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Humans have been cloning plants for centuries by taking cuttings and scientists succeeded in cloning a frog in 1958. This was followed by clones of fish in the 1960s and 1970s but it wasn't until 1997 that the first large animal, a sheep called Dolly, was cloned. Since then, other large animals have also been cloned, including pets. As the technique has been developed, it has been possible to combine cloning with genetic engineering
There are two main techniques of large animal cloning. The first involves separating the stem cells (cells which have not yet become specialized) of a naturally fertilized embryo and implanting the individual cells into a host mother. The second involves removing the nucleus from an unfertilised egg cell and replacing it with the nucleus from a donor animal.
It would be possible to create human clones in the same way but there are many ethical issues.
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