Bekhorot 34: three things to remember

Shmuel permits slaughtering and eating a dying firstborn whose blood was drawn medically, even though the procedure left a lasting blemish.
The Rabbis permit slaughtering an intentionally blemished firstborn only after a separate, natural blemish subsequently develops on the animal.
Rabbinic penalties target the transgressor rather than his property, so a son may slaughter a firstborn his deceased father intentionally blemished.