Genetic Engineering in Halacha – Part 1 by Ariel Caplan


One of the most exciting areas of modern Halachic literature concerns the application of the Halachic system, as laid out in the Mishnah and Gemara almost two millennia ago, to modern-day realities. As technology has progressed – as cars replaced horses, light bulbs replaced candles, and automation replaced certain manual labors – rabbis in each generation have taken on the challenge of applying old rules in new contexts. Halachic concepts must be painstakingly defined with ultimate precision, and careful analysis of both the principles at work and the facts on the ground is used to guide a Poseik to the appropriate conclusion.

One particularly fascinating area is that of genetic engineering, which involves manipulation of an organism’s DNA, the molecule present in each cell of an organism (i.e. living being) that guides the structural development and functioning of the organism. Through genetic engineering, scientists are able to induce structural or functional changes in plants and animals.  Plants can be reprogrammed to grow stronger and healthier, resist bacterial infection and bug infestations, or even produce tastier or more nutritious fruits and vegetables.  Genetic engineering may be done for experimental purposes, such as testing the function of a section of DNA by disabling it. It also plays a role in medicine, as in the usage of bacteria, plants, or small animals to produce a protein or substance needed by humans, such as insulin for diabetics. (While genetic engineering techniques may also be used to alter human characteristics, we will not address the associated halachot and moral dilemmas in this essay, as the issues in that area are very different from the problems that arise in other areas of genetic engineering.) As in all such situations, a clear understanding of the principles and mechanisms at work is essential. Therefore, we will begin with a brief explanation of the genetic engineering process.

The Science Behind Genetic Engineering

The body of any organism, whether human, plant, animal, bacterium, fungus, or anything else, is made up of cells. Each cell contains one or more large molecules called DNA (deoxyribonucleic acid), which constitute a blueprint of the entire organism. These large molecules, which are long double chains of four different nucleic acids known as A (adenine), T (thymine), C (cytosine), and G (guanine), contain functional segments called genes. Genes are instructions for the production of proteins, which are the molecular machines that carry out physical and chemical processes within the cell. On an as-needed basis, functional molecules within the cell “read” the genes and translate the information they contain into proteins.

Using modern technology, it is possible for scientists to isolate a specific section of DNA, create many copies, and introduce this section of DNA into other species. The goal may be simply to insert a gene that encodes a protein that scientists want the species to produce. For example, scientists have created pigs that express a green fluorescent (glow-in-the-dark) pigment-producing protein in their noses and ears, with the result that their noses and ears, instead of being pink, are green and glowing. Sometimes, the goal is more subtle; taking advantage of the complex network of interactions between genes, scientists may introduce a gene that will increase or decrease the expression of an already existing gene (or silence it entirely). In one such case, enhanced expression of a gene encoding a sodium pump, which isolates sodium within a vacuole (storage chamber) inside the cell, has enabled farmers to grow plants which thrive in high-sodium soil.

The introduction of new genetic material can be accomplished in various ways. One method, used in plants and animals, is the use of a gene gun, which directly bombards the target organism with genes encased in metal shells. Another is the intentional infection of a plant with genetically modified bacteria or viruses, which insert their DNA into plant cells. Electroporation, where holes in the cell membranes are created using electric shock, can be used with certain plants to leave openings to the cell through which DNA can seep in. In animals, a variety of compounds can be attached to the DNA, allowing the DNA to pass through the cell membrane and become part of the cell.

When the engineering is geared toward creating a new organism, a cell which has successfully taken in the introduced gene is isolated. This cell is grown under laboratory conditions; in animals, it will be implanted into a female for full development, whereas in plants, it will be grown under laboratory conditions into an entire, new plant.

We should note that there are no specifications as to which organisms can donate to each other (although certain transfers are more difficult to carry out). For example, plants can donate genes to animals or fungi as easily as they can to other plants. Hence, a fish gene put into a plant or a fungus would be perfectly standard practice.

The Right and the Good

As Jews, our actions and policies should always be guided by two considerations, which are described beautifully in Ramban’s comments on the Pasuk (Devarim 6:18), “You shall do what is right and good before Hashem your God.” Based on the Midrash, Ramban explains, “Initially [the Torah] said you should guard [Hashem’s] statutes and laws as He commanded you, and now it says that even in that which He did not command you, apply yourself to doing the good and right in His eyes.” In the continuation of this piece, Ramban focuses on Mitzvot Bein Adam LaChaveiro, but the idea should apply equally to Mitzvot Bein Adam LaMakom. Hence, in any context, we must be alert to both the Halachic constraints placed upon us and the ethical values promoted by the Torah.

In this spirit, we will begin with discussion of the Halachic issues involved in genetic engineering and follow up with an analysis of the moral questions that arise in this area. This week, we will present the basic details of relevant prohibitions. Next week, we will deal directly with the issues that arise in the context of genetic engineering.


If there is a Halachic prohibition against genetic engineering, it would most likely be found among the five prohibitions of mixing species, which fall under the general heading of Kilayim, or “mixture.” These prohibitions emerge from Pesukim in VaYikra and Devarim. VaYikra 19:19 states, “Keep my Chukim [laws]; do not breed your animal as Kilayim, do not plant your field as Kilayim, and a Kilayim garment of Shaatnez shall not be upon you.” Devarim 22:9-11 adds, “Do not plant your vineyard as Kilayim, lest the product of the seed you plant and the crop of the vineyard be forbidden. Do not plow with an ox and a donkey together. Do not wear Shaatnez, [which is] wool and linen together.”

Five prohibitions emerge from these Pesukim: a) breeding animals of different species together, b) plowing with animals of different species together, c) planting a field of any species with mixed seeds in close proximity, d) planting mixed species in a vineyard, and e) wearing a garment which contains mixed wool and linen fibers. To decide if genetic engineering violates halacha, we will have to examine the prohibitions against crossbreeding animals and mixing plants.

Several details of these prohibitions merit special consideration. First, Kiddushin 39a records a statement of Shmuel that the juxtaposition of the prohibitions of crossbreeding animals and mixing plants warrants derivation of details from one to the other. Hence, just as it is forbidden to physically bring animals together, it is forbidden to bring plants together by grafting (a technique where the end of a branch of one plant is attached to the stump of the branch of another, creating a situation where the branch of one type of tree grows out of a different type of tree).

Although the prohibitions emerge from the same words in the Torah, there are important differences between planting species together and grafting. Rambam (Hilchot Kilayim 1:5-6) and Shulchan Aruch (Yoreh Deah 295:1-3) rule that standard Kilayim of plants does not apply to trees, while grafting applies to all plants. Additionally, while the standard Kilayim of plants is prohibited only in Eretz Yisrael (Rambam Hilchot Kilayim 1:1, Shulchan Aruch Y.D. 296:1), grafting is prohibited anywhere (Rambam Hilchot Kilayim 1:5, Shulchan Aruch Y.D. 295:1). It is therefore important to define a prohibited action as one or the other, as this will determine whether or not it is permitted outside of Eretz Yisrael, and whether or not trees are included in the prohibition.

Another significant issue is the specific definition of Kilayim of animals in Bava Metzia 91a. The Gemara there quotes Shmuel as limiting the Torah prohibition to physically forcing the animals to mate by manipulating the relevant organs. As Rambam notes (Hilchot Kilayim 9:1-2), encouraging animals of different species to mate through vocal commands or physical pushing of the whole animal is Rabbinically prohibited, while leaving the two animals in the same pen is completely permissible. This stands in stark contrast to Kilayim of plants, where simply allowing plants which became mixed (such as if a wall between two fields of different species collapsed) to continue to grow in this fashion is prohibited (see Bava Batra 2a). Based on this distinction, an action under consideration will more likely violate Kilayim of plants than Kilayim of animals.

It is also important to note the differences between the general prohibition on mixing seeds and the prohibition of mixing species in a vineyard. Mixing seeds is prohibited at any stage of growth with a penalty of lashes, even for an adult plant; in the context of Kilayim of the vineyard, one only earns a penalty of lashes for planting “wheat, barley, and grape seed [i.e. two species along with the grape seed] in one thrust of the hand” (Kiddushin 39a). (The Rishonim debate whether the entire prohibition or just the penalty of lashes depends on this style of violation.) On the other hand, the products of Kilayim of animals or plants, are permitted for use (Rambam Hilchot Kilayim 1:7 and 9:3, Shulchan Aruch Y.D. 295:7 and 297:5), but the products of Kilayim of the vineyard are forbidden and must be burned, even if the mixture took place after planting (see, for example, Mishnah Kilayim 7:4). It emerges that if we can define an action as Kilayim of the vineyard, the products will be forbidden for use, whereas if it is defined as Kilayim of plants, the products will be permitted for use post facto. (We will not address the post facto Halachic implications of genetic engineering in this series. Still, it bears mentioning that the principle that Kilayim products are usually permitted post facto is very relevant for the potential consumer of genetically engineered meat or produce, or – for that matter – hybridized plants like grapefruit.) The distinction has an additional ramification, as Rambam (Hilchot Kilayim 5:3) and Shulchan Aruch (Y.D. 296:1) record a Rabbinic prohibition to plant Kilayim of the vineyard outside of Eretz Yisrael, even though no such Rabbinic prohibition was enacted for standard Kilayim.

Finally, we should note that Kilayim of plants does not apply across the board to all plants. Rather, as codified in Rambam (Hilchot Kilayim 1:4) and Shulchan Aruch (297:3), it applies specifically to plants fit for human consumption. However, grasses and the like, which are bitter and repulsive, are not subject to the Kilayim prohibitions. (See Mekorot VeTziyunim on Rambam, Frankel edition, Hilchot Kilayim 1:4 as to whether the exception is for specifically bitter species, or even for those which are not bitter if they are not normally eaten.) Hence, any discussion of Kilayim of plants will be limited to a mixture of plants which are derived from fruit-bearing species. However, Shulchan Aruch (Y.D. 295:3) writes that grafting applies equally to fruit-bearing and non-fruit-bearing species. This distinction presents another reason to determine whether a specific action is considered grafting or standard Kilayim, as only actions prohibited under the banner of grafting will be prohibited when it comes to non-fruit-bearing species.


When dealing with any issue in modern Halacha, an understanding of both the Halachic principles and the reality of the situation is essential in arriving at a proper ruling. Through the summaries above, we have hopefully achieved enough understanding of each to begin to address the interaction between the two. Next week, we will explore how recent Poskim have dealt with the Halachic challenges of genetic engineering.

Genetic Engineering in Halacha – Part 2 by Ariel Caplan

The Mesirah Dilemma – Part Three by Rabbi Chaim Jachter