Most of these couples would then decide not to go ahead with the pregnancy. Those who did would be able to prepare for a child with cystic fibrosis.
Currently there are 300 cystic fibrosis babies born each year.
Nicholas Wald, professor of environmental medicine at St Bartholomew's Hospital in central London, is involved in the early stages of setting up a 'couples' screening trial through family doctors in Oxford, allowing testing to take place early in pregnancy. His ideas are at the front line of the application of the new genetic knowledge.
But the full benefits of knowing the identity of genes that cause diseases, or which may predispose people to developing them, are still many years away.
Three per cent of the population carry the cystic fibrosis mutations. These people are normal and will never develop the condition. They do not know they are carriers. But if carriers marry then there is a 25 per cent chance of a child having cystic fibrosis, a 25 per cent chance of a child having no defective gene for cystic fibrosis and a 50 per cent chance of a child being a carrier - like his or her parents.
Professor Wald says that when both the mother- and father-to-be are screened it is only necessary to follow up the couples who are both carriers as only their children are at risk of cystic fibrosis.
Sampling of foetal cells by standard methods will then show if the foetus is affected. It is still not a perfect method, even if all expectant couples are screened. About a third will be missed because of the variability of the cystic fibrosis gene defect. There are about 300 differences, he said.
Population screening of well men and women, who have no disease themselves, is controversial. But testing individuals who have reason to believe - because of the birth of an affected baby or from family history - that they may carry defective genes for a range of ills, is much less so. These are people who already suspect the worst.
There are more than 5,000 inherited disorders caused by single gene defects. Many more genes will predispose their carriers to diseases including cancer and heart disease given some environmental trigger. These are being hunted.
All health regions have genetic testing and counselling centres although they vary great in their range. Most now test for about 20 of the commonest inherited diseases which make up 95 per cent of all the cases. There is effectively no treatment on offer. Trials of gene therapy for cystic fibrosis and an immune disease of childhood are being undertaken. For pregnant women, the option is abortion.
Top of the list of the common gene-defect diseases which are now tested for are cystic fibrosis, muscular dystrophy, haemophilia, thalassaemia, sickle cell disease and Huntington's disease.
'Some hundred or so other rare disorders can also be tested for and over the next 10 or 15 years that will be thousands,' Professor Martin Bobrow, director of the Paediatric Research Unit at Guy's and St Thomas' Hospitals in London, said. 'Genetic traits we know about, that predispose people, that make them more likely to develop a disease, include Alzheimer's, schizophrenia and diabetes,' he said. But testing these groups is not yet a practical proposition as the factors that trigger the onset of illness are not established.
'Medical treatments that would help do not yet exist,' he said.
So despite all the excitement over the identification of new genes, there is still 'a huge gap between finding the gene and then being able to do something it', Alastair Kent, director of the Genetic Interest Group, said. This is an umbrella organisation representing 120 support groups and charities concerned with genetic disorders. 'It is all happening very rapidly. Knowledge is increasing almost as we speak.'
He went on: 'Knowing you carry one of these genes at least gives people the option of deciding what they might do about it. They have the chance to make informed choices. They will know what their chances are for passing on a condition to their children. They may decide not to have children or to have more children.'
Mr Kent added: 'It is knowledge that allows people to make the decisions they feel are most right for them.'
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