitzchak followed the principle of Maaseh Av Siman Labanim. For example, both married their cousins at age 40 and had wives who were barren. The rest of the Parsha serves to illustrate how that rule fits.
However, a third explanation may be given based on a different explanation of the word תולדות. In Perek 2 Pasuk 4, the Torah says ואלה תולדות השמים והארץ בהיבראם. From this Pasuk it is clear that תולדות does not mean children, but rather legacy or that which came after the item, and is what the item is remembered by. Therefore, our Pasuk may be saying that the legacy of Yitzchak is that he is a Ben Avraham. The Peirush Yonatan Ben Uziel on Devarim 6:4 says that the Pasuk of Shema is a statement Bnai Yaakov made to Yaakov before he died. They said, “Do not worry father, we will follow ה' because He is the only God, and our God.” Similarly, Yitzchak is following in his father’s footsteps by recognizing ה' as the only God in the world and trying to spread that message to everyone else, which is his legacy.