Consider the case of the home-grown dentures and the transplanted teeth. At last week's British Association meeting in Belfast, Professor Mark Ferguson, a biologist with a special interest in the allied subjects of teeth and crocodiles, delivered one of those wide-ranging speculative lectures about the potential of genetic engineering which make more cautions colleagues fear a wave of public alarm.
Head of the department of cell and structural biology at Manchester University, Ferguson warned dentists that, within their lifetimes, their art might be transformed by new genetic techniques. Dentures made of human dental material could be grown by programming bacteria equipped with the necessary human gene to manufacture them, he said. Cells from human embryos might be programmed to grow into new teeth.
The use of embryos to patch up other human beings is among the most controversial of the host of ambiguous possibilities raised by the new lines of research. Even if practicable, it would be banned in this country at the moment, and would be deeply repugnant to many people. Researchers are on tenterhooks that public revulsion over one area such as this may block wider inquiries which contain an immense potential for good.
The most exciting possibilities of all are in the medical field. As other illnesses yield to treatment, an increasing proportion of patients, especially children, suffer from genetic disabilities. Although it will probably never be possible for anyone to trade in their old genes for a new set - they would be trading in their identities - genetic techniques give hope of dramatic new means of prevention, diagnosis and treatment.
If the defective gene is identified, parents in high-risk groups can be reassured that their unborn child is safe, or forewarned if it is affected. Understanding the way the gene acts chemically may point to ways of counteracting the often mysterious malfunctions it induces. In diseases with a strong genetic component, like diabetes and coronary disease, individuals predisposed to it could be warned in time to adapt their way of life and given regular health tests.
In some cases, such as sickle cell anaemia, a transfusion of the child's own tissue back into its bone marrow with the missing gene spliced in might equip it with the missing defences. Experiments in this field are expected in the United States in the next year or two. But major strides have already been made in the past few months with the two most important genetic diseases affecting children in Britain - muscular dystrophy, subject of research by US scientists in Boston, and cystic fibrosis, being fought by Dr Bob Williamson's team at St Mary's, Paddington.
"Once you understand what the defective gene does, you can go to the drug designers and ask for a medicine that will interact with the defective protein and cause it to function properly," Williamson says. "No one can say how long it will be before we have a useful treatment, but it will certainly be in the lifetime of young people who now have cystic fibrosis. The protein will be known within a few months or years - but it is sure to be 10 years or more before a drug comes on the market, because safety tests have to be so thorough."
Because all living things are related, skills developed with animals or plants - with ethically acceptable aims - inevitably open up possibilities of similar activity in the human context which would not be acceptable. American researchers are reported to have produced pigs engineered to grow faster and supply leaner bacon for low-cholesterol rashers, and a genetically-tailored virus designed to attack insect pests is on the point of experimental release in Britain. But there are practical as well as ethical dangers, with the possibility of some mutated virus, plant or animal, which might spread uncontrollably.
In Britain, agricultural experiments are closely supervised by the Health and Safety Commission's genetic advisory committee, and research in the medical sphere by a voluntary licensing authority set up by the Medical Research Council and the Royal College of Obstetricians and Gynaecologists.
In practice, the dangers are less imminent than science fiction would suggest. As yet, it has proved impossible to transplant more than one gene from one species into another without causing the embryo to abort. The prospects of cows as big as elephants and sheep growing silk instead of wool are severely limited by practical difficulties. The American pigs built to give lean meat have so little fat that they must be mollycoddled to ensure that they do not die of cold.
Similarly, most altered plant or bacterial strains are likely to be so specialized that they will not thrive outside artificial conditions. But the theoretical risk remains, and the penalties of a mistake are incalculable.
"They have had some success in America, but they have had problems - arthritis in the joints and so on," says Professor Barry Cross, director of the Agriculture and Food Research Council's research into animal physiology and genetics. "Evolution is no fool - it eliminates the weak bits. If you jump millions of years by sticking in a gene without knowing all the complexities, you are liable to come unstuck."
If scientists are able to redesign the shape and size and animals by altering their genes, it is highly probable - in principle - that they could do the same to selected strains of future human beings. It is not difficult to think of useful properties which it might be nice to splice in, from increased resistance to the common cold to athletes bred for success in the Olympics.
But this is a banned area of inquiry, and is likely to remain so for the foreseeable future. "Until these systems are up and going in livestock and have ceased to attract any sort of notoriety, it will be out of the question even to consider attempting them in the human context," Cross says.
"As of now, intelligence genes have not been identified," says a researcher in a different area of human genetics. "But it will be medically very valuable to study the genes for mental handicap - and by studying those one will willy-nilly get information about genes that affect mental function. But everyone in the field would regard 'genius manufacture' as unethical."
Professor Martin Bobrow of Guy's Hospital does not believe that we shall ever see human engineering, in the sense of modifying an individual's basic characteristics. "If you mess around with one part of an organism you affect the whole balance of it, perhaps harmfully, in unpredictable ways," he says. "And, if they were human beings, you couldn't just destroy your failed experiments.
"In any case, we don't have clearly defined objectives. Most people would reckon that it is a disadvantage to be extremely short. But to be extremely tall is not necessarily an advantage. As for intelligence, the fact that there is such a wide natural range may suggest that in practice there is no adaptive advantage in being intelligent - otherwise we would all be near the top of the range.
"The fact is that our state of ignorance about these rather complex issues is quite substantial. To extrapolate from changing the hormone balance in mice to human intelligence is a bit like throwing a spanner into a spacecraft and expecting to improve its operation."
Any structural change permanently impolanted in the genes of human beings would create an open-ended problem for the future, as the subjects of the experiment passed their characteristics on to their children. Even if the results could be predicted, the outcome would be, in effect, to create artificial races of man.
