Cloning is a natural phenomenon in species as diverse as armadillos, poplar trees, aphids, and bacteria. Identical twins are clones. Biologists have been cloning some organisms, such as carrots, for decades. Attempts to clone animals have been far less successful. They began long before the February 1997 announcement of the birth of Dolly, a sheep cloned from a mammary gland cell nucleus of a six-year-old sheep.
Oxford University developmental biologist John Gurdon cloned frogs in the 1960s, but in a limited way. He showed that a nucleus from a tadpole's intestinal lining cell could be transferred to an enucleated fertilized egg and support development to adulthood, and that a nucleus from an adult cell could support development as far as the tadpole stage. However, he was unable to coax a nucleus from an adult amphibian's cell to support development all the way to adulthood. In the 1980s several companies tried to commercialize cloning of livestock from nuclei taken from embryos or fetuses. The efforts failed because the cloned animals were nearly always very unhealthy newborns and did not survive for long. Currently, livestock cloning is limited to research, although some companies offer tissue preservation services in anticipation of future advances in commercial livestock cloning. There is no reason to believe that human clones would fare any better in terms of health or survivability than most cloned animals do.
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