ספר בראשית

Genesis ‎25,19 “And these are the generations of Yitzchok, the son ‎of Avraham; Avraham begot Yitzchok.” (We have been told in ‎Genesis 21,12 ‎כי ביצחק יקרא לך זרע‎, “for your seed (descendants) ‎will be known through Yitzchok.” G’d told Avraham already ‎before Yitzchok was born that although he had another son, ‎Yishmael, his descendants would always be associated with ‎Yitzchok. It was understood that this promise was contingent on ‎Yitzchok becoming a righteous person, a tzaddik, future ‎generations would not trace themselves back to their ancestor ‎Avraham but each generation would only trace itself back to their ‎immediate forbears, i.e. their fathers. In other words, the new ‎element provided by our verse above is that even Yitzchok’s ‎offspring would trace themselves back to their founding patriarch ‎Avraham. We learn from here also that it is up to the “son” to ‎demonstrate by his deeds that he was not only descended from ‎his father but could claim previous generations as his “roots.” ‎When we consider this, the word ‎תולדה‎ is no longer an adjective, ‎an attribute of a person which he came by naturally, without any ‎input of his own, but it is a tribute to the person so described, ‎meaning that he is a worthy descendant of his illustrious forbears.‎