Parashat Toladot opens as follows: "These are the 'toladot' of Yitzchak: Avraham begot Yitzchak."
Although the term "toladot" generally refers to offspring, many translators and commentators prefer to translate this phrase as "This is the story of Yitzchak..." The reason is clear: if the verse opens, "These are the offspring of Yitzchak," then we expect the verse to continue with Yitzchak's children, not the already-known fact that Avraham was Yitzchak's father!
One may, however, retain the straightforward meaning of the word "toladot." On a simple, "peshat" level, the opening verse may serve to introduce the entire parasha, not necessarily the continuation of the verse. Indeed, the rest of the parasha does in fact tell of Yitzchak's progeny. "Avraham begot Yitzchak" may be seen as a parenthetical comment, inserted before the discussion regarding the offspring of Yitzchak.
Although this may be the simplest explanation, the peculiar structure of this opening verse calls for additional levels of interpretation, along the lines of "derash." Rav Moshe Feinstein zt"l suggests that the verse here tells us that the primary "offspring," or legacy, of Yitzchak involved his being Avraham's son. His most significant contribution to the world in general and the development of God's nation in particular was his emulation of his father. "This is the legacy of Yitzchak - Avraham begot Yitzchak," Yitzchak was truly Avraham's son, the ultimate inheritor of his teachings.
It should be added that, as has been pointed out by many, Yitzchak innovates very little throughout his lifetime, at least as appears from the Chumash itself. As opposed to Avraham, who developed a revolutionary theological system and emerged as an internationally renowned religious figure, Yitzchak seems to introduce very little. Yet, his contribution may be the most significant of all. It was he who stabilized the new beliefs of Avraham, who ensured that the name and legacy of Avraham would survive the centuries and millennia and not be left as a curious historical anecdote. Yitzchak's dedication to that for which his father stood guaranteed its safe passage through the ocean of time, to this very day.
Rav Moshe applied this idea to what he perceived as an exaggerated tendency to innovate, to introduce new ideas and fresh concepts. He felt that too many organizations, many of which were motivated by sincere aspirations for the sake of Torah and Am Yisrael, emerged claiming some new idea to sell. What had been lost, felt Rav Moshe, was a commitment to perpetuity, to simply transmitting the Jewish heritage to the next generation. This constituted Yitzchak's primary contribution, and this constitutes the primary responsibility of every generation.