Cloning is essentially a laboratory method of producing offspring that is nearly genetically identical to its parent, bypassing ordinary reproductive methods.

In February 1997, Dr. Ian Wilmut of the Roslin Institute in Scotland reported that he and his colleagues had successfully cloned a sheep, producing a lamb they named Dolly. Cloning is an extraordinarily difficult process. Dolly was born after 277 unsuccessful attempts.

The method used to create Dolly is called somatic cell nuclear transfer. Professor Wilmut's team took a donor sheep egg cell and removed its nucleus. They then removed cells from the udder of a different ewe and fused the nucleus of one of those udder cells with the egg. Thus, the reconstructed egg, from one sheep, contained a full complement of nuclear DNA from another sheep. That egg then formed an embryo, and the embryo was implanted in a surrogate mother sheep, which carried it to term. Dolly was thus not created through the union of a ram's sperm and an ewe's egg, but was the genetic product of a single female ewe.

Although more then 99.9% of the DNA in the cloning process is derived from the donor somatic cell, some DNA from the egg remains, this DNA resides in mitochondria. Thus, although cloning results in near genetic identity, there are still some differences between cloned animals at the genetic level.