A clone is a group of genetically identical cells or organisms derived from a single cell or individual. Cloning (the producing of clones) has been commonplace for many years in agriculture. A plant can be propagated (cloned) by simply taking a small piece of the original plant and growing a new one from it. The new plant is thus an exact genetic copy of the original one. Such propagation is also possible with simple animals such as flatworms: cut a flatworm in two, and the tail grows a new head and the head grows a new tail. However, such simple techniques do not work with higher animals, such as sheep or humans.

In the now-famous “Dolly” experiments, cells from a sheep (donor cells) were fused with unfertilized sheep eggs from another sheep (recipient cells) from which the natural genetic material was removed by microsurgery. Then the genetic material from the donor cells was transferred into the unfertilized eggs. Unlike unfertilized eggs, these laboratory-made eggs had a complete set of chromosomes and genes. Unlike eggs fertilized naturally (with sperm), the laboratory-made eggs received genetic material from only one source. The eggs then started to develop into embryos. The developing embryos were transplanted into a female sheep (the surrogate mother), where they developed naturally. One of the embryos survived, and the resulting lamb was named Dolly. As expected, Dolly was an exact genetic copy of the original sheep from which the donor cells were taken, not of the sheep that provided the eggs.

Research on cloning continues, but cloning of humans is technically difficult and ethically controversial. Studies suggest that cloned higher animals (and thus humans) are more likely to have serious genetic defects than normally conceived offspring. Governments have attempted to make cloning humans illegal. However, cloning need not only be used to create a whole organism. It can, theoretically, also be used to create a single organ. Thus, one day a person may be able to receive “spare parts” manufactured in the laboratory, using the person's own genes.

Whether a cell used for a clone produces a specific type of tissue, a specific organ, or an entire organism depends on the potential of the cell—that is, how highly the cell has developed into a particular type of tissue. For example, certain cells (stem cells) have the potential to produce a wide variety of tissue types or even possibly an entire organism. They have not yet differentiated into specific types of tissues. Other cells have differentiated and can develop into only those specific tissue types. Stem cells have stimulated interest because of their potential to generate tissue that can replace diseased or damaged tissues. Because stem cells tend to be less differentiated, they can thus potentially replace a wide or unlimited variety of types of tissue.