Of all the terms coined by scientists which have entered popular vocabulary, ‘clone’ has become one of the more emotive. Strictly speaking a clone refers to one or more offspring derived from a single ancestor, whose genetic composition is identical to that of the ancestor. No sex is involved in the production of clones, and since sex is the normal means by which new genetic material is introduced during procreation, clones have no choice but to have the same genes as their single parent.
In the same way, a clone of cells refers simply to the descendants of a single parental cell. As such, adult organisms can be viewed as clones because all their parts stem from the single cell which is the fertilised egg. Likewise, many tumours are clones, derived from one aberrant cell which no longer obeys the normal rules of growth control. The offspring of organisms which reproduce asexually, like corals, are also clones; as are identical twins produced by the natural, or sometimes deliberate, splitting of a single embryo.
Members of a clone are genetically identical and genetic identity has given cloning an additional more technical meaning: namely the rocedures used to create a new organism whose genetic constitution is a replica of another existing individual. Such a feat can be achieved by substituting the nucleus, which contains the genes, from one of the cells making up that individual’s body, for the nucleus of a fertilised egg. Since our genes dictate to a large extent what we look like, how we behave and what we can and cannot do, having identical genes, as identical twins do, ensures something more than mere similarity.
Novelists and film makers have not been slow to exploit the imagery fforded by cloning. Limitless numbers of identical beings manufactured from existing or previous generations has obvious dramatic potential, although seldom of a reassuring nature. Clones traverse the cinema screen as crowds of dehumanised humans destined for monotonous drudgery, as invincible armies of lookalikes from outer space, as replicas of living megalomaniacs and, in the ultimate fantasy, as the resurrected dead – troupes of little Hitlers and herds of rampaging dinosaurs.
Of course, this is science fiction. Nonetheless here is just a whiff of plausibility, a whisker of scientific credibility; enough to plant an indelible vision of what might be, or even what could be. So it is easy to understand why the arrival earlier this year of Dolly, the sheep developed from an egg whose own genes had been replaced by those from an adult udder cell, was seen as the first incarnation of a sinister future.
Dolly was a clone of the sheep (her genetic mother) who provided the udder cell. The package of genes in the nucleus of that udder cell contained exactly the same repertoire of genes as all the rest of her mother’s cells and o Dolly’s genetic makeup was to all intents and purposes identical to her mother’s. No sperm had had the opportunity to add its genetic pennysworth. However, there was nothing radically new, neither technically nor conceptually, in the way in which Dolly was made.
Almost all films and documentaries on cloning still show the same footage, produced more than twenty-five years ago during unsuccessful attempts to clone rabbits, of a nucleus being injected into an egg. What was novel about Dolly was that she was the first unequivocal mammalian clone. Lower vertebrates had been cloned in the early 1960s hen it was shown that a nucleus taken from an adult frog cell transplanted to a frog egg whose own nucleus had been destroyed was able to direct the development of that egg into a swimming tadpole.
Indeed, it was this experiment that first indicated that the genetic content of all our cells, despite the profound differences between a skin cell and kidney cell, must be more or less the same and retain all the genetic information necessary for an egg to develop into a whole organism. While cloning can offer the scientist important answers to fundamental questions about our enes, it has a much older and very natural history which long precedes the sophistications of the modern laboratory. The word ‘clone’ comes from the Greek klwn, meaning twig, and there is a very good reason for this.
For example, every chrysanthemum plant you buy at a Garden Centre is a clone of some distant and probably long dead chrysanthemum which once supplied a side-shoot for rooting. Likewise, whenever you divide an overgrown shrub or successfully cultivate a houseplant cutting you are cloning. In each case you are deliberately propagating a copy of the parent, and eventually over successive ears and many hours in the greenhouse, producing a multitude of plants (clones) all genetically identical to the prized parent.
Elm trees and other suckering plants clone themselves naturally, sending out subterranean roots from which new plants, of identical genetic constitution, will sprout. Deliberate cloning is as old as horticulture itself. Thousands of years before anyone understood the physical nature of heredity, specific genetic constitutions were preserved through cloning because they bestowed on the plant desirable qualities such as disease-resistance, high yield and predictable growth.
Cloning is as important to the production of fine wine, the supply of rubber and the fruit harvest as it is to the variety of an English country garden. Furthermore, natural cloning is not confined to plants: microbes and some insects frequently propagate themselves by producing genetically identical offspring without recourse to sex. The toothless mammal, the armadillo, gives birth not to identical twins but to genetically identical octuplets: every litter a batch of eight clones. There is nothing a priori unnatural about cloning.