Cloning Animals: How Dolly Changed Zoology

Cloning is the production of a genetically near-identical copy of an animal from a single cell, without fertilisation. The most famous clone in history is a sheep called Dolly, born in Scotland in 1996. Dolly was made from a single cell taken from the udder of an adult ewe, and her birth proved something that many biologists had thought impossible: that the nucleus of a fully grown, specialised adult cell still contains all the instructions needed to build a whole animal. A skin cell, a udder cell, or a muscle cell is not a dead end. Given the right conditions, its DNA can be switched back to the beginning and run the entire programme of development again, from a single cell to a lamb.

Cloning is older than Dolly, and it is also older than the laboratory. Nature clones animals all the time. Identical twins are natural clones: two animals with the same genome, made when one embryo splits in two. Some animals reproduce by cloning themselves outright, producing daughters that are genetic copies of their mothers, with no male involved at all. Gardeners have cloned plants for centuries by taking cuttings, and every cutting is a genetic copy of its parent. What Dolly changed was not the idea of a clone but the source of the copy. Before her, clones came from embryos or from eggs. Dolly came from an adult, and that single fact changed genetics, medicine, farming, conservation, and the argument about what a clone really is.

This article covers the whole subject at full depth: the natural clones that need no laboratory, the methods that make clones in one, step by step, the long road to Dolly and the animals cloned since, the reasons cloning so often fails, the question of whether Dolly aged too fast, the real uses in farming and medicine, the conservation cases that have already saved animals from extinction, and the ethics that still divide people. The science is explained in plain language, and every claim about success and failure is given with the numbers that support it, because cloning is a subject where the numbers matter more than the headlines.

Cloning at a Glance

A clone is an animal that is genetically almost identical to another animal, called the donor. The word "almost" matters, and it is the first thing to understand. A clone is not a photocopy. It carries the donor's nuclear DNA, the DNA in the nucleus of the cell, which is nearly all of the animal's genome. But it does not carry the donor's mitochondrial DNA, the small amount of DNA in the cell's mitochondria, because the mitochondria come from the egg that the clone grows in, and that egg belongs to a different animal. A clone also does not share the donor's life: it is born at a different time, in a different womb, eating different food, meeting different microbes, and its genes are switched on and off by a different environment. Two animals can share a genome and still differ, just as identical twins differ, in fingerprints, in personality, in health, and in luck.

Cloning is also not the same as genetic engineering. A clone is not a new kind of animal. It is a copy of an animal that already existed, with the same genes in the same arrangement, and no new gene added. Genetic engineering, by contrast, changes the genes themselves, adding a gene from another species or switching one off. The two are often confused, because the same laboratories do both, and because a cloned animal can also be genetically engineered, but they are different operations. Cloning copies a genome. Genetic engineering edits one.

There are three kinds of cloning, and keeping them apart clears up most of the confusion around the subject. Reproductive cloning makes a whole animal, a lamb, a calf, a puppy, that is born and lives its life. That is what Dolly was. Therapeutic cloning makes an embryo and then takes stem cells from it, and the embryo is never allowed to develop into a baby; the stem cells are the product, and they can be used to grow tissues for medicine. Gene cloning, also called DNA cloning or molecular cloning, makes copies of a single gene or a short stretch of DNA, and it is a routine laboratory technique used in every genetics laboratory in the world. When newspapers say "cloning" without qualification, they almost always mean reproductive cloning, and that is the subject of most of this article, but the other two matter, and they are covered in their own sections.

The numbers are the honest measure of cloning. In the best modern work, roughly one to five in every hundred cloned embryos become a healthy live animal, and the rest are lost: they fail to develop, they miscarry, or the newborn dies soon after birth. Dolly herself was one live birth out of 277 attempts. Those numbers have improved since 1996, and cattle cloning now succeeds more often than sheep cloning did, but cloning has never become routine, and it is still far less reliable than ordinary breeding. Every use of cloning, in farming, in pets, in conservation, has to be weighed against that failure rate, and against the welfare of the animals that carry the failed pregnancies.

Natural Clones: Cloning Without a Laboratory

Long before any laboratory made a clone, nature was doing it, and the most familiar natural clones are identical twins. Identical twins form when a single fertilised egg splits into two embryos in the first few days after fertilisation, and each embryo then develops into a separate animal with the same genome. In humans, about one in every 250 births is a pair of identical twins. In cattle, identical twins occur naturally in a small number of births. In the nine-banded armadillo, something stranger happens: a single fertilised egg routinely splits into four embryos, so every litter is a set of four identical quadruplets, all the same sex, all genetically identical. The armadillo does by habit what a cloning laboratory does by design, and it has been doing it for millions of years.

Some animals go further and reproduce by cloning as their normal way of life. This is called parthenogenesis, from the Greek for "virgin birth", and it means that a female produces offspring from unfertilised eggs, and the daughters are genetic copies of their mother. The whiptail lizards of the American south-west are the classic case: the whole species is female, there are no males at all, and every lizard is a clone of her mother. The water flea Daphnia switches between cloning and sexual reproduction with the seasons: in a good summer, the females clone themselves, producing daughters that are copies of themselves, and when the pond begins to dry or the food runs out, they switch to sexual reproduction and produce males and resting eggs that can survive the winter. The marbled crayfish, discovered in the German pet trade in the 1990s, is a single clone: every marbled crayfish in the world is genetically identical, descended from one female, and the species has spread across Europe and Madagascar as a single genetic copy of itself.

Even animals that reproduce sexually can produce clones by accident. A human identical twin is a clone, and so is a twin calf. A polyp of a jellyfish buds off copies of itself. A starfish that loses an arm can regrow the whole animal from the arm, and the new animal is a clone of the old one. The planarian flatworm, famous in regeneration research, can be cut into pieces, and each piece grows into a whole worm, genetically identical to the original. These are all clones, made without a laboratory, and they show that cloning is not an invention but a capacity that life already possesses, and that the laboratory has only learned to trigger on demand.

Plants are the masters of cloning, and the contrast with animals is instructive. A gardener who takes a cutting from a rose bush and roots it has cloned the rose: the new plant is genetically identical to the old one, and every rose of that variety in every garden in the world is a clone of the original plant. A strawberry plant sends out runners, and each runner is a clone. A potato is a clone of the plant that made it. Plants clone easily because their cells stay flexible: a leaf cell can become a root, and a root cell can become a shoot, and the whole plant can be rebuilt from a piece. Animal cells are different. Once an animal cell has become a skin cell or a muscle cell, it has switched off most of its genes, and switching them back on again is the hard problem that cloning had to solve. That is why Dolly mattered: she proved that an animal cell, like a plant cell, could be reset.

What Cloning Is Not

The first misunderstanding is that a clone is the same animal, reborn. It is not. A clone shares the donor's nuclear genome, but it is a different animal: it is born at a different time, it grows in a different womb, it is raised by a different mother, and its genes are switched on and off by a different environment. The clone of a prize-winning dog will look like the dog, and it will share the dog's genes for coat colour and size, but it will not share the dog's memories, its training, or its temperament, because those are made by experience, not by DNA. Pet cloning companies are careful to say this, and owners who expect their dead pet back are disappointed. A clone is a twin born later, not a resurrection.

The second misunderstanding is that a clone is a perfect copy. It is not, and the reason is the mitochondria. A clone is made by putting the donor's nucleus into an egg from another animal, and the egg brings its own mitochondria, with their own small genome. The clone's mitochondrial DNA therefore comes from the egg donor, not from the nuclear donor, and the clone is a genetic mixture: nuclear DNA from one animal, mitochondrial DNA from another. The difference is small, about 37 genes out of roughly 20,000, but it is real, and it affects how the animal makes energy. In cattle, the mitochondrial difference is one reason cloned calves can differ from their donors in growth and health, even when their nuclear DNA is identical.

The third misunderstanding is that cloning creates new species or new genes. It does not. A clone is a copy of an animal that already existed, and its genes are the donor's genes, in the donor's arrangement. Cloning cannot make a new kind of animal, and it cannot bring back a species whose DNA is gone, because a clone needs a living cell with an intact nucleus, and a species that is truly extinct leaves no such cell behind. This is the limit of de-extinction, and it is why the mammoth has not been cloned, although its DNA survives in frozen carcasses: the cells are dead, their nuclei are broken, and no intact nucleus exists to copy.

The fourth misunderstanding is that cloning is the same as in vitro fertilisation. It is not. In vitro fertilisation joins a sperm and an egg in a dish, and the embryo that results has genes from two parents, just as it would in the body. Cloning uses no sperm at all: the nucleus of a single body cell is put into an egg, and the embryo that results has the genes of one animal. The two techniques are used together in laboratories, and both involve embryos in dishes, but they answer different questions, and they are regulated differently in almost every country.

The last misunderstanding is that a clone is made instantly, or that it grows faster than a normal animal. A clone is born as a baby, and it grows up at the normal speed. Dolly was born as a lamb, and she grew into a ewe over the same months as any other lamb. Cloning does not copy an adult: it copies the instructions for making an animal, and the instructions have to be run from the beginning, cell by cell, through pregnancy and birth and growth, exactly as they are in an ordinary animal.

Somatic Cell Nuclear Transfer, Step by Step

Somatic cell nuclear transfer, usually shortened to SCNT, is the method that made Dolly, and it is still the main way animals are cloned. The name describes the operation: a somatic cell is an ordinary body cell, one that is not a sperm or an egg, and nuclear transfer means moving its nucleus into another cell. The procedure has seven steps, and each one is a piece of delicate laboratory work, carried out under a microscope with a needle finer than a hair.

The first step is to take a donor cell. Any body cell will do, but the ones most often used are skin cells, because they are easy to take without harming the animal, and udder cells, because those were the cells used for Dolly. The donor cell is grown in a dish, and then it is starved: the nutrients in its dish are reduced, and the cell stops dividing and goes quiet, entering a resting state called G0. The starving matters, because a resting nucleus is easier to reset than a busy one. The donor cell's nucleus, with its full set of chromosomes, is the package that will be copied.

The second step is to take an egg. The egg comes from a female of the same species, usually collected after hormonal treatment that makes her release many eggs at once. The egg is a large cell, and its nucleus is visible under the microscope. The nucleus is sucked out with a fine glass needle, a step called enucleation, and the egg is left with its cytoplasm, its mitochondria and its machinery, but with no nucleus of its own. An enucleated egg is an empty house: it has everything needed to run development, except the instructions.

The third step is the transfer itself. The donor cell, or just its nucleus, is placed next to the enucleated egg, or slipped inside it, and the two are fused together with a brief electric pulse. The pulse does two things at once: it opens the membranes so that the donor nucleus enters the egg, and it activates the egg, waking it up as if it had been fertilised. The egg now has a full set of chromosomes, all from the donor, and it begins to divide. The first division is the hardest: the donor nucleus has to forget that it was a skin cell or an udder cell, and remember how to be an embryo, and the egg's cytoplasm has to do the forgetting, by stripping the chemical marks off the donor's DNA and putting embryonic marks in their place. This step is called reprogramming, and it is where most clones fail.

The fourth step is culture. The reconstructed embryo is kept in a dish for a few days, in a warm, carefully balanced fluid, and it is watched as it divides: two cells, four, eight, sixteen, then a hollow ball of cells called a blastocyst, with about a hundred cells and an inner cluster that will become the animal. Most embryos stop before this point. They divide once or twice and then stall, because the reprogramming did not work, and the embryo cannot switch on the genes it needs. The embryos that reach the blastocyst stage are the ones that go forward.

The fifth step is transfer to a surrogate mother. A blastocyst is placed into the womb of a female of the same species, who has been hormonally prepared to receive it, and the pregnancy begins. The surrogate is not the genetic mother of the clone: she provides the womb, the blood supply and the milk, but the clone's genes come from the donor. Several embryos are usually transferred at once, in the hope that at least one will take, and most do not. The pregnancies that fail are lost early, often without the surrogate ever showing signs of pregnancy.

The sixth step is birth. A cloned animal is born like any other, usually by caesarean section, because cloned pregnancies are more likely to run into trouble, and the newborn is watched closely. Cloned newborns are often large, and they often have problems with their lungs, their hearts or their immune systems, and many die in the first days. The ones that survive grow up normally, and they can breed normally, and their own offspring are made the ordinary way, by fertilisation, and are not clones.

The seventh step is the test that the clone really is a clone. The laboratory checks the clone's DNA against the donor's, usually by looking at short repeated sequences called microsatellites, which vary between individuals, and by checking the mitochondrial DNA, which should match the egg donor rather than the nuclear donor. A true clone matches the nuclear donor at every microsatellite, and matches the egg donor in its mitochondria, and that pattern is the fingerprint that proves the copy.

Embryo Splitting and Other Ways to Clone

Somatic cell nuclear transfer is the best-known method, but it is not the only one. The oldest method is embryo splitting, and it is the laboratory version of identical twinning. An embryo in its first few days, when it is a ball of eight or sixteen cells, is split into two halves, and each half is placed into a surrogate, where it develops into a separate animal. The two animals are identical twins, made on purpose. Embryo splitting has been used in cattle since the 1980s, and it is still used, because it is simpler and more reliable than nuclear transfer: the embryos are already embryos, so there is no reprogramming to fail, and the success rate is far higher. Its limit is that it can only make a copy of an embryo, not of an adult, and it can only make two copies, or at most four, from a single embryo.

A second method is blastomere separation, which is embryo splitting taken one step further. The cells of a very early embryo are separated from each other, and each cell is placed on its own, where it grows into a whole embryo. The method works because, in the first few divisions, each cell of the embryo is still totipotent, meaning it can still become any part of the animal, and a single cell can still build a whole body. Hans Driesch showed this in 1891, when he separated the cells of a sea urchin embryo and watched each one grow into a complete larva, and the result proved that the early embryo is not a machine with fixed parts but a flexible system that can rebuild itself from a piece.

A third route is parthenogenesis in the laboratory, in which an unfertilised egg is activated and made to develop on its own, producing a daughter that is a near-copy of its mother. The method works in some species and not in others, because mammalian development needs genes from both a father and a mother: some genes are switched on only when they come from the father, and others only when they come from the mother, a phenomenon called genomic imprinting, and an embryo with only maternal genes usually fails. In mice, researchers have made parthenogenetic embryos by manipulating the imprinted genes, but the animals are not healthy, and the method has never been a practical way to clone a mammal.

A fourth route does not clone an animal at all, but it is often confused with cloning, and it matters for medicine: induced pluripotent stem cells, or iPSCs. In 2006, Shinya Yamanaka in Japan showed that an adult skin cell could be turned back into a stem cell, a cell that can become any tissue, by switching on just four genes. The resulting cells are not a clone of the animal, and they are not an embryo, but they carry the animal's genome, and they can be grown into skin, heart muscle, or nerve cells in a dish. Yamanaka won the Nobel Prize in 2012, shared with John Gurdon, whose frog work had pointed the way, and iPSCs are now the main alternative to therapeutic cloning, because they make stem cells without making an embryo at all.

Method	What is copied	Success rate	Main use	Limit
Somatic cell nuclear transfer	An adult body cell	About 1 to 5% of embryos	Copying an adult animal	Reprogramming often fails
Embryo splitting	An early embryo	High, often over 50%	Identical twins in cattle	Only copies embryos, not adults
Blastomere separation	A single early cell	Moderate	Research on early development	Only works in the first divisions
Therapeutic cloning	An embryo, for stem cells	Low, like SCNT	Making patient-matched stem cells	Needs human embryos; heavily regulated
Induced pluripotent stem cells	No embryo at all	Routine in the laboratory	Stem cells without embryos	Makes cells, not whole animals

Before Dolly: Frogs, Carp and the Long Road

Dolly did not come from nowhere. The road to her began in 1952, when Robert Briggs and Thomas King, working at the Institute for Cancer Research in Philadelphia, took the nucleus from a frog embryo and put it into a frog egg whose own nucleus had been removed, and the egg developed into a tadpole. It was the first nuclear transfer, and it proved that an egg could run development from a transplanted nucleus. But Briggs and King used nuclei from embryos, and when they tried nuclei from adult frogs, the eggs failed. The conclusion, for forty years, was that an adult nucleus could not be reset, and that development was a one-way street.

The next step came in 1962, when John Gurdon, at Oxford, took the nucleus from the intestinal cell of a tadpole and put it into an enucleated frog egg, and the egg developed into a tadpole, and in a few cases into an adult frog. Gurdon's frogs were the first clones made from a differentiated cell, a cell that had already become a particular kind of tissue, and they showed that the nucleus of a specialised cell still held the whole programme. Gurdon's work was not believed at first, and it took years of repetition before the field accepted it, but it is now recognised as the founding experiment of cloning, and Gurdon shared the 2012 Nobel Prize for it. His frogs were not made from a fully adult animal, though: they came from tadpole gut cells, and the question of a truly adult donor stayed open.

In 1963, the Chinese biologist Tong Dizhou took the same idea to fish, and made the first cloned carp, by putting the nucleus of an adult carp cell into an enucleated carp egg. The carp grew into an adult fish, and Tong's work showed that the method worked in a vertebrate other than a frog. Tong worked through the Cultural Revolution, in difficult conditions, and his results were little known outside China for years, but they are now recognised as an early landmark, and the carp was the first cloned fish.

Through the 1970s and 1980s, the work moved to mammals, and the results were frustrating. Nuclei from mouse embryos could be transferred and could make mice, but nuclei from adult mice could not, and the failure was blamed on the mammalian egg, which seemed less able to reset a nucleus than a frog's egg. In 1984, the Danish embryologist Steen Willadsen made the first cloned sheep, but from an embryo, not from an adult: he split a sheep embryo and made identical twin lambs, and then he made a lamb by nuclear transfer from an embryonic cell. In 1986, Willadsen made the first cloned cow the same way. The embryonic barrier was broken, but the adult barrier stood, and by the early 1990s, most textbooks said that cloning an adult mammal was impossible in principle.

What changed was a small, unglamorous insight about starving cells. At the Roslin Institute in Scotland, Keith Campbell, a cell biologist, argued that the reason adult nuclei failed was that they were busy dividing, and that a dividing nucleus could not be reset. If the donor cell was starved into a resting state, he reasoned, its nucleus would be quiet, and the egg would have a chance to reprogramme it. Ian Wilmut, the embryologist who led the Roslin team, tested the idea, and the combination of a starved adult cell and an enucleated egg worked. The team made two lambs from embryonic cells first, Megan and Morag, born in 1995, and then they tried an adult cell, and the result was Dolly.

Dolly the Sheep

Dolly was born on 5 July 1996, at the Roslin Institute near Edinburgh, in Scotland. She was a Finn Dorset ewe, and she was made from a single cell taken from the mammary gland, the udder, of a six-year-old Finn Dorset ewe. The cell was starved into a resting state, its nucleus was put into an egg from a Scottish Blackface ewe whose own nucleus had been removed, the reconstructed egg was given an electric pulse, and the resulting embryo was placed into the womb of a third ewe, a Scottish Blackface surrogate. Dolly was the only live lamb from 277 reconstructed embryos, and she was the first mammal ever cloned from an adult cell.

Her name was a joke, and the team never hid it. The donor cell came from a mammary gland, and the team named the lamb after Dolly Parton, the singer, because of the famous association between her name and that part of the body. The joke was told in every newspaper in the world, and it helped make Dolly famous, but it also hid how serious the work was. The Roslin team had spent years on the problem, and Dolly was the proof that an adult nucleus could be reset, which was one of the biggest questions in biology.

The announcement came in February 1997, in the journal Nature, and the reaction was immediate and worldwide. Newspapers called it the most important biological discovery since the double helix, and politicians called for bans on human cloning within days. The President of the United States, Bill Clinton, asked for a report on the ethics of cloning within ninety days. The Pope condemned it. The British government set up an inquiry. The public argument that followed, about whether humans would be cloned, and whether they should be, has never really stopped, and it is still the first thing most people think of when they hear the word cloning.

Dolly lived for six and a half years, and she lived most of them as an ordinary sheep, at Roslin, with the other sheep. She was mated with a Welsh Mountain ram called David, and she gave birth to six lambs, naturally, and the lambs were healthy, which showed that a clone could breed normally, and that her own children were not clones. Her first lamb, Bonnie, was born in 1998. Dolly became a celebrity: she was photographed, filmed, and visited, and she was the most famous sheep in the world, and probably the most famous animal of the twentieth century after a few film stars.

Her health became a question in her last years. At five, she developed arthritis in her hip and her elbow, which is early for a sheep, and she was treated with anti-inflammatory drugs. When she died, on 14 February 2003, it was from a lung infection, a progressive pneumonia caused by a virus called Jaagsiekte sheep retrovirus, which is common in sheep and which she had probably caught from the flock. She was put down because the infection was not responding to treatment, and she was six and a half years old, which is younger than the average lifespan of a Finn Dorset ewe, about eleven or twelve years. Her body was stuffed and put on display in the National Museum of Scotland, in Edinburgh, where it still stands.

The question of whether Dolly aged too fast is covered in its own section below, because the answer is more interesting than a simple yes or no. What matters here is that Dolly lived, bred, and died as a sheep, and that her birth changed the subject. Before Dolly, cloning an adult mammal was thought impossible. After Dolly, it was a technique, with a success rate, a cost, and a list of uses, and the argument moved from whether it could be done to whether it should be done, and for what.

Cloned Livestock

After Dolly, the first animals to be cloned in numbers were farm animals, and cattle led the way. In 1998, two years after Dolly, Japanese researchers cloned two calves, and in the same year, an American team cloned calves from adult cells, proving that the method worked in cattle as well as in sheep. Since then, thousands of cattle have been cloned, more than any other animal, because a single outstanding bull or cow can be worth a great deal of money, and cloning offers a way to copy it. A prize dairy cow that gives far more milk than her herdmates, or a bull whose daughters are exceptional, can be cloned, and the clone carries the same genes for milk and for growth.

Pigs were cloned in 2000, and goats in 1999, and both are now cloned regularly, for farming and for research. Pigs matter because they are close to humans in the size of their organs, and cloned pigs are used in research on organ transplantation, including the long-running work on growing human-compatible organs in pigs. Goats matter because they can be made to produce medicines in their milk: a goat that has been genetically engineered to make a human protein in its milk can be cloned, so that a whole herd of identical goats makes the same medicine, and the first drug made this way, an anticlotting protein called antithrombin, was approved for use in 2006.

The food from cloned animals has been argued about more than the animals themselves. In 2008, the United States Food and Drug Administration published a risk assessment that concluded that meat and milk from cloned cattle, pigs and goats, and from their offspring, are as safe to eat as meat and milk from conventionally bred animals, and it allowed them to enter the food supply. The European Food Safety Authority reached a similar conclusion about safety, although the European Union has kept a ban on the food in place for other reasons, and cloning for food remains rare. In practice, very little cloned meat reaches a shop: cloning is expensive, and a cloned cow is worth far more as a breeding animal than as meat, so the clones are kept for breeding, and it is their ordinary, non-cloned offspring that are sold for food.

The economics are the reason cloning has stayed small in farming. Cloning a cow costs thousands of dollars, and the success rate is low, so it only pays when the animal being copied is worth a great deal. A cloned bull can be worth hundreds of thousands of dollars, and a cloned dairy cow with exceptional milk yield can repay the cost many times over, but an ordinary cow cannot. For most farmers, the cheaper way to improve a herd is still the old way: choose the best bull, mate it with the best cows, and let Mendel's laws of inheritance do the work. Cloning is a tool for the exceptional animal, not for the average one.

Cloned Pets and Famous Clones

The first cloned pet was a cat called CC, short for CopyCat, born in 2001 at Texas A and M University. CC was a calico, and she was cloned from a calico called Rainbow, but she did not look like Rainbow: Rainbow was a tortoiseshell and white, and CC was a tabby and white, because the coat pattern of a calico is decided by which X chromosome is switched off in each patch of skin, and that switching is random, so two animals with the same genes can have different patterns. CC was the first proof that a clone is not a visual copy, and she lived a long and healthy life, and she had kittens of her own, naturally.

The first cloned dog was Snuppy, an Afghan hound, born in 2005 at Seoul National University in South Korea, in the laboratory of Hwang Woo-suk. Dogs were hard to clone, because their eggs are difficult to collect and their reproductive cycle is awkward, and Snuppy took more than a thousand attempts. He was the only live puppy from 1,095 reconstructed embryos. Snuppy lived for ten years, and he was the only dog cloned in that laboratory that survived, but he proved that dogs could be cloned, and the work has since moved to companies that clone pets for owners.

Pet cloning is now a business, and the best-known company is ViaGen Pets, in Texas, which clones cats and dogs for owners who have lost a pet, or who want a copy of a living one. The price is high, tens of thousands of dollars for a dog, and the process starts with a small sample of the pet's skin, which is grown in a dish and frozen. When the owner is ready, the cells are used to make a clone, and the puppy or kitten is delivered at about two months old. The company is careful to tell owners that the clone will look like their pet but will not be their pet: it will have the same genes, but it will be a different animal, with its own personality, and the owners who understand that are the ones who are satisfied.

Other famous clones include the first cloned horse, Prometea, born in Italy in 2003, who was the clone of her own surrogate mother, which made her the first clone to be carried by the animal she was copied from. The first cloned mule, Idaho Gem, was born in the same year, in the United States, and he went on to race, which showed that a clone could compete as an athlete. The first cloned camel, Injaz, was born in Dubai in 2009. The first cloned wolf, Snuwolf, was born in South Korea in 2005, and the first cloned monkeys, Zhong Zhong and Hua Hua, two long-tailed macaques, were born in China in 2018, and they were the first primates ever cloned by somatic cell nuclear transfer, which matters because primates are the closest relatives of humans, and the method that works in a monkey is the method that would work in a person.

Clone	Animal	Year	Why it mattered
Dolly	Sheep	1996	First mammal cloned from an adult cell
CC (CopyCat)	Cat	2001	First cloned pet; coat differed from donor
Prometea	Horse	2003	Clone carried by her own donor
Idaho Gem	Mule	2003	First cloned mule; raced as an athlete
Snuppy	Dog	2005	First cloned dog; 1 live birth from 1,095 embryos
Injaz	Camel	2009	First cloned camel
Zhong Zhong and Hua Hua	Long-tailed macaque	2018	First cloned primates

Cloning for Conservation

Cloning has one use that no other technique can match: it can bring back the genes of an animal that is already dead. A zoo that has lost its last male of a rare species can clone him from cells frozen years earlier, and the clone can breed and put his genes back into the population. The cells are stored in frozen zoos, collections of skin samples and sperm kept in liquid nitrogen at minus 196 degrees Celsius, and the San Diego Zoo's Frozen Zoo holds samples from more than a thousand species. A clone made from a frozen sample is a genetic rescue, and it is the only way to recover genes that would otherwise be gone forever.

The first endangered animal to be cloned was a gaur, a wild ox of Asia, in 2001. The calf, called Noah, was born to a domestic cow as a surrogate, and he lived for two days before dying of dysentery, which is a common illness in newborn calves and was not caused by the cloning. The second was a banteng, another wild ox, in 2003, and the calf lived and grew up in the San Diego Zoo. Both were proofs of principle, and both showed the same problem: a wild animal gestated by a domestic surrogate faces a mismatch, because the surrogate's womb, her milk and her microbes are those of a different species, and the calf can suffer for it.

The most important conservation clone so far is Elizabeth Ann, a black-footed ferret, born in December 2020. The black-footed ferret is one of the rarest mammals in North America: it was thought extinct in the 1970s, and every living ferret descends from just seven animals found in Wyoming in 1981. Elizabeth Ann was cloned from a female called Willa, who died in 1988 and whose cells had been frozen for more than thirty years. Willa's genes are not in the living population, because she never bred, so Elizabeth Ann carries genetic diversity that no living ferret has, and her offspring, when she breeds, will put that diversity back into the species. She is the first clone of an endangered species in the United States, and she is healthy and living at the National Black-Footed Ferret Conservation Center in Colorado.

The second landmark came in the same year, with Kurt, a Przewalski's horse, born in August 2020 at the San Diego Zoo. Przewalski's horse is the last truly wild horse, and it was extinct in the wild until it was reintroduced in Mongolia in the 1990s. Kurt was cloned from a stallion called Kuporovic, who died in 1998 and whose cells had been frozen. Kuporovic's genes are valuable because he was a founder of the captive population, and his genes are underrepresented in the living horses, so Kurt's future offspring will widen the gene pool. Kurt is the first clone of a Przewalski's horse, and he is the first clone of any endangered species to be made from cells stored in a frozen zoo.

Cloning cannot save a species on its own. A clone is one animal, and a species needs a population, with habitat, with food, and with enough genetic diversity to survive disease and change. A clone can add diversity to a breeding programme, but it cannot replace the habitat that was lost, and it cannot fix a population that has collapsed to a handful of animals. The ferret and the horse show what cloning can do, and they also show its limit: it is a tool for genetic rescue, not a substitute for conservation, and it works best when it is used alongside the ordinary work of protecting habitat and breeding animals in the usual way.

Why Cloning So Often Fails

The honest number in cloning is the success rate, and it is low. In the work that made Dolly, 277 reconstructed embryos produced one live lamb, a success rate of less than half of one percent. In cattle, the best modern work gets a live calf from roughly one to five in every hundred embryos transferred, and in dogs it can be worse: Snuppy was one live puppy from 1,095 embryos. The numbers vary between species and between laboratories, and they have improved since 1996, but cloning has never become reliable, and the reason is not a single problem but a series of them, at every step of the process.

The first and biggest problem is reprogramming. An adult cell has switched off most of its genes, and the egg has to switch them back on again, and it has to do it in the right order and at the right time. The switching is done by chemical marks on the DNA, called epigenetic marks, and the egg has to strip the adult marks off and put embryonic marks on. When it fails, the embryo stops dividing, or it divides but cannot switch on the genes it needs to become a blastocyst. Most cloned embryos fail at this stage, and they fail silently, in a dish, before any pregnancy begins. The egg of a frog is good at reprogramming, which is why Gurdon's frogs worked, and the egg of a mammal is worse, which is why mammalian cloning is harder.

The second problem is the placenta. A cloned embryo that reaches the blastocyst stage still has to implant in the womb and build a placenta, and the placenta of a cloned pregnancy is often abnormal: it can be too large, or too small, or poorly attached, and the blood supply to the foetus suffers. Many cloned pregnancies are lost in the first weeks for this reason, and the losses are often invisible, because the surrogate simply comes back into season and the pregnancy is never detected. In cattle, cloned pregnancies are more likely to be lost late as well, and the calves that survive to birth are often delivered by caesarean section, because the pregnancy has run long or the calf is too large.

The third problem is large offspring syndrome. Cloned calves and lambs are often born far heavier than normal, sometimes twice the normal weight, and the extra size causes difficult births and weak newborns. The cause is again epigenetic: the genes that control growth are among the ones most often mis-set during reprogramming, and a foetus whose growth genes are switched on too strongly grows too fast. Large offspring syndrome is the most visible problem in cloned livestock, and it is one of the reasons cloned pregnancies are watched so closely and delivered early by caesarean.

The fourth problem is the health of the newborn. Cloned animals that survive birth often have problems in their first days: weak lungs, a weak immune system, an enlarged liver or heart, and a tendency to infection. Many die in the first week, and the ones that survive usually recover and grow normally, but the early losses are high, and they are the welfare cost of cloning. The animals that do survive to adulthood are generally healthy, and they can breed normally, but the road to adulthood is harder than for an ordinary animal.

Stage	What goes wrong	Usual outcome
Reprogramming	Adult gene marks not stripped; embryo cannot switch genes on	Embryo stops dividing in the dish
Blastocyst formation	Inner cell mass fails to form properly	No embryo to transfer
Implantation	Placenta abnormal or poorly attached	Pregnancy lost early, often unnoticed
Gestation	Large offspring syndrome; overgrown foetus	Difficult birth, caesarean needed
Newborn period	Weak lungs, weak immunity, enlarged organs	Death in the first days is common
Adulthood	Mostly healthy if the newborn period is survived	Normal breeding; offspring are not clones

The failure rate is the reason cloning is regulated as it is, and it is the reason the welfare argument is the strongest argument against it. Every cloned animal that is born represents many embryos that were not, and many pregnancies that were lost, and the surrogates that carried them bore the cost. The people who defend cloning argue that the failures are falling, and that the uses, in medicine and in conservation, justify the cost. The people who oppose it argue that the cost is too high for a technique that copies an animal that could be bred in the ordinary way. Both sides are arguing about the same numbers, and the numbers are the reason the argument matters.

Did Dolly Age Too Fast?

For years after Dolly's death, the most argued question about her was whether she had aged too fast, and the evidence was her telomeres. Telomeres are the protective caps on the ends of chromosomes, and they shorten a little with every cell division, so a cell's telomeres are a rough measure of how old it is. When Dolly's telomeres were measured, they were shorter than those of other sheep of her age, and they matched the telomeres of the six-year-old ewe that had donated her nucleus. The finding was reported in 1999, and it was taken to mean that Dolly had been born old: that her cells had started life at the age of her donor, and that she would age faster and die younger than an ordinary sheep.

The arthritis fed the same worry. Dolly developed arthritis in her hip and her elbow at five, which is young for a sheep, and the early arthritis was read as another sign of premature ageing. When she died at six and a half, of a lung infection, the death was also read that way, although the infection was a common sheep disease, and the sheep she lived with carried the same virus. The story that formed was that Dolly was a prematurely old sheep, and that cloning carried an ageing cost, and the story was repeated in every newspaper.

The later evidence points the other way. In 2007, researchers at the University of Nottingham made four more clones from the same cell line that had produced Dolly, the same mammary cells from the same donor ewe, and the four ewes, called Debbie, Denise, Dianna and Daisy, were kept and studied as they aged. They lived to seven, eight and nine years, in good health, with no early arthritis and no sign of premature ageing, and their telomeres were normal for their age. The conclusion was that Dolly's short telomeres and her early arthritis were not an inevitable result of cloning, and that they may have been caused by her individual circumstances, including the fact that she lived indoors for much of her life, which is hard on a sheep's joints, and that she was overweight, which is also hard on them.

The honest answer is that Dolly's ageing is still not fully settled, and that one sheep cannot prove a rule. Her telomeres were short, and that is a fact. Her sisters lived long and healthy lives, and that is also a fact. What the two facts together show is that cloning does not always produce an old animal, and that the ageing of a clone depends on more than the age of its donor, including the reprogramming of the nucleus, the conditions of the pregnancy, and the life the animal lives afterwards. The telomere question is still open, and it is one of the reasons cloned animals are followed through their whole lives, rather than being judged on their first years.

Uses in Agriculture and Medicine

The most practical use of cloning in farming is copying an outstanding animal. A dairy cow that gives far more milk than her herdmates, or a bull whose daughters are exceptional, is worth copying, and the clone carries the same genes for milk yield and for growth. The clone is not used for meat: it is used for breeding, and its ordinary offspring, made by fertilisation, carry half its genes, and the herd improves faster than it would by ordinary selection. The same logic applies to pigs bred for lean meat, to sheep bred for wool, and to horses bred for racing, although racing authorities in most countries do not allow clones to race, and the cloned racehorse is still a curiosity rather than a competitor.

Cloning is also used to multiply genetically engineered animals, and this is where it matters most for medicine. A goat can be engineered to make a human medicine in its milk, but engineering one goat is slow and expensive, and the goat is female and can only have a few kids. Cloning the goat makes a herd of identical animals, all making the same medicine in their milk, and the medicine can be collected, purified and sold. The first drug made this way was antithrombin, a protein that stops blood from clotting, made in the milk of cloned goats and approved in Europe in 2006 and in the United States in 2009. It is used to treat people who are born without enough antithrombin, and it was the first medicine made by a cloned animal to reach patients.

Pigs are the most important animals in the medicine work, because their organs are the right size for humans. Researchers are cloning pigs that have been engineered to remove the pig genes that cause the human immune system to reject a pig organ, and the aim is to grow hearts, kidneys and livers in pigs that can be transplanted into people. In 2022, the first pig heart was transplanted into a human patient, in the United States, and the patient lived for two months, which was long enough to show that the approach could work, and short enough to show how far it still has to go. Cloned pigs are also used to study human disease, because a cloned pig with a human gene for cystic fibrosis, or for Huntington's disease, develops the disease in the same way a person does, and the animal can be used to test treatments.

Cloning is also used to preserve the genetics of animals that cannot breed. A prize stallion that has been gelded cannot have foals, but his cells can be cloned, and the clone can breed. A bull that has died can be cloned from frozen cells, and the clone can carry his genes forward. A dog that has been neutered can be cloned, and the clone can breed. In each case, cloning recovers breeding value that would otherwise be lost, and the value is genetic, not sentimental: the clone is not the same animal, but it carries the same genes, and the genes are what the breeder wants.

Therapeutic Cloning and Stem Cells

Therapeutic cloning is the other kind of cloning, and it does not make a baby. The procedure starts exactly like reproductive cloning: a donor nucleus is put into an enucleated egg, and the egg is allowed to develop into a blastocyst, a hollow ball of about a hundred cells. But instead of placing the blastocyst into a surrogate, the inner cells are taken out and grown in a dish, and those cells are embryonic stem cells, which can become any tissue in the body. The stem cells carry the donor's genes, so they are a genetic match for the donor, and they can be grown into skin, heart muscle, or nerve cells that the donor's body will not reject, because they are, genetically, the donor's own cells.

The promise is personalised medicine. A patient with Parkinson's disease could have skin cells taken, and those cells could be used to make stem cells, and the stem cells could be grown into the nerve cells that Parkinson's destroys, and the nerve cells could be put back into the patient's brain, where they would not be rejected, because they are the patient's own cells. A patient with diabetes could have stem cells grown into insulin-making cells. A patient with a spinal injury could have stem cells grown into nerve cells. The work is still experimental, and no therapeutic cloning treatment has yet reached ordinary patients, but the principle has been proved in animals, and in 2013, researchers in the United States made the first human embryonic stem cells by therapeutic cloning, from the skin cells of a baby with a genetic disease.

The method is heavily regulated, because it uses human embryos, and the regulations vary widely. In the United Kingdom, therapeutic cloning is legal under licence, and the Human Fertilisation and Embryology Authority oversees it. In the United States, federal funding for it has been restricted at various times, and several states have banned it. In many countries it is banned outright. The ethical argument is about the embryo: the blastocyst is destroyed when the stem cells are taken, and the people who oppose the work say that the embryo is a human life, while the people who support it say that a blastocyst of a hundred cells, with no nervous system and no capacity to feel, is not a person, and that the medicine it could make justifies the work.

The practical alternative, and the one that has largely taken over, is the induced pluripotent stem cell, the iPSC. An iPSC is made by taking an adult skin cell and switching on four genes, and the cell turns back into a stem cell, without any egg and without any embryo. The iPSC carries the donor's genes, just as a therapeutic clone would, and it can be grown into any tissue, and it raises none of the ethical problems of embryo use. iPSCs are now used in most stem cell laboratories, and they are the reason therapeutic cloning is done far less often than it was. The two methods are covered together in our article on stem cells in animals, and the comparison there sets out where each one is used.

Ethics and Animal Welfare

The ethical argument about cloning has three parts, and they are worth keeping separate, because they are often run together. The first is animal welfare. Cloning has a high failure rate, and every cloned animal that is born represents many embryos that did not develop, many pregnancies that were lost, and many surrogates that carried pregnancies that failed. The surrogates can suffer, because cloned pregnancies are more likely to be difficult, and the newborns can suffer, because they are more likely to be born large, or weak, or ill. The welfare argument is the strongest argument against cloning, and it is the one that the people who do the work take most seriously, because it is measured in animals, not in ideas.

The second part is identity. A clone is not the same animal as its donor, and the people who buy a cloned pet expecting their dead dog back are mistaken, and the companies that sell the service have a duty to say so. The clone will look like the dog, and it will share the dog's genes, but it will be a different animal, with its own experiences and its own character, and the owner who does not understand that will be disappointed. The identity argument also matters for conservation: a cloned ferret is not the ferret that died, and the species is not saved by a single clone, and the people who fund cloning need to understand that the clone is a tool for breeding, not a replacement for the animal that was lost.

The third part is diversity. If cloning is used widely in farming, and the same few animals are copied again and again, the gene pool narrows, and the population becomes more vulnerable to disease and to change. A herd of clones is a herd with no genetic variation, and a disease that kills one clone can kill them all. The same logic applies to conservation: a clone adds diversity only if it carries genes that the living population lacks, and if the same clone is copied many times, it narrows the pool rather than widening it. The answer is to use cloning sparingly, and to keep ordinary breeding alongside it, so that the gene pool stays wide.

The counter-argument is that the uses justify the cost. A cloned ferret that carries lost genes is worth the embryos that failed, because the species is at stake. A cloned pig that grows a heart for transplant is worth the failures, because a human life is at stake. A cloned cow that makes a medicine in its milk is worth the cost, because the medicine cannot be made any other way. The argument is not about whether cloning is perfect: it is about whether the benefit is worth the animals that are lost in the making, and different people weigh that question differently, and the laws of different countries reflect the different answers.

Human Cloning: Where the Line Is Drawn

Reproductive cloning of a human being, making a born human clone, is banned in almost every country in the world, and it is the one point on which the argument is nearly settled. The United Nations adopted a declaration in 2005 calling on states to prohibit it. The United Kingdom, France, Germany, Japan, Australia, Canada and most other countries have laws against it. The United States has no federal law against it, but the Food and Drug Administration has said it would not allow it, and several states have banned it. The reason for the ban is not a single argument but a combination: the failure rate, which would mean many dead embryos and many damaged babies, the welfare of the child, who would be born as an experiment, and the identity question, which is sharper for a person than for a sheep, because a human clone would grow up knowing it was copied, and would live with the expectations of the person it was copied from.

The technical possibility is real. The method that made Dolly, and that made the cloned monkeys Zhong Zhong and Hua Hua in 2018, would work in a human, in the sense that a human egg could be enucleated, a human nucleus could be put in, and an embryo could be made. The monkeys matter because they are primates, and the method that works in a macaque is the method that would work in a person, and the 2018 result was reported with the same mixture of scientific interest and public alarm that followed Dolly. No human clone has been made, and no credible laboratory has tried, and the ban is one reason, but the other reason is the failure rate: a method that loses most of its embryos and damages many of its newborns is not a method anyone could defend using on a human child.

Therapeutic cloning in humans is a different question, and the law treats it differently. Making a human blastocyst to take stem cells from it is legal in some countries, including the United Kingdom under licence, and banned in others, and the argument turns on the status of the embryo. The people who support it say that a blastocyst of a hundred cells, with no nervous system, is not a person, and that the medicine it could make, for Parkinson's disease, for diabetes, for spinal injury, justifies the work. The people who oppose it say that the embryo is a human life, and that its destruction cannot be justified by any benefit. The argument is not settled, and it is the live edge of the cloning debate, because the science is ready, and only the law and the ethics stand in the way.

How the Methods Compare

The methods can be compared on four questions: what they copy, how often they work, what they are used for, and what they cost in animals. Somatic cell nuclear transfer copies an adult, and it works a few percent of the time, and it is used for livestock, pets and conservation, and its cost is high, in embryos and in pregnancies. Embryo splitting copies an embryo, and it works more than half the time, and it is used for identical twins in cattle, and its cost is low, because the embryos are already embryos. Therapeutic cloning copies an embryo for stem cells, and it works as rarely as nuclear transfer, and it is used for medicine, and its cost is the embryo itself. Induced pluripotent stem cells copy no embryo at all, and they work routinely, and they are used for stem cells, and their cost is a dish of skin cells. The comparison shows why the field has moved: the methods that copy an adult are powerful and unreliable, and the methods that make stem cells without an embryo are reliable and limited, and the choice between them depends on whether the product has to be an animal, or whether a cell will do.

Method	Product	Needs an embryo?	Needs a surrogate?	Main limit
Reproductive cloning (SCNT)	A live animal	Yes, then implanted	Yes	1 to 5% success; welfare cost
Embryo splitting	Identical twins	Yes, the embryo itself	Yes	Only copies embryos, not adults
Therapeutic cloning	Stem cells	Yes, destroyed for cells	No	Embryo ethics; heavily regulated
Induced pluripotent stem cells	Stem cells	No	No	Makes cells, not whole animals

The other comparison is between a clone and an ordinary animal. A clone is not better than the donor, and it is not worse: it is the same genome, run again, in a different body and a different life. The clone of a prize cow will give the same milk, if it is fed the same way, and the clone of a fast horse will run as fast, if it is trained the same way, but the clone has no advantage over the donor, and it has the disadvantage of the failures that were needed to make it. Cloning is not a way to improve an animal: it is a way to copy one, and the copy is worth making only when the animal is exceptional, or when its genes are otherwise lost, or when the medicine it can make cannot be made any other way.

Key Takeaways

• Cloning makes a genetically near-identical copy of an animal from a single cell, without fertilisation. The copy carries the donor's nuclear DNA but the egg donor's mitochondrial DNA, so it is not a perfect copy.
• Nature clones animals already: identical twins, parthenogenesis in whiptail lizards and Daphnia, and regeneration in planarians are all natural cloning, and plants clone themselves by cuttings and runners.
• Somatic cell nuclear transfer, the method that made Dolly, puts the nucleus of an adult body cell into an egg whose own nucleus was removed, fuses them with an electric pulse, and grows the embryo in a dish before placing it in a surrogate.
• Embryo splitting is the laboratory version of identical twinning, and it is simpler and far more reliable than nuclear transfer, but it can only copy an embryo, not an adult.
• Dolly, born on 5 July 1996 at the Roslin Institute, was the first mammal cloned from an adult cell, made from a mammary cell of a six-year-old ewe, and she was the only live lamb from 277 attempts.
• Cloned livestock, led by cattle, is used to copy outstanding animals for breeding, and cloned goats make medicines in their milk, including antithrombin, approved in 2006.
• The first cloned pet was CC the cat in 2001, and the first cloned dog was Snuppy in 2005; the first cloned primates were the macaques Zhong Zhong and Hua Hua in 2018.
• Conservation cloning has rescued lost genes: Elizabeth Ann, a black-footed ferret cloned in 2020 from cells frozen since 1988, and Kurt, a Przewalski's horse cloned in 2020, both carry diversity no living animal has.
• Cloning usually fails: roughly 1 to 5 in every 100 cloned embryos become a healthy live animal, and the failures come from reprogramming errors, abnormal placentas, large offspring syndrome, and weak newborns.
• Dolly's telomeres were shorter than expected for her age, but the four ewes cloned from the same cell line in 2007 lived long and healthy lives, so premature ageing is not an inevitable result of cloning.
• Therapeutic cloning makes stem cells from an embryo and never makes a baby, and induced pluripotent stem cells now do the same job without any embryo at all.
• Reproductive cloning of humans is banned in almost every country, and the reason is the failure rate, the welfare of the child, and the identity question, while therapeutic cloning is legal in some countries and banned in others.

Short Questions

1. What is animal cloning in simple words?

See answer

Animal cloning is making a genetically near-identical copy of an animal from a single cell, without fertilisation. The best-known method is somatic cell nuclear transfer: the nucleus of an adult body cell, such as a skin cell, is put into an egg whose own nucleus has been removed, the egg is activated with an electric pulse, and the embryo that grows is placed into a surrogate mother, where it develops into a baby that carries the donor's genes. The clone is not a perfect copy: its mitochondrial DNA comes from the egg donor, not from the nuclear donor, and it is a different animal, born at a different time and shaped by a different life, just as identical twins share a genome but are different people.

2. How was Dolly the sheep made?

See answer

Dolly was made at the Roslin Institute in Scotland by somatic cell nuclear transfer. The team took a cell from the mammary gland of a six-year-old Finn Dorset ewe, starved it into a resting state, and put its nucleus into an egg from a Scottish Blackface ewe whose own nucleus had been removed. The reconstructed egg was given an electric pulse to fuse it and activate it, the embryo was grown in a dish, and it was placed into a Scottish Blackface surrogate. Dolly, born on 5 July 1996, was the only live lamb from 277 attempts, and she was the first mammal ever cloned from an adult cell, which proved that an adult nucleus could be reset and run the whole programme of development again.

3. Why is a clone not exactly identical to its donor?

See answer

A clone is not exactly identical to its donor for two reasons. First, its mitochondrial DNA comes from the egg donor, not from the nuclear donor, because the egg that the clone grows in brings its own mitochondria, and the clone's mitochondrial genome, about 37 genes, is the egg donor's, not the nuclear donor's. Second, the clone lives a different life: it is born at a different time, in a different womb, raised by a different mother, eating different food and meeting different microbes, and its genes are switched on and off by that different environment. The first cloned cat, CC, showed this clearly: she was cloned from a calico, but her own coat pattern was different, because the switching of the X chromosome in each patch of skin is random.

4. Why does cloning so often fail?

See answer

Cloning so often fails because reprogramming an adult nucleus is hard. An adult cell has switched off most of its genes, and the egg has to strip those chemical marks off and put embryonic marks on, in the right order and at the right time, and when it fails, the embryo stops dividing in the dish. The embryos that survive that step can still fail: their placentas are often abnormal, their pregnancies are often lost, and the newborns are often born too large, a condition called large offspring syndrome, or too weak, with poor lungs and weak immunity. Roughly 1 to 5 in every 100 cloned embryos become a healthy live animal, and Dolly herself was one live lamb from 277 attempts.

5. What is Elizabeth Ann, the cloned ferret?

See answer

Elizabeth Ann is a black-footed ferret, born in December 2020, who was cloned from a female called Willa, who died in 1988 and whose cells had been frozen for more than thirty years. The black-footed ferret is one of the rarest mammals in North America, and every living ferret descends from just seven animals found in Wyoming in 1981, so the species has very little genetic diversity. Willa never bred, so her genes are not in the living population, and Elizabeth Ann carries diversity that no living ferret has. When she breeds, her offspring will put that diversity back into the species. She is the first clone of an endangered species in the United States, and she is healthy and living at the National Black-Footed Ferret Conservation Center in Colorado.

6. Is human cloning legal?

See answer

Reproductive cloning of a human being, making a born human clone, is banned in almost every country in the world. The United Nations adopted a declaration in 2005 calling on states to prohibit it, and the United Kingdom, France, Germany, Japan, Australia, Canada and most other countries have laws against it. The reasons are the failure rate, which would mean many dead embryos and damaged babies, the welfare of the child, who would be born as an experiment, and the identity question, because a human clone would grow up knowing it was copied. Therapeutic cloning, which makes a blastocyst to take stem cells from it and never makes a baby, is treated differently: it is legal under licence in some countries, including the United Kingdom, and banned in others.

Frequently Asked Questions

What is animal cloning?

Animal cloning is the production of a genetically near-identical copy of an animal from a single cell, without fertilisation. The main method is somatic cell nuclear transfer: the nucleus of an adult body cell is put into an egg whose own nucleus has been removed, the egg is activated, and the embryo is placed into a surrogate mother. The clone carries the donor's nuclear DNA but the egg donor's mitochondrial DNA, so it is not a perfect copy, and it is a different animal, born at a different time and shaped by a different life.

How was Dolly the sheep cloned?

Dolly was cloned at the Roslin Institute in Scotland by somatic cell nuclear transfer. A cell from the mammary gland of a six-year-old Finn Dorset ewe was starved into a resting state, its nucleus was put into an enucleated egg from a Scottish Blackface ewe, the egg was activated with an electric pulse, and the embryo was placed into a surrogate. Dolly, born on 5 July 1996, was the only live lamb from 277 attempts, and she was the first mammal ever cloned from an adult cell.

Why do clones often die or fail?

Clones often fail because reprogramming an adult nucleus is difficult: the egg must strip the adult chemical marks off the donor's DNA and put embryonic marks on, and when it fails, the embryo stops dividing. The embryos that survive can still fail because their placentas are often abnormal, their pregnancies are often lost, and the newborns are often born too large, a condition called large offspring syndrome, or too weak, with poor lungs and weak immunity. Roughly 1 to 5 in every 100 cloned embryos become a healthy live animal.

Can cloning save endangered species?

Cloning can help save endangered species by rescuing genes that would otherwise be lost, but it cannot save a species on its own. Elizabeth Ann, a black-footed ferret cloned in 2020 from cells frozen since 1988, carries genetic diversity that no living ferret has, and her offspring will put that diversity back into the population. Kurt, a Przewalski's horse cloned in the same year, does the same for his species. But a clone is one animal, and a species needs a population, with habitat and food, so cloning works best alongside the ordinary work of protecting habitat and breeding animals in the usual way.

Is a cloned pet the same as the original pet?

No, a cloned pet is not the same as the original pet. It shares the donor's nuclear genes, so it will look like the pet and share its coat colour and size, but it is a different animal: it is born at a different time, raised by a different mother, and shaped by different experiences, so its personality and behaviour will differ. Its mitochondrial DNA also comes from the egg donor, not from the nuclear donor. The first cloned cat, CC, showed this clearly: she was cloned from a calico, but her own coat pattern was different, because the switching of the X chromosome in each patch of skin is random.

Did Dolly age too fast?

Dolly's telomeres, the protective caps on the ends of her chromosomes, were shorter than expected for her age, and she developed arthritis at five and died at six and a half, which was read as premature ageing. But the four ewes cloned from the same cell line in 2007, Debbie, Denise, Dianna and Daisy, lived to seven, eight and nine years in good health, with no early arthritis and normal telomeres for their age. The evidence together shows that cloning does not always produce an old animal, and that Dolly's ageing may have been caused by her individual circumstances, including living indoors and being overweight, rather than by cloning itself.

Related reading

• DNA Structure and Replication Explained Simply
• Mendel's Laws of Inheritance Explained
• All Genetics Articles

MCQs

1. What is the key step in somatic cell nuclear transfer?

• A. Joining a sperm and an egg in a dish
• B. Splitting an embryo into two halves
• C. Putting the nucleus of an adult body cell into an egg whose own nucleus was removed
• D. Taking a cutting from an adult animal

Answer: C

Explanation: Somatic cell nuclear transfer moves the nucleus of an adult body cell into an enucleated egg, and the egg is then activated so that it develops into an embryo carrying the donor's nuclear DNA, which is the method that made Dolly.

2. Dolly the sheep was cloned from which kind of cell?

• A. An embryonic stem cell
• B. A sperm cell
• C. An egg cell
• D. An adult mammary gland cell

Answer: D

Explanation: Dolly was made from a single cell taken from the mammary gland of a six-year-old Finn Dorset ewe, and her birth proved that the nucleus of a fully grown adult cell still held all the instructions needed to build a whole animal.

3. How many reconstructed embryos did it take to produce Dolly?

• A. 277, with one live birth
• B. 12, with six live births
• C. 1,095, with one live birth
• D. 50, with 25 live births

Answer: A

Explanation: Dolly was the only live lamb from 277 reconstructed embryos, a success rate of less than half of one percent, which shows how unreliable cloning was even in the experiment that proved it possible.

4. Which animal was the first cloned pet?

• A. Snuppy the dog
• B. CC the cat
• C. Prometea the horse
• D. Injaz the camel

Answer: B

Explanation: CC, short for CopyCat, was born in 2001 at Texas A and M University, and she was the first cloned pet; her coat pattern differed from her donor Rainbow, because the switching of the X chromosome in each patch of skin is random.

5. Why is a clone not genetically identical to its donor?

• A. Because the clone has different parents
• B. Because its mitochondrial DNA comes from the egg donor
• C. Because the clone is always a different species
• D. Because cloning changes the nuclear genes

Answer: B

Explanation: A clone's nuclear DNA comes from the donor, but its mitochondrial DNA comes from the egg that it grew in, and that egg belongs to a different animal, so the clone is a genetic mixture with about 37 mitochondrial genes from the egg donor.

6. What is large offspring syndrome in cloned animals?

• A. Cloned animals that grow up too quickly
• B. Cloned calves and lambs born far heavier than normal, causing difficult births
• C. Cloned animals that produce too many offspring
• D. Cloned animals with extra limbs

Answer: B

Explanation: Cloned calves and lambs are often born far heavier than normal, sometimes twice the normal weight, because the genes that control growth are among those most often mis-set during reprogramming, and the extra size causes difficult births and weak newborns.

7. Elizabeth Ann, the cloned black-footed ferret, matters because she carries what?

• A. Genes from the first ferret ever born
• B. The genes of her surrogate mother
• C. A completely new set of ferret genes
• D. Genetic diversity that no living ferret has

Answer: D

Explanation: Elizabeth Ann was cloned from Willa, a female who died in 1988 and never bred, so her genes are not in the living population, and Elizabeth Ann carries diversity that no living ferret has, which her offspring will put back into the species.

8. What did the measurement of Dolly's telomeres suggest?

• A. They were longer than those of other sheep
• B. They were exactly the same as a newborn lamb's
• C. They were shorter, matching the age of the donor ewe
• D. They were missing entirely

Answer: C

Explanation: Dolly's telomeres were shorter than those of other sheep of her age, and they matched the telomeres of the six-year-old ewe that donated her nucleus, which was read as evidence that she had been born old, although her sister clones later lived long and healthy lives.

9. What is therapeutic cloning used to make?

• A. A whole cloned animal
• B. Embryonic stem cells, not a baby
• C. A new species
• D. Identical twin embryos

Answer: B

Explanation: Therapeutic cloning makes a blastocyst and then takes embryonic stem cells from it, and the embryo is never allowed to develop into a baby; the stem cells can be grown into tissues that genetically match the donor.

10. Which were the first primates ever cloned by somatic cell nuclear transfer?

• A. Zhong Zhong and Hua Hua, long-tailed macaques
• B. Dolly and Bonnie, sheep
• C. CC and Rainbow, cats
• D. Snuppy and his littermates, dogs

Answer: A

Explanation: Zhong Zhong and Hua Hua, two long-tailed macaques born in China in 2018, were the first primates ever cloned by somatic cell nuclear transfer, which matters because primates are the closest relatives of humans.