Cloning: Where Do We Draw The Line?

September 4, 2025
Author: José Antonio García Iturriaga
Versión en español

In May 2025, the Universidad Autónoma Metropolitana of Xochimilco successfully cloned a sheep, placing Mexico on the map of reproductive biotechnologies. Beyond the scientific achievement, long-standing questions that have divided researchers, bioethicists, and legislators resurfaced: How far should cloning go? Which uses are acceptable, and which ones pose risks to society?

Cloning, once considered science fiction, is now a reality in livestock farming, species conservation, and even biomedical research. However, with each breakthrough, new ethical and social dilemmas surge. What seemed unattainable yesterday is now happening in our country, demanding that we stay informed.

In this context, opening public debate is essential. Cloning can be a valuable tool to preserve local breeds, guarantee food security, and even advance regenerative medicine, though it can also cross delicate lines. This is why it is important to understand how it works and what applications it has, since it is an issue that involves society as a whole.

In biology, cloning refers to the reproduction of organisms in the absence of sexual interaction. As a result, the offspring are not a mixture of their parents’ characteristics, but rather genetically identical copies of the organism from which they originated. These copies are produced using in vitro techniques, of which the best-known are: somatic cell nuclear transfer (SCNT), embryo splitting, and simplified variants of SCNT (Handmade Cloning). ¹

• Somatic cell nuclear transfer (SCNT): An oocyte is taken, and its nucleus is removed. This egg without a nucleus is then implanted with the nucleus of a somatic cell (for example, a skin cell). Finally, the reconstructed oocyte is activated through electrical or chemical stimulation and implanted into the recipient female. The newborn will be a clone of the nucleus donor.

This method was used to create Dolly the sheep in 1996. ²

• Embryo splitting: This technique consists of dividing an embryo at a very early stage, when its cells are still totipotent, that is, that it is still capable of generating a complete organism, including extraembryonic tissues. There is no nuclear reprogramming; it is simply the partition of the same embryo. Essentially, it replicates the natural process of monozygotic twinning.

It is mainly used to multiply valuable embryos. ¹

• Simplified variants of SCNT (Handmade Cloning, HMC): This is a simpler and less expensive version of SCNT. In this method, the zona pellucida of the oocyte is removed, and the oocyte is manually divided into two (cytoplasts). These two halves are then fused with a donor somatic cell that provides the nucleus. ³

Throughout history, these techniques have led to scientific milestones that have evolved year after year and generated various advances in cloning. The following chronology summarizes some of the most relevant experiments:

• 1996: Birth of Dolly the sheep. She was the first mammal cloned using the SCNT technique at the Roslin Institute, demonstrating that the genes of a nucleus from a mature and differentiated somatic cell are capable of reverting to an embryonic state, creating a cell that can develop into any tissue of an animal.⁴

• 1999: Transgenic cloned goats. SCNT was combined with genetic engineering, where a human gene was introduced into laboratory-grown goat cells, resulting in transgenic clones. This experiment gave rise to “animal pharmaceuticals.” ⁵

• 2000: First cloned pigs. The same institute that carried out Dolly’s cloning announced that it had cloned pigs using the same technique. Since they many anatomical and physiological similarities with humans, they became ideal candidates for xenotransplantation (transplantation of organs from one species to another). ⁶

• 2001: Cloning of a gaur (“Noah”). The gaur population was in decline by the end of the 20th century. The SCNT technique was used, using with domestic cows as surrogate mothers and cells from a male gaur that had died in 1993, preserved in liquid nitrogen (interspecies cloning). Noah died two days after birth due to an infection. This experiment showed that it was possible to revive endangered species using stored cells and surrogate mothers from related species. ⁷

• 2005: First cloned dog (“Snuppy”). Although several mammal species had already been cloned, the cloning of dogs was a special milestone due to their complex reproductive biology (irregular reproductive cycles and post-ovulation oocyte maturation). Snuppy was born using SCNT, lived for 10 years, and successfully reproduced. This experiment opened the debate on pet cloning. ⁸

• 2013: SCNT stem cells. SCNT was applied not to clone babies, but rather to obtain pluripotent cell lines genetically identical to the donor. This achievement opened new possibilities for regenerative medicine and marked a major leap for therapeutic cloning (different from reproductive cloning).⁹

• 2018: First primates cloned by SCNT. Previous attempts to clone primates had failed due to difficulties with epigenetic reprogramming. A team from the Institute of Neuroscience of the Chinese Academy of Sciences adjusted the technique to overcome earlier obstacles and created Zhong Zhong and Hua Hua. Because primates are very close to humans, their birth sparked discussions about the limits of cloning and the need for clear ethical frameworks. ¹⁰

• 2024: First rhesus monkey cloned to reach adulthood. Cloning efficiency in primates remained very low, and many did not survive the neonatal stage. The major barrier laid in the placental trophoblast. In this case, the cloned trophoblast was replaced with a healthy one obtained from a normally fertilized embryo. For the first time, a primate reached adulthood, representing a crucial advance for generating genetically identical biomedical models, while also intensifying the ethical debate. ¹¹

Although animal cloning has been developing for decades in other countries, Mexico had not reported a viable mammal clone until recently. In May 2025, the Universidad Autónoma Metropolitana of Xochimilco, together with the Universidad Autónoma de Chapingo, announced the successful cloning of sheep using SCNT. Two sheep were cloned, and one of them was presented publicly at two months of age, apparently healthy, without deformities or anomalies. ¹²

This achievement positions Mexico among the few Latin American countries with applied cloning capacity (this list includes Argentina, Brazil, Peru, and Colombia). ¹³ It also opens the door to genetic improvement in Mexican livestock farming, as well as the conservation of local breeds.¹² 

Given these advances, it is relevant to examine the current position of countries regarding cloning. Globally, human reproductive cloning is prohibited in all countries, either through specific national laws or international treaties.

Some of these treaties include:

• Universal Declaration on the Human Genome and Human Rights, UNESCO: Article 11 states that “practices contrary to human dignity, such as reproductive cloning of human beings, shall not be permitted.” ¹⁴

• Additional Protocol to the Oviedo Convention, Council of Europe: Article 1 prohibits “any intervention seeking to create a human being genetically identical to another human being, whether living or dead.”¹⁵

Several countries allow therapeutic cloning for research purposes, while others explicitly prohibit it. In any case, there is a universal consensus in rejecting human cloning for reproductive purposes due to its ethical, social, and human dignity implications. Some of the regulatory frameworks in key countries regarding these techniques are:

1. United Kingdom: The HFEA can authorize SCNT protocols for research.¹⁶

2. Australia: Since 2006, SCNT has been permitted under license and must be reviewed by national ethics and research committees.¹⁷ 

3. Japan: Prohibits the creation and transfer of cloned embryos into a human uterus, regulates specified embryos (those created through non-conventional techniques) and hybrid/chimeric embryos (mixes of human and animal cells)¹⁸. 

4. Israel: Periodically updates its cloning-related laws and regulates research protocols.¹⁹

5. Canada: The Assisted Human Reproduction Act prohibits all forms of human cloning, including therapeutic cloning.²⁰ 

6. United States: There is no total federal ban, but restrictions on funding exist, alongside state-level laws (some states prohibit all cloning, while others allow research and therapeutic cloning).²¹

7. China: Restricts the manipulation of human embryos but permits and funds animal cloning.

8. Mexico: The General Health Law prohibits human cloning in Article 100 bis, although animal cloning is legal and regulated within agricultural research.²²

Beyond the legal regulations and national stances, cloning opens a field of ethical questions that cannot be ignored. Bioethical dilemmas have become inseparable from scientific research, and cloning sparks debates because it touches on issues such as dignity, identity, and kinship. The ability to copy a human being identical to another risks reducing the person to a technical product rather than a unique and unrepeatable subject. This idea raises profound questions about what it means to be someone in a world where genetic singularity is no longer unquestionable.

Another important bioethical concern relates to animal welfare. Despite reports of numerous healthy clones that have lived for many years, cloning remains a low-efficiency procedure in which most embryos do not survive and perinatal loss rates are high. This highlights not only a technical problem but also an ethical dilemma, since these procedures often expose animals to painful interventions and leading lives with malformations or even metabolic failures.

The issue touches on issues of justice and access. For example, cloning competition horses or even pets has become a service available only to those who can afford the high costs involved. This socioeconomic bias turns biotechnology into a privilege rather than a shared resource. Thus, the value of cloning ends up being measured more by its market price than by its role in an ecosystem or community. This poses the risk that science may orient itself more toward a luxury market than toward the common good, displacing urgent priorities.

Talking about cloning means talking about limits. It is not only about what science can achieve, but about how far we are willing to allow it to go. Human reproductive cloning is a clear example, since there is a universal consensus that this line must not be crossed, as it would compromise dignity, identity, and kinship. However, when discussing cloning for research, therapeutic, or even animal purposes, that line becomes more flexible. Low efficiency, animal suffering, and the commercial use of clones are reminders that the boundary is not fixed and must be redefined with each new advance.

The bioethical challenge is to learn to recognize the point at which knowledge stops expanding horizons and begins to put fundamental values at risk. Every new technique forces us to ask ourselves: Does this contribute to the common good, or is it just another market product? Should we really take this step if we do not yet know the long-term consequences for the beings involved? And what kind of society do we want to be, one that celebrates the uniqueness of each life, or one that normalizes its reproduction as just another product?

The real challenge lies in maintaining an open and informed dialogue that allows society to decide where the line is and why it is worth respecting.

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José Antonio García Iturriaga is a graduate of Medicine from Universidad Anáhuac México Norte, with a strong interest in medical genetics and bioethics. He is currently completing his social service at CADEBI, where he participates in projects that integrate clinical ethics with health initiatives. His long-term goal is to specialize in medical genetics. This article was assisted in its writing using ChatGPT, an artificial intelligence tool developed by OpenAI. 

The opinions expressed in this blog are the sole responsibility of the authors and do not necessarily represent the official stance of CADEBI. As an institution committed to inclusion and plural dialogue, at CADEBI we promote and disseminate a diversity of voices and perspectives, convinced that respectful and critical exchange enriches our academic and educational work. We value and encourage all comments, responses, or constructive criticism you may wish to share.