The Torah in Parashat Shemini introduces the law forbidding eating the meat of animals that do not feature two specific characteristics: it chews its cud, and has split hooves (11:3). In the subsequent verses, the Torah makes it clear that an animal must feature both properties for its meat to be permissible for consumption; an animal with one characteristic but not the other is forbidden.
A number of Acharonim addressed the conceptual question as to how these two characteristics are to be viewed. Namely, are these features themselves the reason why an animal is permissible, or do they merely reflect this status? The first approach contends that a cow, for example, is kosher because it chews its cud and has split hooves; according to the second approach, a cow is kosher for some other, perhaps unknown, reason, and these two properties are simply indicators of the animal’s permissible status.
Rav Menachem Zemba, in Zera Avraham (14:24), notes that this question likely hinges on a debate among the Tanna’im recorded in Masekhet Bekhorot. The Mishna (5b) establishes the rule that “ha-yotzei min ha-tahor tahor” – the offspring of a kosher animal is permissible for consumption, even if it has the physical properties of a non-kosher animal. For example, if a cow gave birth to a creature which looks like a donkey, the meat of that offspring is permissible, even though it does not have the physical properties required of a kosher animal. Since it was born to a cow, it is considered a member of a kosher species and is thus permissible for consumption, despite its physical properties. The Gemara (6b), however, cites Rabbi Shimon as disputing this ruling. In his view, an animal that does not feature the two characteristics of a kosher animal may not be eaten, regardless of its biological origins. The same is true in the reverse case, where a non-kosher animal, such as a horse, delivers a child resembling a kosher animal, such as a cow. According to the majority view, the offspring may not be eaten, since it was born to a non-kosher animal, whereas Rabbi Shimon would permit eating this animal, since it features the characteristics of a kosher animal.
It seems, Rav Zemba writes, that these two views reflect the question of whether the properties of a kosher animal should be viewed as a siman – an indicator of its kosher status – or a siba – the reason for this status. The majority opinion likely views these properties as merely indicators of a kosher species. As such, once we know that an animal was born to a member of a kosher species, its own physical properties are irrelevant. After all, these properties are not inherently required for an animal to be deemed kosher; they are needed only to determine the animal’s status. And so if this status can be determined based on the animal’s mother, we have no need to examine its own properties. Rabbi Shimon, however, appears to have understood that the properties of a kosher animal are the reason for an animal’s permissible status. As such, according to Rabbi Shimon, an animal must feature these characteristics for it to be deemed kosher, and its biological origins are irrelevant.
Halakha follows the majority view, that the offspring of a kosher animal is kosher, and the offspring of a non-kosher animal is not kosher, regardless of the child’s physical properties (Shulchan Arukh, Y.D. 79:2).
A possible modern application of this issue would be genetically modified organisms. If scientists modify the genes of the fetus of a kosher animal such that it does not feature the properties of a kosher animal, it would, at first glance, be permissible, in line with the accepted view that the determining factor is an animal’s biological origins, and not its own physical characteristics. Conversely, if the offspring of a non-kosher animal is modified such that it chews its cud and has split hooves, it would, presumably, nevertheless be forbidden for consumption, as it was born to a non-kosher animal. (Needless to say, this issue requires a far more comprehensive discussion.)