Parashas Vayeishev · Passuk 37:2
These are the generations of Yaakov: Yosef, being seventeen years old, was feeding the flock with his brothers; and he was a lad with the sons of Bilhah, and with the sons of Zilpah, his father’s wives; and Yosef brought their evil report unto their father.
Torah Temimah points to Bava Basra 123a.
English: Rabbi Yonasan said: The birthright was fit to issue from Rachel, as it is stated “These are the generations of Yaakov — Yosef,” but Leah preceded her through mercy.
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The verse’s focus on Yosef is taken as evidence that the birthright properly belonged with Rachel’s line.
The birthright was originally destined for Rachel’s firstborn. Leah’s earlier childbirth, granted through divine mercy, transferred it, yet the verse still marks Yosef as the primary “generations of Yaakov.”