Does DNA hold all the clues?

What sex is this parrot? Either wait for it to lay eggs - or test its DNA; The OJ trial: DNA evidence appeared to point the finger; The body of Bible John exhumed: what was the truth of the killings? DNA decides

What do the following have in common: a mysterious dog which defecates by night on suburban lawns in Leicester; a car worker who wants to exhume a dead aristocrat; puzzled parrot-fanciers who cannot work out the sex of their birds; two Scottish families fighting over ownership of a Burmese cat; a Bangladeshi man trying to persuade the British immigration authorities that his son really is his son so he can enter the UK; a registered falconer who insists that his birds were bred in captivity and not taken illicitly from the wild; the man arrested for the murder of the French hitchhiker Celine Figard?

The answer is that the fate of all of them may turn on the outcome of DNA fingerprinting, forensic science's greatest breakthrough since the original inky science of fingerprinting was perfected at the turn of the century.

In recent months, DNA testing has emerged as a tool in a whole range of endeavours, using a sample obtained from saliva, hair-root, semen, blood, bones or any other part of the body. Police use it to screen large numbers of people in the hunt for perpetrators of serious crimes. Mothers use it to finger reluctant fathers in maintenance cases, and fathers in situations where they are denied access to their children. Courts use it to determine ownership of disputed animals.

The Russian authorities even persuaded the Duke of Edinburgh to have a DNA test so that they could test the bones in graves which they suspected to be those of Prince Philip's relatives - the family of the last Tsar of all the Russias, murdered in 1917.

There is even a use for DNA testing in supermarkets. Tesco and Marks & Spencer are trying out a DNA-fingerprinted spray which, triggered by an alarm, will douse shoplifters in their stores and which will mark the intruders as well as the goods and will rub off on to receivers, remaining visible to police under ultra-violet light for six months.

All this new-found power to prove depends upon the double-helix structure of deoxyribonucleic acid (DNA) which is found in the chromosomes of all body cells. This is the chemical that carries the coded instructions which lay down an individual's genetic blueprint; with the exception of identical twins, no two people have the same genetic code.

The possibility of using DNA as an identification tool came about in 1984 when Professor Sir Alec Jeffries, a geneticist at Leicester University, discovered among the three billion base units which make up a DNA molecular chain the bits which distinguished between individuals. The technique of DNA profiling which he then developed is far more versatile than fingerprinting. In the case of birds a DNA profile can now even be detected from the feathers, thanks to a technique developed by Dr David Parkin, senior lecturer in genetics at Nottingham University. His work has been the basis for a nationwide police campaign against collectors who claim they have bred rare birds like peregrine falcons, goshawks, golden and sea eagles from parents held legally under Department of the Environment registration, but who may have removed them from the wild. Suspicions that many such birds are taken illicitly from the wild could not, until recently, be proved.

"Before the DNA test was developed it was often difficult to prove a breeder was lying," says Dr Parkin. He has now been approached by experts from the dog world who suspect that a top Highland White terrier breeder is passing off bought-in puppies and claiming they are sired by his champion dog. There is even one irate character locally who wants to use DNA testing on dog dirt to find out whose animal has been misbehaving on his lawn.

"You could genetically fingerprint dog turds," Dr Parkin sighs. "There is bound to be some mucus on them containing cells from the bowel of the animal. But really you'd be better off sitting up and looking out of the window until you spotted it. Unless, that is, you've got more money than sense."

In any case the technique is not quite as magical and all-encompassing as many people seem to believe. Although everyone's DNA code is unique, DNA profiling rests on the probability of an exact match. To analyse the three billion units of an individual's entire DNA double helix would be a herculean task. So instead, forensic scientists sample segments of around 100,000 units which they hope will be typical. Analysing a part rather than the whole leaves open the possibility of error.

"All the DNA scientist is able to look at is a minute proportion of the total DNA," says Dr Patrick Lincoln, reader in haemogenetics at St Bartholomew's and the Royal London School of Medicine and Dentistry, who specialises in DNA tests in criminal and paternity cases. "If the two samples appear to match," he says, "the scientist then has to make a judgement based on a mathematical assessment of how often an individual with matching DNA occurs in the population." But there is no magic machine, the scientist has to look at what comes out and interpret it. There is always a chance that two samples will be wrongly linked.

All this can make a test hard to explain in court where juries can be falsely convinced of its strength. Questions have been raised by statisticians about the adequacy of the base material upon which this "match probability" is calculated. The chance of error is often expressed as one in many hundreds of thousands or even millions. Figures of this kind have a dramatic quality which could influence a jury strongly.

"To say there is just a million to one chance that the accused person is not guilty makes things sound pretty certain. But there are 50 million people in Britain, so that means there are 49 other people who could be the culprit," says John Wadham, director of Liberty, the civil liberties watchdog, which is concerned about DNA testing. Juries might be overly impressed by the statistics, the organisation fears, whatever caution the judge gives them.

And a lay perception of how probability is determined can be misleading. "Suppose it is a gene for blue eyes," says Dr Parkin. "That would be much less significant in, say, Finland than South Africa."

Fears have been raised of some major miscarriage of justice. Only last month police in Wales re-opened the investigation into the murder of Lynette White, a prostitute hacked to death seven years ago on St Valentine's day. Today, three years after three men were cleared by the Court of Appeal of her murder, police are investigating claims that at least two original suspects may have been wrongly eliminated from inquiries because of inadequate DNA and blood-group testing. And in the OJ Simpson trial, though the prosecution successfully presented complicated DNA evidence (which might have been enough to convict in any normal case), the defence was equally effective in undermining its value by claiming that the police crime laboratory was contaminated.

Civil liberties activists are also concerned at the opening of a Home Office National DNA Database. Under the 1994 Criminal Justice Act, police have the right to take a DNA sample from anyone who commits a recordable crime - "and that includes shoplifting and not paying your fare on the train," says John Wadham.

These are not reservations which Dr Parkin shares. "I'd be happy to give a sample to a national database - what have law-abiding people to fear?" he asks.

In any case, in practice DNA testing is as decisive in ruling out suspects as in pointing the finger at them. Earlier this month the body of the Scottish killer Bible John was exhumed 16 years after his death. DNA tests on his remains finally cleared the husband of one of the killer's victims almost 27 years after his wife was strangled.

Techniques of testing are becoming ever more sophisticated. In the past 18 months a technique has been developed called Polymerase Chain Reaction which, says Dr Lincoln, can perform the tests with less material or material of poorer quality. The new approach also yields far more accurate results. These days, says Dr Parkin, it would be possible for police to offer an IRA suspect a cigarette and after he or she has left the room get an adequate sample for testing from the cigarette tip.

The commercial world is only now awakening to the implications of all this. University Diagnostics Ltd is one of the growing number of commercial DNA testing firms. In addition to paternity tests at pounds 127, it also offers would-be parents tests (at pounds 98 a couple) to see if either is a carrier of the cystic fibrosis gene. The company's testing on animals has, over the past three years, virtually eliminated from pigs a gene which made them lean but highly stressed - removed, says its managing director, Dr Paul Debenham, not by genetic engineering but by ordinary selective breeding methods informed by the extra information DNA testing brings.

His firm is currently working on other areas: on a test to sex parrots (something which even top breeders are unable to do, except by waiting to see which one has the eggs), a test to detect the cryptosporidium parvum water parasite - which causes severe gastro-enteritis and which conventional water treatment doesn't remove - and a food diagnostic test to ascertain whether the contents of a tin of, say, Angus beef is Angus or some inferior kind of beef.

"The possibilities are endless," says Dr Debenham. "The applications may only be limited by the human imagination."

The secrets of a cell: how the testing works

To extract DNA, a hair root (or other sample) is placed in a test tube with reagents to break open the cells and release the DNA they contain. Next, solvents are added to dissolve the membranes, cell walls, fat and other tissue, leaving only the DNA behind.

The result will be only a little DNA, so a biomedical technique known as Polymerase Chain Reaction is employed to hijack the DNA's natural replication ability to make it multiply itself until there is enough material to test. This is then X-rayed on to X-ray film, where it produces a pattern that can be read like a barcode.

The time required for DNA tests varies widely. The test for cystic fibrosis, for example, is relatively simple and takes two days. A test to establish paternity, which involves matching up the bar code from progeny to those from parents (every band in the child should be present in one of the parents, allowing a 1 per cent difference for mutations), takes about four weeks. Tests to establish individual identity can take even longer.

Future DNA tests might discover which individuals are predisposed to certain illnesses - and why. They may suggest different kinds of treatment for the asthma provoked by the house mite and the asthma caused by dust. They might mean that some heart patients will be told to lay off chip butties whereas others will be told to avoid stress.