Cattle, mice and pigs have been cloned since scientists in Scotland announced the birth of Dolly the sheep in 1996, the first mammal cloned from an adult. In the simplest technique, scientists use an electrical shock to fuse a skin cell from the animal to be cloned to an egg cell whose genes have been removed. The combined cell starts to grow into an embryo that is genetically identical to the skin-cell donor, and that embryo is transferred to a surrogate mother's uterus. Most pregnancies with clones end in spontaneous abortion, apparently because of abnormalities in the embryo or the placenta, which connects the embryo to the womb. But surprisingly, and important for those who wish to clone humans, the procedure appears to be quite safe for the surrogate mothers.
Most miscarriages go unnoticed because they occur so early in pregnancy.
Clones have grown up, mated and given birth to normal offspring. That all suggests that if a human clone were to survive fetal development, it would probably go on to lead a physically normal life, scientists said. Psychological health might be another matter. Scientists and philosophers have questioned whether a child who is a genetic twin of a parent might suffer from an intergenerational identity crisis. Some also worry that cloned children might be burdened by parental expectations that they turn out like the person from whom they were cloned - an unrealistic hope because the clone would grow up in different circumstances.