Genesis 25:19

"These are the generatations of Yitzchak": Yaakov and Eisav who are spoken of (further on) in the passage

"Avraham fathered Yitzchak": Since the Scriptures wrote "Yitzchak son of Avraham",it was compelled to say "Avraham fathered Yitzchak". For the scoffers of the generation said that Sarah became pregnant from Avimelech, for after all, she resided many years with Avraham and didn't become pregnant from him. What did the Hakadosh Baruch Hu do? He formed Yitzchak's visage to resemble that of Avraham, and so all testified "Avraham fathered Yitzchak". And this is what is written here, that Yitzchak was the son of Avraham, for there was testimony that Avraham fathered Yitzchak.

These are the generations of Izhak bar Abraham. And because the appearance of Izhak resembled the appearance of Abraham, the sons of men said, In truth Abraham begat Izhak.

AND THESE ARE THE GENERATIONS OF ISAAC, ABRAHAM’s SON. Scripture says this of Esau and Jacob, is not to be understood in the broad sense of “generations” but in the more specific sense of “children.” Thus the verse reads, And these are the children of Isaac, namely, Esau and Jacob mentioned further on. Compare Ramban at beginning of Seder Noach. Isaac’s sons who are mentioned further on. Scripture further mentions the circumstances of their birth.

אברהם הוליד את יצחק, according to the plain meaning of the text this statement was necessary to prevent the wrong impression being created by the previous statement in verse 12 where Ishmael is described merely as having been born by Hagar the Egyptian woman. Here when Avraham’s principal son, Yitzchok is the subject of the paragraph, it was important to repeat who had fathered him. He was the son of Avraham’s true wife, his lifetime companion, and G’d had predicted in Genesis 21,12 that Avraham’s descendants would always be known through his son Yitzchok and his descendants. We have a similar verse in Chronicles I 1,28 describing Yitzchok and Ishmael as Avraham’s sons, [but not in the chronological order of their births, to remind us that the principle of preference for the firstborn was ignored. Ed.] In connection with the sons of Keturah, described as the concubine of Avraham, Chronicles does not even spell out that Avraham fathered Keturah’s sons. In fact, in verse 34 of that chapter in Chronicles, the fact that Avraham fathered Yitzchok is repeated once more, no such repetition being made concerning Ishmael.

ואלה תולדת יצחק, his biography and his descendants.

אברהם הוליד את יצחק, only Yitzchok is truly called Avraham’s seed.

ואלה תולדת אברהם יצחק בן אברהם אברהם. Whereas the Torah when listing Ishmael’s descendants had restricted itself to a brief summary, now when reporting about Yitzchok, the Torah elaborates a great deal more both about Yaakov and about his brother Esau.

אברהם הוליד את יצחק, we are told in Baba Metzia 86 that Yitzchok’s facial features were so similar to those of his father that anyone ever encountering Yitzchok immediately knew that he must be the son of Avraham. The reason that this had been arranged so by G’d was that when someone claims to have become a father in his old age, some people tend to doubt the “father’s” claim, assuming that the woman who had born that child must have been unfaithful to her husband, or that at least the baby now presented as this father’s was in fact a foundling. Yitzchok’s amazing similarity to his father precluded anyone from making such spurious accusations. Another reason that the Torah repeated something which we all knew, i.e. that Avraham had sired Yitzchok, was that Yitzchok possessed the same virtues and wonderful qualities which Avraham excelled in, so that it was clear who must have been his father not only physically, but that the same father also transmitted a spiritual legacy to his son.

יצחק בן אברהם, אברהם הוליד את יצחק, “Yitzchok the son of Avraham; Avraham sired Yitzchok. The numerical value of the letters in the word הוליד, is equivalent to the numerical value of the word דומה, “to be a look a like;” it suggests that Avraham and Yitzchok’s facial features made them look as if they were clones.