science's most reliable and potent weapons. The Pitchfork case, an extraordinary virtuoso demonstration of the technique, set the tone for years to come: genetic fingerprinting's ability to acquit the innocent, even in the face of what might seem overwhelming evidence of guilt; its ability to flush out the guilty just by the threat of its use; its amazing precision and reliability — if properly used; its reliance on small samples of bodily tissue, even nasal mucus, spit, hair or bone from a long-dead corpse.
Genetic fingerprinting has come a long way in the decade since the Pitchfork case. In Britain alone, by mid-1998 320,000 samples of D N A had been collected by the Forensic Science Service and used to link 28,000 people to crime scenes. Nearly twice as many samples have been used to exonerate innocent people. The technique has been simplified, so that single sites of minisatellites can be used instead of many. Genetic fingerprinting has also been amplified, so that tiny minisatellites or even microsatellites can be used to give unique 'bar codes'. Not only the lengths but the actual sequences of the minisatellite repeats can be analysed to give greater sophistication. Such D N A typing has also been misused or discredited in court, as one might expect when lawyers are involved. (Much of the misuse reflects public naivety with statistics, rather than anything to do with the D N A : nearly four times as many potential jurors will convict if told that a D N A match has a chance probability of 0.1 per cent than if told one in a thousand men match the D N A
- yet they are the same facts.8)
D N A fingerprinting has revolutionised not just forensic science but all sorts of other fields as well. It was used to confirm the identity of the exhumed corpse of Josef Mengele in 1990. It was used to confirm the presidential parenthood of the semen on Monica Lewinsky's dress. It was used to identify the illegitimate descendants of Thomas Jefferson. It has so blossomed in the field of paternity testing, both by officials publicly and by parents privately, that in 1998 a company called Identigene placed billboards by freeways all over America reading: ' W H O ' S T H E F A T H E R ? C A L L
1-800-DNA-TYPE'. They received 300 calls a day asking for their S E L F - I N T E R E S T 1 3 5
$600 tests, both from single mothers trying to demand child-support from the 'fathers' of their children and from suspicious 'fathers'
unsure if their partner's children were all theirs. In more than two-thirds of cases the D N A evidence showed that the mother was telling the truth. It is a moot point whether the offence caused to some fathers by discovering that their partners were unfaithful outweighs the reassurance others receive that their suspicions were unfounded. Britain, predictably, had a fierce media row when the first such private service set up shop: in Britain such medical technologies are supposed to remain the property of the state, not the individual.9
More romantically, the application of genetic fingerprinting to paternity testing has revolutionised our understanding of bird song.
Have you ever noticed that thrushes, robins and warblers continue singing long after they have paired up in spring? This flies in the face of the conventional notion that bird song's principal function is the attraction of a mate. Biologists began DNA-testing birds in the late 1980s, trying to determine which male had fathered which chicks in each nest. They discovered, to their surprise, that in the most monogamous of birds, where just one male and one female faithfully help each other to rear the brood, the female's mate quite often with neighbouring males other than their ostensible 'spouses'.
Cuckoldry and infidelity are much, much commoner than anybody expected (because they are committed in great secrecy). D N A fingerprinting led to an explosion of research into a richly rewarding theory known as sperm competition, which can 'explain such trivia as the fact that chimpanzee testicles are four times the size of gorilla testicles, even though chimpanzees are one-quarter the size of gorillas. Male gorillas monopolise their mates, so their sperm meets no competitors; male chimpanzees share their mates, so each needs to produce large quantities of sperm and mate frequently to increase his chances of being the father. It also explains why male birds sing so hard when already 'married'. They are looking for
'affairs'.10
C H R O M O S O M E 9
D i s e a s e
A desperate disease requires a dangerous remedy.
On chromosome 9 lies a very well-known gene: the gene that determines your ABO blood group. Since long before there was D N A fingerprinting, blood groups have appeared in court.
Occasionally, the police get lucky and match the blood of the criminal to blood found at the scene of the crime. Blood grouping presumes innocence. That is to say, a negative result can prove you were not the murderer absolutely, but a positive one can only suggest that you might be the murderer.
Not that this logic had much impact on the California Supreme Court, which in 1946 ruled that Charlie Chaplin was most definitely the father of a certain child despite unambiguous proof from the incompatibility of their blood groups that he could not have been.
But then judges were never very good at science. In paternity suits as well as murder cases, blood grouping, like genetic fingerprinting, or indeed fingerprinting, is the friend of the innocent. In the days of D N A fingerprinting, blood-group forensics is redundant. Blood D I S E A S E 137
groups are much more important in transfusion, though again in a wholly negative way: receiving the wrong blood can be fatal. And blood groups can give us insights into the history of human migrations, though once more they have been almost entirely superseded in this role by other genes. So you might think blood groups are rather dull. You would be wrong. Since 1990 they have found an entirely new role: they promise understanding of how and why our genes are all so different. They hold the key to human polymorphism.
The first and best known of the blood group systems is the A B O
system. First discovered in 1900, this system originally had three different names with confusing consequences: type I blood, according to Moss's nomenclature was the same as type IV blood according to Jansky's nomenclature. Sanity gradually prevailed and the nomenclature adopted by the Viennese discoverer of the blood groups became universal: A, B, AB and O. Karl Landsteiner expressively described the disaster that befell a wrong transfusion thus: 'lytischen und agglutinierenden Wirkungen des Blutserums'. The red cells all stick together. But the relation between the blood groups was not simple. People with type A blood could safely donate to those with A or A B ; those with B could donate to those with B and A B ; those with AB could donate only to those with A B ; and those with O blood could donate to anybody - O is therefore known as the universal donor. Nor was there any obvious geographic or racial reason underlying the different types. Roughly forty per cent of Europeans have type O blood, forty per cent have type A blood, fifteen per cent have type B blood and five per cent have type AB
blood. The proportions are similar in other continents, with the marked exception of the Americas, where the
