תולדות

“These are the children of Yitzchak ben Avraham. Avraham fathered Yitzchak” (Bereishis 25:19). The question is, after the Torah mentions that Yitzchak was ben Avraham, why does it need to say after that Avraham fathered Yitzchak? If I tell you Yitzchak is ben Avraham, most geniuses, even simple folks like me and you, could figure out that Avraham fathered Yitzchak.