משנה אויף במדבר 18:17

If a ewe gave birth to what looked like a kid, or a [female] goat gave birth to what looked like a lamb, it is exempt from [the law of] the first born. But if it some of the signs of [its mother] it is liable [to the law of the first born].

The substitute of a first-born and an animal tithed, their young and the young of their young until the end of time, they are all treated like a first-born and an animal tithed, and are eaten by the owners when blemished. What is the difference between a first-born and an animal tithed [on the one hand] and other dedications [on the other]? All [blemished] dedications are sold in the market, killed in the market, and weighed by the pound, but not a first-born and an animal tithed. They [other dedications] and their substitutes are redeemed, but not a first-born and an animal tithed. They [other dedications] come from outside the land [to the land], but not a first-born and an animal tithed. [If] they however came from [outside the holy land] unblemished, they are offered, if blemished they are eaten by their owners with their blemishes. Rabbi Shimon: what is the reason? Because a first-born and an animal tithed have a remedy wherever they are, whereas all other dedications, although a blemish has occurred in them, remain holy.