1. Vegetative reproduction.

Many horticultural plants are routinely propagated vegetatively by cuttings. Rooted shoots are genetically identical to the plants from which they were derived. Some plants and lower animals regularly reproduce vegetatively by budding. Dandelions and poplar trees may spread clones over an acre or more by sprouting new shoots from outlying parts of their roots. If lowly animals such as Hydra fail to bud in an aquarium a scalpel can divide them into two regenerating halves. Mammals, including humans, lack these capacities.

An alternative form of vegetative reproduction is through an aberrant sexual cycle in which the egg ends up with the same genotype as the female parent. Diverse plants circumvent the random shuffling of their genes during the meiotic (sexual) cycle in various ways. The general term for the vegetative production of seeds in higher plants is apomixis. The specific process in animals is parthenogenesis. Parthenogenetic all-female races of amphibians, reptiles and fish occur in nature and a parthenogenetic strain even of domestic turkeys has been produced. But it is unlikely that parthenogenetic births in humans will be possible anytime soon, because of specialized features of the female sexual cycle in mammals.

2. Twinning.

"Identical" twins are natural clones arising when two embryos develop from one fertilized egg. About one in sixty human births are twins. Genetic factors predispose some people to a higher likelihood of bearing twins, so twins seem to run in some families. Rates of twinning are much higher in some non human species; Armadillo babies, for example, always come as genetically identical quadruplets.

Twins can be produced artificially in laboratory animals by surgically dividing the very early embryo and implanting the two halves separately, even in different surrogate mothers. Probably this could be done for humans as well, although it has not been attempted so far. Embryos can be divided at or after the two cell stage or after the egg has cleaved into a ball of undifferentiated cells. Potentially it might be possible to culture the undifferentiated embryonic cells to allow an indefinite number of cloned individuals of one genotype.

3. Cloning offspring from a somatic cell nucleus of an existing adult animal.

Cloning offspring from a somatic cell nucleus of an existing adult animal. This is how Dolly, the celebrated cloned sheep was recently produced (Nature 385:810, 1997). Cells were taken from the udder of another sheep and cultured in aspecific way in-vitro. The nucleus was removed from a fertilized egg from one sheep by micro pippette, and the nucleus was transferred from the cultured udder cell into the enucleated egg. This reconstituted egg was cultured to an implantation stage and transferred into the womb of a surrogate mother sheep where it grew into a normal animal. Dolly had exactly the same genotype as the sheep which provided the cells from its udder.

The success of this experiment astonished most biologists because nuclear transplantation into eggs had been investigated extensively in frogs thirty years ago, and no adult frogs had ever been obtained using nuclei from cells of a mature animal. The classic experiments of Briggs and King showed that nuclei from somatic cells of very young frog larvae could support development into a normal adult, but nuclei from progressively older larvae gave poorer and poorer rates of success. Most embryos developed severe abnormalities and died. Similiar results had also been obtained from transplantations of a nuclei from one particular cell type taken at various stages in its differentiation to a final adult cell type. No adult frogs have ever been obtained using nuclei from cells of a mature animal.

So most biologists had concluded that cells undergo undisclosed nuclear changes as an animal matures, preventing their nuclei from supporting normal embryogenesis. And they were right. Sheep cells do loose this capacity by the adult stages but the change is reversible and Wilmut et al. figured out how to do just that by culturing the adult cells under conditions that modified their rate of cellular division.

Programs to clone other species of farm animals (cats, pigs, cows, other animals) from nuclei of adult cells are already in progress. Some advocates predict that Wilmut's techniques will be extended to humans within a few years, and one organization is already soliciting customers for a human cloning service that they propose to establish. But these projections are unlikely, as it will take many years to perfect the technique even for sheep. The experiment which produced Dolly was only part of a longer-term project to devise an efficient, practical protocol for cloning adult agricultural animals. Dolly was the one successful outcome among dozens of failed attempts. Nevertheless, the consensus among biologists is that, if geneticists can clone progeny from somatic cells of an adult sheep they eventually will be able to do the same for humans.