Originally a botanical term meaning asexual reproduction, or reproduction by "budding" rather than by genetic recombination involving an egg and a sperm. Animals can be cloned either by splitting an embryo during cleavage (nature does this in producing monozygotic twinning) or by inserting an adult cell nucleus into a suitably primed secondary oocyte, a process called somatic cell nuclear transfer (and generally still very inefficient, resulting in a new developing embryo in at most a few percent of attempts). See also therapeutic cloning and reproductive cloning.

The use of somatic cell nuclear transfer or embryo splitting to secure pregnancy and a new individual intended to be virtually genetically identical to the person who donated the cell nucleus used or genetically identical to siblings resulting from embryo splitting (which in effect is the intentional creation of identical twins, or triplets etc.). Practiced in animals, especially farm animals. Considered ethically abhorrent because of strong ethical arguments within the fields of deontological ethics (including a duty not to intentionally violate the biological principle of sexual reproduction involving genetic reassortment and individual difference), teleological ethics (including the very high risk of severe birth defects with SCNT) and utilitarian ethics (where arguments are based on considerations by and for society generally).

The use of somatic cell nuclear transfer to produce embryonic stem cells suitable for differentiation into tissues that are a perfect match to treat disease in the person who provided the cell nucleus used. Ethically controversial.

Replication of a diploid organism after combination of haploid male and female gametes, each of which has been produced through the process of meiosis, thus assuring a reassortment of the alleles present among the two parents, in the form of a new, unique, individual genome. Preserved with in vitro fertilisation despite the absence of sexual intercourse. The opposite to cloning.

(SCNT) Transfer of the nucleus of a somatic cell into a secondary oocyte or zygote from which the egg or zygote's chromosomes have been removed, in such a way that cleavage still occurs, in effect causing the cell's nucleus to 'bud', or reproduce without genetic recombination, and hence a form of asexual reproduction or cloning now commonly used for reproductive cloning in animals and, it is hoped, for therapeutic cloning to create embryonic stem cells for treatment of serious degenerative disease and cancers.

A relatively undifferentiated cell in any tissue responsible for replenishing cells lost from a tissue through normal wear and tear, in the case of adult stem cells, or responsible for virtually all of a fetus's cells, in the case of embryonic stem cells; characterised by being able to replicate (through mitosis) seemingly for ever, either in the body or in tissue culture, without wearing out the ends of the chromosomes. Embryonic stem cells are controversial because they are derived from human embryos, sometimes by the process of somatic cell nuclear transfer, which involves creating embryos or embryo-like forms of a kind that could be used for reproductive cloning. The word stem comes from analogy with the stem of a plant giving rise to branches and further branches, representing the ever-increasing number of different paths a stem cell can differentiate into.