Yesterday, in a turning point for genetic science, American and British researchers announced they had found the genetic lesion that causes the degenerative disease of the nervous system Huntington's chorea. The discovery marks the end of a 10- year international hunt for the gene and opens the way to understanding exactly what is going wrong in those affected by the disease.
Next week, the Royal Manchester Children's Hospital will launch the country's first programme of testing for those who might be at risk of passing on to their children the defective gene that causes cystic fibrosis.
By the early years of the next century, researchers participating in the international 'Human Genome Project' expect to have teased out and analysed every one of the hundred thousand genes that make up humanity's genetic inheritance. 'Genetic medicine' is upon us; sooner than most
The techniques of the genetic surgeons have the potential to alleviate immense human misery. More than 4,000 different types of 'single gene defect' are known to afflict humanity. Each of us is reckoned to carry about half a dozen potentially lethal flaws in the genetic inheritance we received from our parents. Fortunately most have no effect on health, because we inherit most genes in duplicate: a defective gene inherited from a father will not usually manifest itself as disease, if the corresponding copy inherited from a mother is intact. Huntington's chorea is a rare exception, where someone who inherits one defective copy of the gene will get the disease.
Problems arise when two people carrying the same genetic lesion meet and marry: there is then a risk that their children will inherit both damaged copies and suffer from the disease. More than 242 million people around the world are at risk of passing on to their children the crippling blood diseases of sickle cell anaemia or thalassaemia. About 200,000 babies are born each year with such diseases of the blood. The commonest genetic disease in the UK is cystic fibrosis, where one in 25 carries the gene and one in 2,000 babies is born with the disease.
In the face of this crying need, in the light of Carly Todd's life-saving operation, who can doubt the bona fides of the genetic scientists and surgeons? If the consequences of the new genetics were confined to clinics and operating theatres, then there would be no problem. But what is going on in research laboratories and medical consulting rooms will percolate into wider society, where it will escape the control of well-intentioned researchers and clinicians.
The record of totalitarian regimes in attempting to apply what is known about human genetics is appalling and well documented. What is less well known is that democratic societies, too, have an unhappy record in coping with new knowledge about human genetics.
In the late Sixties, the US instituted a programme of testing newborn children for sickle cell anaemia - a disease that predominantly afflicts the black population. The programme quickly gave rise to discrimination and distress.
In many states, the test failed to distinguish between children who carried the trait - who were perfectly healthy because they had only one copy of the defective gene - and those who had the disease, having inherited the defect from both mother and father. Blacks with the trait were falsely characterised as weak, and doctors wrongly recommended to parents that the children should not undertake heavy physical exercise.
There was discrimination at work: the belief got about that blacks with the trait had a defect in the oxygen- carrying capacity of their blood, and they were banned from jobs with airlines and refused entry to the US Air Force Academy.
Most pernicious of all, in a country that relies upon private health insurance, the results of the compulsory tests were entered on the children's medical records, marking them for the rest of their lives. Private health insurance is basically a gamble: I bet that I will fall sick; the insurer bets that I won't. And one way to ensure that one's clients do not fall sick is to exclude those who have a genetic predisposition to illness. Thus, in the US, genetic testing threatens to create a genetic underclass denied health insurance because of the composition of its genes.
One of the saddest aspects of the technology today is that it can offer only diagnosis; apart from a few rare conditions, such as that afflicting Carly Todd, treatment or cure are not yet in sight. Diagnosis, therefore, has little effect on the overall national incidence of genetic disease and it cannot diminish the burden such diseases place on the health-care system, so a national health service, catering for the entire community, is largely unaffected.
But the British cannot relax, for the problems of genetic diagnosis affect any insurance system. Much of this country's population looks forward to a retirement in which they enjoy a private pension in addition to the state benefit. The time may come when the providers of private pension plans demand that applicants supply the results of genetic testing.
The problem may seem remote, but it will become very real when genetic diagnosis moves beyond the diseases such as cystic fibrosis, where a defect in only one gene is involved, to conditions such as heart disease, where several genes work in concert to produce individual susceptibility. Tests for such 'polygenic' conditions are some years away, yet society needs to decide now how to handle them.
Tests for polygenic disease will only reveal a probability that someone might suffer from the ailment; such tests do not say, as the ones for single gene diseases may do, 'you have the gene, you will get the disease'. This highlights perhaps the most profound aspect of the new genetics. Perniciously and imperceptibly, by virtue of newspaper reports and half-remembered school biology, most of us are being led to believe that genes are to human biology what Newton's Law of Gravitation is to cosmology: an iron hand that inexorably determines what will happen in the future. There is 'a gene for' cystic fibrosis and there is a gene for sickle cell anaemia. But someone who has a genetic propensity to heart disease may reduce the chance of the disease manifesting itself by altering diet or taking more exercise.
There is no iron hand determining our future. The road to understanding human nature does not lie through our genes. As the nature of more and more human genes is uncovered by the researchers, there is one truth to which we must cling: that we are, all of us, more than the sum of our genes.
Tom Wilkie's book on the moral implications of the new genetics, 'Perilous Knowledge', will be published by Faber and Faber in May.
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