Tiferet is associated with Jacob and Yesod with Joseph. What began with the father, bearing 12 sons was completed in Joseph, who established the proper connections.
“The Shekinah was thus leagued with Abraham, Isaac, and Jacob together with Joseph, inasmuch as the two latter are one in essence, each one being the image of the other, as indicated in the words: “These are the offspring of Jacob: Joseph” (Genesis 37:2).”
Tiferet, which ‘stands in’ for six of the seven middot, is considered the ‘body.’ Yesod, through the aspect of ‘connection’ is associated with the covenant of circumcision, the ‘Brit Milah,” or “Brit” for short.
“In Kabbalah, the third day, tiferet (“beauty”), is the origin of the sixth day, yesod (“foundation”). Tiferet and Yesod totally integrate in the secret of the “middle line” – “the body and the brit are considered one.”
The Mystical Signficance of the Hebrew Letters: Tzadik, The Faith of the Righteous One, Rabbi Yitzchak Ginsburgh