A controversial issue is the possible application of new genetic engineering techniques to produce human clones. So far, genetic engineering and cloning have been used to clone plants, single-celled organisms, amphibians and simple mammals. This led to significant advances in agriculture, industry and medicine. New genetic engineering techniques have allowed scientists to clone more complex mammals and have opened up the possibility of cloning humans. While this technology offers many potential benefits, the prospect of human cloning has raised many practical, ethical, and religious dilemmas currently debated by society. At the moment, the actual cloning of humans does not seem likely to occur in the near future. Genetic engineering is the modification of an organism's DNA, genetic material to eliminate unwanted traits or produce desirable traits. The first form of genetic engineering dates back to the scientist Gregor Mendel who experimented with peas. He selected only peas with the most desirable characteristics to obtain a healthier and stronger pea (McCuen 8). This method, called selective breeding, is still used today with plants and animals to increase food production. Corn plants are selectively bred to produce a larger, tastier kernel. Another type of genetic engineering called hybridization or crossbreeding involves breeding animals of different species to obtain the most desirable traits of both. Male donkeys are crossed with mares to produce mules, which make excellent working animals (Levine 1). In 1938, Hans Speman proposed cloning a mammal by transplanting the nucleus of an adult cell into a fertilized egg. This process is called nuclear transfer and was first used to clone a frog in 1952 (Sinha 59). Using this process, nuclear DNA from a donor frog's body cell was injected into the egg cell of a recipient frog whose nuclear genetic material had been removed. The fused cells divided just like a normal fertilized egg and formed an embryo genetically identical to the donor frog. In 1980 mice were successfully cloned using a similar procedure. The nucleus of a body cell from an embryo taken from a pregnant mouse was inserted into a fertilized egg from another mouse whose nucleus had been removed. The cell was grown in vitro until it divided and became an embryo. It was then implanted into another mouse and allowed to mature.