Why does the Torah say ואלה תולדת יצחק?

Question

Parashat Toldot starts with the words

ואלה תולדת יצחק בן אברהם; אברהם הוליד את יצחק

These are the generations of Yitzhak the son of Avraham; Avraham gave birth to Yitzhak.

Normally when the Torah says eileh toldot XYZ it then proceeds to give a list the descendants of XYZ. In this case, though, the Torah does something completely different. First it mentions Yitzhak's father, and then it switches back to story-telling mode to tell about the births of Yitzhak's sons instead of giving a long list of descendants.

Why does the Torah stray from it's normal MO of listing a long chain of descendants?

Answer 1

I talk about the meaning of toldos

as

The use of "Eilah Toldos", which is usually translated as "These are the generations" or "these are the products" is found only in sefer Breishis (except for one case in Sefer Bamidbar 3:1) and only at the nodal points of an era. Each time it appears, it marks the end of an era and the beginning of a new one. The initial statement "Eilah Toldos" gives the name of the "person" at the cusp of the node, points to what has triggered the new era and is followed by a summary of what will be the significance of that era up to its end.

. . .

He has two sons, Yishmael and Yitzchak. They are regarded as two separate streams of humanity and will therefore be considered the branching point. That is why we have Lech Lecha 25:12 and Toldos 25:19.

יב. וְאֵלֶּה תֹּלְדֹת יִשְׁמָעֵאל בֶּן אַבְרָהָם אֲשֶׁר יָלְדָה הָגָר הַמִּצְרִית שִׁפְחַת שָׂרָה לְאַבְרָהָם:

12. Now these are the generations of Ishmael the son of Abraham, whom Hagar the Egyptian, the maidservant of Sarah, bore to Abraham.

    יט. וְאֵלֶּה תּוֹלְדֹת יִצְחָק בֶּן אַבְרָהָם אַבְרָהָם הוֹלִיד אֶת יִצְחָק:

    19. And these are the generations of Isaac the son of Abraham; Abraham begot Isaac.

Each one starts a completely different line of history. Just as the Torah drops the sons of Noach to follow Shem, and the children of Shem to follow Terach, we now end the summary of Yishmael with the indication that his descendants have their own history and we follow Yitzchak. Avraham was an individual. Yitzchak and Yishmael create families that will become nations. It is Yitzchak who is regarded as the true heir of Avraham and who we will follow.

Thus Yitzchak as the transitional connection between Terach and Yaakov is listed with his most significant trait. This is not that he gave birth to Ya'akov and Eisav, but that he continued and strengthened the line of Avraham by "purging" the traits exemplified by Yishmael. He wove the gevurah of his mother with the chesed of his father into the second of the "three fold string"

Rav Munk in The Call of Torah" goes into this and points out the difference between Yishmael and Yitzchak.

Also note that the children of Keturah are mentioned and then dropped. They are not a תּוֹלְדֹת but continue on just like the remainder of the nations.

We continue with Yaakov as a node and a start of a line himself, purging the bad traits of Eisav and creating the third element of the "three fold cord" that can no loger be broken.

Answer 2

On that Pasuk, Rashi writes, "על ידי שכתב הכתוב יצחק בן אברהם הוזקק לומר אברהם הוליד את יצחק, לפי שהיו ליצני הדור אומרים מאבימלך נתעברה שרה, שהרי כמה שנים שהתה עם אברהם ולא נתעברה הימנו, מה עשה הקב"ה, צר קלסתר פניו של יצחק דומה לאברהם, והעידו הכל אברהם הוליד את יצחק, וזהו שכתב כאן יצחק בן אברהם היה, שהרי עדות יש שאברהם הוליד את יצחק:" Rashi gives two explanations.

1. Only after Hashem named Avram Avraham, he had Yitzchak.

2. People might have doubted that Yitzchak was Avraham's son because only after Sarah was taken by Avimelech that she had a baby. So Hashem made Yitzchak look so much like Avraham, that everyone had to say, "אַבְרָהָ֖ם הוֹלִ֥יד אֶת־יִצְחָֽק:" (Avraham begot Yitzchak!).