Cloning is fundamentally different from ordinary reproduction. It involves taking an adult cell from a living person, slipping it into an egg cell whose genetic material has been removed and allowing the genetic material of the adult cell to direct the development of a new embryo, then foetus, then person who is the identical twin of the person who provided the initial cell. It enables a living person to be reborn as a duplicate of themselves. Scientists and infertility specialists envision certain specialized circumstances in which it might be acceptable to clone humans, for example, to provide donor organs and bone marrow.
Since Scottish researchers created Dolly in 1997 by placing an adult ewe's genes in an empty egg and triggering it to develop into an embryo, other scientists have managed to clone worms, mice and cattle using the same methods. But the failure rate has been alarmingly high at 98 percent. Often the attempts have resulted in failed births or clones with genetic defects such as extra large organs or weak hearts or immune systems.
Despite a scientist's announcement that he and his colleagues would be ready to place a cloned embryo in a woman's womb within 18 months, the technique has hardly been perfected.