The Torah says Elazar son of Aharon married a daughter of Putiel, and she bore Phinehas (Exodus 6:25). Who is this Putiel that the Torah mentions nowhere else? Targum Pseudo-Jonathan on Exodus 6:25 answers without hesitation: Putiel is Jethro, Moses' father-in-law, the priest of Midian who left idolatry to serve the God of Israel.
The name Putiel, the midrashic reading suggests, comes from the Hebrew pitem, to fatten — Jethro once fattened calves for idolatry before he turned. His daughter, then, a granddaughter of a former idolater, becomes the mother of Phinehas, the priest whose single act of zeal halted the plague at Baal-Peor (Numbers 25).
The meturgeman is doing something daring here. He is saying that the priest who becomes the great defender of Israelite purity against foreign seduction was himself born from a convert household. The grandson of a former pagan priest becomes the zealot for the God of Israel. Phinehas's ancestry is not a shame to hide; it is proof that the covenant is open to anyone who chooses it fully.
The takeaway: a single ancestor's turn toward God can launch a dynasty of holiness. Jethro left Midian, and his great-grandson stood in the breach for all Israel.