I talk about the meaning of toldos
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
יב. וְאֵלֶּה תֹּלְדֹת יִשְׁמָעֵאל בֶּן אַבְרָהָם אֲשֶׁר יָלְדָה הָגָר הַמִּצְרִית שִׁפְחַת שָׂרָה לְאַבְרָהָם:
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:
- 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.