Cloning is the production of one or more individual plants or animals (whole or in part) that are genetically identical to an original plant or animal. In cloning there are three different procedures with three different goals. They are Embryo cloning, Adult DNA cloning and Therapeutic cloning.
First of is the Embryo cloning this is a technique which produces identical twins or triplets. One or more cells are removed from a fertilized embryo and encouraged to develop into one or more duplicate embryos. Twins or triplets are then formed, with identical DNA. This has been done for many years on various species of animals; only very limited experimentation has been done on humans.
Second, Adult DNA cloning is intended to produce a duplicate of an existing animal. The DNA from an ovum is removed and replaced with the DNA from a cell removed from an adult animal. Then, the fertilized ovum, now called a pre-embryo, is implanted in a womb and allowed to develop into a new animal. It is specifically forbidden by law in many countries. It has the potential of producing a twin of an existing person. Previous animal studies show that has produced several genetic defects, such as the sheep Dolly for this reason many medical ethicists consider it to be an immoral procedure when done on humans.
Finally, Therapeutic cloning is a procedure whose initial stages are identical to adult DNA cloning. However, the stem cells are removed from the pre-embryo with the intent of producing tissue or a whole organ for transplant back into the person who supplied the DNA. The goal of therapeutic cloning is to produce a healthy copy of a sick person's tissue or organ for transplant. Researches have showed that nerve cells derived from human embryonic stem cells and transplanted into paralyzed rats have enabled the animals to walk again. .
Whatever their differences, there are major ethical concerns about all three types of cloning, when applied to humans.