Scientists have successfully cloned tadpoles, sheep, pigs, monkeys, cats and horses. Reproductive cloning creates genetic duplicates of living creatures. To clone a dog, for example, a scientist takes an egg cell from a female dog and empties it out. She then fills the egg with DNA from another dog. If the egg eventually develops into a puppy, it'll be genetically identical to the donor dog.
Cloning efforts usually fail. The success rate, at time of publication, is between 0.1 and 3 percent. After placing genetic material in the egg, scientists implant the egg in a female. Implantation may fail or the mother may miscarry the clone. People who object to human cloning frequently cite the failure rate as one of the problems. Even if the cloning succeeds, the clone may grow up with abnormally large organs or other health problems.
Some scientists see cloning as a way to preserve endangered species. The wild Sumatran rhino population, for instance, is down to fewer than 300. Successfully cloning some of the rhinos could restore the population. This only works if the clones grow up and reproduce, however. Researchers have cloned the endangered black-footed cat, but many of the kittens died before reaching maturity. Successful cloning requires knowing a species' reproductive biology. With some animals, the details of reproduction are unknown.
One of the reasons reproductive cloning fails so often is that filling an egg with foreign DNA isn't a natural process. The egg may reject its new contents, or the genes may not trigger cell division and growth at the proper times. Another problem is that as cells age, the ends of their chromosomes shrink until they make it impossible for the cell to divide. Cloned cells from older animals start out with the shortened chromosomes, which may cause clones to die young.
As clones are genetically identical, this could make them valuable in researching new drugs. A lab testing drugs on cloned rats, say, wouldn't have to worry about genetic differences affecting the results. Farm researchers could clone animals that have particularly high milk yield or lean meat, though the cost at time of publication is prohibitive. Scottish researchers have genetically modified sheep to produce medically useful proteins in their milk, then cloned the sheep.