The Limits of Cloning: How Many Times Can You Clone a Clone?

The 2006 film 'The Prestige' features a character who repeatedly clones himself as part of a magic trick, presenting a scenario of seemingly endless copies. This cinematic depiction is compared to the real-world phenomenon of repeated photocopying, where each successive copy degrades in quality.

In reality, the limits of cloning are being actively tested by science, particularly in mice. A landmark paper published in 2026 documented a new record of 58 generations of clones, but as researchers continued to push these limits, various problems began to emerge.

The term 'clone' was introduced in 1903 by plant physiologist Herbert Weller, who derived it from the Greek word 'klon,' meaning 'twig' or another plant cutting. Generally, cloning refers to the asexual creation of genetically identical copies of DNA sequences, living cells, or entire organisms.

Humans have engaged in cloning practices, such as growing plants from cuttings, long before the term existed. Furthermore, animals have naturally reproduced clonally through mechanisms like regeneration or virgin birth, as observed in species including flatworms, starfish, and some lizards. While humans cannot regenerate entire bodies from a single body part, artificial cloning has diverse applications, from cultivating organs for transplants to growing synthetic meat and aiding species conservation efforts, all of which are subjects of ongoing research and ethical debate.

Early cloning research primarily relied on embryonic cells, specifically those from very young embryos that were only a few cells large. These early embryonic cells possess the unique ability to develop into any tissue type, which is crucial for growing a complete organism with specialized parts. Cloning an animal from an older, specialized cell required scientists to 'reprogram' it, effectively reverting it to an immature, stem-like state, a significantly more challenging task.

A major breakthrough occurred in 1996 with Dolly the sheep, who became the first fully developed mammal cloned from adult tissue, specifically a cell from a mammary gland. Although not a complete clone due to mitochondrial DNA originating from the enucleated egg cell rather than the donor cell, Dolly was a success, living long enough to reproduce sexually and give birth to six lambs. Her birth followed 276 failed attempts of the same experiment, underscoring the technical difficulties involved in cloning from specialized adult cells. Despite Dolly not being serially cloned herself, scientists have applied serial cloning to other adult cloned animals.

Before 2004, serial cloning had only been successfully achieved in mice. However, in that year, scientists published a study detailing the successful cloning of a bull to the second generation (G2). This work built upon a 2000 study where researchers cultivated skin cells from a generation-zero Japanese beef bull, transferred their nuclei into fertilized eggs, and implanted the resulting embryos into surrogate cows, producing six cloned generation-one (G1) calves. The 2004 publication reported the successful production of one healthy G2 bull from a G1 calf.

Despite this success, when the team attempted to produce a third generation (G3), only approximately 20% of the embryo transfers reached the fetus stage, and all of these spontaneously aborted. This outcome challenged the notion that mammals could propagate their genetic lines indefinitely through serial cloning without sexual reproduction. Nevertheless, researchers persisted, and the G3 threshold in mammals was eventually surpassed.

A significant milestone in mammalian cloning was reached by 2013 when a mouse experiment, initiated in 2005, successfully produced almost 600 new mice across 25 generations. This success was largely attributed to the identification of specific small molecules that proved highly effective in reprogramming and reverting specialized cells back to an immature state, a crucial step for cloning.

These small molecules work by mechanically tweaking structural differences in specialized cells, removing accumulated chemical tags on DNA, and reconfiguring how DNA is packaged within the nucleus, enabling the cell to give rise to all tissues of a new body. Researchers also benefited from using cumulus cells for cloning, as other studies had previously discovered their nuclei are easier to reset than those of other specialized cells, such as skin cells. By the 25th generation, the cloned mice appeared fertile and capable of sexual reproduction, and the success rate did not show a clear decline, instead fluctuating. While some genetic errors appeared, they were not always harmful and did not accumulate irreversibly, leading scientists to cautiously consider the possibility of indefinite serial cloning under ideal conditions.

The mouse cloning study, which began in 2005, continued until 2026, ultimately producing over 1,200 mice across a record-breaking 58 generations. However, after the 25th generation, the success rates for producing live cloned pups began a downward trend, dropping to almost zero by the 58th generation, indicating that the complex hurdles became insurmountable.

Researchers were unable to pinpoint a single failure point; rather, the decline was attributed to the gradual accumulation of various genetic mutations and consequent problems with cell division, a phenomenon referred to as 'mutational load.' A critical issue observed in later generations was abnormal placenta development, which significantly hindered the survival of pups.

Serial clones accumulate harmful mutations at a significantly faster rate than sexually reproduced individuals. This difference is largely due to sexual reproduction's reliance on recombination, a process during which chromosomes shuffle genes as egg and sperm cells are formed. Recombination ensures each sex cell receives a unique, shuffled version of half an animal's genome, and importantly, it helps to randomly break up and prevent the propagation of harmful mutations.

Cloning bypasses this essential gene shuffling process. Consequently, after the 25th generation in the mouse study, genetic errors began to affect larger segments of the genome. These increasingly large errors eventually became deleterious, raising the likelihood of disease or disorder to the point where subsequent generations were no longer viable. Additionally, abnormal placenta development was frequently observed in later generations, further impeding the successful term birth of healthy pups. Despite these challenges, researchers maintain cautious optimism that a deeper understanding of all contributing factors could potentially enable serial cloning to continue indefinitely.

The findings of the mouse cloning study offer insights into prominent evolutionary theories. The authors suggest their results contradict the Red Queen Hypothesis, which posits that sexual reproduction is necessary for increasing genetic diversity and propagation, by demonstrating that asexual copying can introduce non-harmful genetic mutations that contribute to diversity. However, they also affirm that their work supports Muller’s ratchet theory.

Muller’s ratchet theory suggests that, in the absence of genetic recombination, successive generations of asexual reproduction will inevitably accumulate so many mutations that the clonal lineage will eventually go extinct. While some species in nature, such as certain Amazonian fish, have evolved mechanisms to circumvent this limitation and have cloned themselves for approximately 100,000 years, these solutions do not appear applicable to mammals. Therefore, for mammals, including mice, dogs, and potentially humans, Muller’s ratchet theory holds true, indicating a natural limit to indefinite serial cloning.