אלה תולדות יצחק בן אברהםת אברהם הוליד את יצחקץ
These are the offspring of Yitzchak son of Avraham, Avraham fathered Yitzchak.There are those, such as רש"י who connect this with the incident in G'ror when Avimelech kidnapped Sarah. The mockers of that generation pretended that Avrohom was "too old" to father a son with a women ten years yonger than him (since Sarah was only 90). Rashi quotes the Medrash that Hashem made Yitzchak resemble Avraham exactly so that he would be shown to be Avrohom's son. However, there is an alternate explanation.
Rabbi Silbur explained that this is similar to the description of Aharon's sons as the sons of "Aharon and Moshe". That is, a student is also his teacher's "son". Yitzchak was not only the "physical" child of Avrohom, but also the spiritual heir as well. Avrohom was the teacher who formed his son's spiritual and mental being. This enabled Yitzchak to continue the development of what would become the "Children of Israel" and fom the second leg of the triplet that would support us.
We say that the world is supported by three "legs': Torah, Avodah, and Gemilus Chasadim. We also say that the three patriarchs each exemplified a different trait.
Avrohom - Chesed or Midas Harachamim (emotionalism)
Yitzchak - Gevurah or Midas Hadin (rationality or logic)
Yaakov - Emes or Torah - the ability to meld the two traits into a whole that allows life to continue.
We are told that Yishmael and Eisav took the trait of their fathers (Chesed [emotionalism] and Gevurah logic]) and attempted to make them the cornerstone of their being without balancing it with other traits. As a result, they became flawed and could not be accepted as part of the line of descent that would become "Hashem'People". Avraham who had the trait of Chesed married Sarah who exhibited Gevurah when it was needed. The child of chesed (Yitzchak) was gevurah and he married Rivkah who exhibited the trait of Chesed so that he too was able to maintain a balance. He was able to absorb the teaching of his father and apply it to his life, becoming the second of the "three stranded rope" that would not break.
"Rabbi Scroll" points out that where Avrohom had many students, Yitzchak had just one, Yaakov. Yaakov had to absorb the traits of his father and his grandfather (who taught him until he was 15). He too had to learn to balnce and control the traits that he inherited from his parents.
Another point that needs to be made is that the Torah is very careful at the points which it uses the term "Toldos" regarding a person. The creation of the universe, Adam, Noach, Terach, Yitzchak. These are the significant break points in history.
Another point that we see is that Yitzchok and Yaakov brought back into the family the traits of Nachar and Haran Avrohom's brothers, sinc Milkah, Nachar's wife was the daughter of Haran. Similarly, we have the medrash that Iskah, Haran's other daughter, was Sarah. Thus, Yischak was actually the culmination of the line of Terach, and Yaakov was the full manifestion of the development of this line. When Yaakov married Leah and Rachel, he created the final aspect of this line. We see this in the other lines that are mentioned, as each of the final product has 12 children. For example, consider Nachar who had 8 children by his wife and 4 by his concubine, similar to Yaakov who had 8 children by his wives and four by the maid servants.
Avrohom and Yitzchak had children that inherited their "nature" but failed to absorb the "nurture" and continue the teachings. Avraham had to reject Yishmael so that he could properly bring up Yitzchak to follow him. THere is a medrash that Yitzchak was afraid that Eisav was "too red" and that circumcision would be a health hazard. As a result, he planned to wait until he was 13 years old, like Yishmael, before giving him a bris milah. At that point, Eisav refused, thereby making himself ineligible for continuing the line of Avraham. Yitzchok married Rivkah so that he would be able to balance his trait of Gevurah with his father's (and his wifes's) trait of Chesed. Yaakov absorbed both traits and married Rachel and Leah so that he was able to balance the "Olam Haemes" with the ability to live among the world as it is (as his wives managed to do).
We see therefore, that to succeed, me must balance "nature" and "nurture" and attempt to avoid the problems that giving too much emphasis to eother might cause.