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A clone is a genetically identical copy of a living organism. Human clones are nothing new; they occur naturally in the recognizable form of identical twins. Not until the successful cloning of a sheep called Dolly, however, has the possibility of intentionally producing an identical copy of a human been considered seriously.
Production of a human clone would most likely utilize the same basic methodology that resulted in Dolly. A human egg would be retrieved from an individual, and its genetic material (DNA) would be removed and replaced with DNA derived from any adult human cell type. This would bypass the need for fertilization of the egg by the sperm in order to obtain a full complement of DNA. In a laboratory dish, the egg would then undergo several cell divisions. Placed into a uterus, the resulting embryo would grow and, with luck, develop until birth.
Although this cloning technique is conceptually and procedurally simple, its success rate has been extremely low. The birth of one Dolly, for instance, required the preparation of 277 re-nucleated eggs, followed by the implantation of 29 resulting best embryos. The low success rate can be attributed to the difference between the young DNA of a normally fertilized egg and the genetic material of the re-nucleated egg, which is mature and of defined destiny – it has already committed itself to a particular physiological role. In order for proper fetal development to occur, mature DNA must be coaxed into reverting to its youthful state, a complex process that will be difficult to achieve for the human species.
Beyond the practical difficulties of intentional human cloning, scientists, politicians, and others have raised serious ethical concerns about this practice. For example, there is a chance that the cloning procedure would adversely affect the developing embryo. There also might be deleterious effects on the long-term health of the clone. Proponents of human cloning counter that human sexual reproduction is not a risk-free affair either. Even if human cloning could be made safe, the motivations behind producing human clones do not fall clearly on one side of the ethical divide. While proponents stress potentially appealing motivations – a man and a woman who are both carriers of a genetic disease can use cloning to assure that their child will not be afflicted with the disorder – critics argue that the practice could and likely would be used for unethical ends, such as to select for certain traits, such as hair/eye color or sexual orientation, for example.