Science: The normal, the abnormal and the geneticist: Bernard Dixon reports on the rising ethical challenge of the DNA revolution

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THE POWER of modern genetics was vividly displayed last week, with the announcement that American researchers had discovered a possible genetic component to homosexuality.

It is but the latest of what is turning into a torrent of advances in our knowledge of human genetic anatomy. With appropriate timing, some 3,000 geneticists from around the world will gather next month in Birmingham's International Convention Centre for the 17th International Congress of Genetics. In the 40 years since Francis Crick and Jim Watson showed how the DNA double helix could account for heredity, the science of genetics has been transformed almost beyond recognition. Yet those very advances have also provoked suggestions that some of its exponents are becoming extravagant in their ambitions.

Before Crick and Watson ushered in a new era, geneticists knew exactly what they were doing. They studied the patterns in which countless characteristics were inherited in animals and plants. Those patterns indicated that genes, discrete units, carried these characteristics from one generation to the next. Meanwhile, other biological and medical scientists were pursuing different aspects of living things - biochemists studied the chemical changes inside cells, cancer researchers investigated cancer, neuroscientists probed the workings of the brain.

Over the past decade, however, all these disciplines have been revolutionised by ideas and techniques from molecular genetics. We now know that genes are stretches of DNA. They carry the instructions for making particular proteins - ranging from the components of muscles to hormones (such as insulin). Many diseases have been traced to proteins with

abnormal structures resulting from corresponding errors or mutations in the DNA.

Over the past decade, scientists have located many of these mutated genes, including those responsible for cystic fibrosis, and they have determined the sequences of units in the mutated DNA. This permits carriers of abnormal genes to be identified. Couples at risk of producing an affected child can decide on abortion - or in some cases have implanted in the woman's womb a foetus known not to carry the defective gene.

Genetic engineers can also transfer desirable genes from one type of cell to another and clone them. This has allowed drugs such as human growth hormone and certain vaccines to be manufactured more safely than before.

These practical developments illustrate the revolution that has affected virtually all of the biomedical sciences. Biochemists now clone and sequence the genes responsible for metabolic changes. Cancer researchers focus on the oncogenes that trigger malignant changes. Neuroscientists locate genes for the 'receptors' that respond to chemical messages in the nervous system.

The entry of molecular genetics into so many other sectors has been accompanied by a significant trickle of concern that this powerful and practically successful science may be overreaching itself. Last year, the British Medical Journal carried an editorial headed 'The allure of genetic explanations', warning of social and political dangers in simplistic attempts to account for human behaviour solely in terms of genes. Several recent books have also attacked what their authors see as reductionist tendencies in contemporary genetics and their practical application.

Ruth Hubbard and Elijah Wald's Exploding the Gene Myth, for example, examines possible abuses of genetic screening in insurance and employment. And it considers other scenarios: 'The development of tests to detect genes, or substances whose metabolism they affect, opens the door for the invention of an unlimited number of new disabilities and diseases.

'For any metabolite or other trait that has a normal distribution in the population, some people can be defined as having 'too much' and others 'not enough' . . . Pharmaceutical companies and physicians stand to make a good deal of money from inventing new diseases as fast as new diagnostic tools are developed that

can spot or predict their occurrence.'

Most geneticists will see such ideas as far-fetched, to say the least. Nevertheless, their craft commands tremendous power and promises great benefits and problems. As well as transforming other sciences, genetics is posing many challenges for society. For this reason, the Birmingham congress will have an unusually extensive public awareness programme.

This should be a congress to remember.