Dna: The Secret of Life

by Watson, James

  More interesting than the relatively few differences we see among the races is what we all have in common – what it is that makes us so different from our closest relatives. As we have seen, our lineal split from the chimpanzee about 5 million years ago has barely given us enough time apart to evolve a 1 percent genetic difference. But in that 1 percent lie the critical mutations that make us the remarkable thinking, speaking creatures we are. It may be debated whether other species possess some limited form of consciousness, but clearly none of them has produced a Leonardo da Vinci or a Francis Crick.

  The chromosomes of humans and chimpanzees are very similar. Chimpanzees, however, have 24 pairs whereas we have 23. It turns out that our chromosome 2 was produced by the fusion of two chimpanzee chromosomes. There are also differences in the human and chimpanzee versions of chromosomes 9 (bigger in humans) and 12 (bigger in chimpanzees) and several examples of inversions (or flips) within chromosomes that differ in humans and chimpanzees. Whether these chromosomal differences will prove significant is hard to say.

  The relative merits are not much clearer at the biochemical level, where so far we know of only two differences between humans and chimpanzees. Difference 1: In both species a sugar molecule called sialic acid appears on the outside of every cell. But while the molecule is subtly modified in chimpanzees through the action of an enzyme, in humans, the gene encoding that enzyme is always mutated: no enzyme is produced, and human cell-surface sialic acid is unmodified. We have no clue at all as to whether this is significant. Difference 2: This one, discovered in 2002 by Svante Pääbo's group, is more suggestive: a difference in FOXP2, a gene known to be involved somehow in human language. (Because mutations in the human version have been found to cause linguistic impairment, FOXP2 has been misleadingly dubbed by the media as "the grammar gene.") Out of a chain running 715 amino acids, just two changes distinguish humans from chimpanzees and gorillas, whose FOXP2 proteins are identical. In fact, these amino acids are identical in all mammals tested except for humans. Moreover, statistical analysis of the pattern of DNA variation in and around the gene suggests that natural selection may have had a role in shaping the protein during human evolution. It is therefore tempting (but premature) to suggest that FOXP2 is the evolutionary equivalent of a smoking gun – a glimpse of a critical step in the origin of language.

  Pääbo's lab has also pioneered a promising and original approach to identifying other genes that may encode the critical difference(s). Using DNA microarrays, which determine what genes are switched on in a particular tissue (see chapter 8), Pääbo has compared patterns of gene expression – which genes are switched on – in humans, chimpanzees, and macaque monkeys for three different tissues: white blood cells, liver, and brain. As would be expected on the basis of their close relationship, humans and chimpanzees fall out close to each other for both blood cells and liver. However, the pattern of gene expression in the brain tells a totally different story: the human brain is very different from those of the chimpanzee and macaque. Perhaps this is not entirely surprising: most of us would not need a laboratory full of equipment to figure out that human brains are distinct from chimpanzee brains. The research's significance lies instead in its ability to provide us with an inventory of the genes whose expression differs between human and chimpanzee brains. Even that will be only a start at best. It is unlikely that, even once we have a full catalog of the underlying mechanisms, we shall understand precisely how they set us apart. Our humanness is likely much more difficult to describe than even a precisely detailed list of controlled molecular events. But in our search for its genetic underpinning we are now at least beginning to assemble a list of suspects.

  As I write this, the chimpanzee genome project is nearing completion. When it is done, the DNA making up the 1 percent difference that King and Wilson identified will be revealed. My guess is that they will be proved right: the critical differences will lie not in the genes themselves but in their regulation. Humans, I suspect, are simply great apes with a few unique – and special – genetic switches.

  Molecular biology's grandest mission is surely to answer questions about ourselves and our origins as a species. But each human soul yearns to know its own story as well as that of its kind. DNA can provide a more individualized account of ancestry as well. In a sense, written in my DNA molecules is the history of my evolutionary lineage, a narrative that can be viewed at different levels. I can situate the sequence of my mtDNA into Cann and Wilson's human family tree, or I can look in greater detail at my known family's past. My Y chromosome and mtDNA will tell different stories – my mother's side, and my father's.

  I was never interested in genealogy. But my family – like many, I suspect – had its own in-house archivist in the form of my aunt Betty, who spent a lifetime worrying about who was related to whom and how. It was she who found that the Watsons – of lowland Scots stock – first appeared in the United States in 1795 in Camden, New Jersey.

  And it was she who insisted that some paternal ancestor of mine designed Abe Lincoln's house in Springfield, Illinois. But I've always been more interested in my Irish side, my maternal grandmother's family. My mother's grandparents fled Ireland during the great potato famine of the 1840s, ending up in Indiana, where her grandfather, Michael Gleason, died in 1899, the year my mother was born. On his gravestone it says he had come from a town in Ireland called Glay.

  On a visit to Ireland, I tried to find out more about my great-grandfather at the County Tipperary Records Office, whose quarters in Neneagh, twenty miles from Limerick, had formerly been a prison. My sleuthing was singularly unsuccessful. Finding no record at all of "Glay," I could only conclude that name as spelled on the tombstone of my probably illiterate ancestor was fanciful. Thus ended my only brush with genealogical research, until recently. Now that the framework of the human family tree has been laid out by Cann and others, I am keen to see where I fit in. Companies like Bryan Sykes's Oxford Ancestors represent the new face of genealogical research, with high-tech laboratories to replace dusty archives. With a sample of my DNA, Oxford Ancestors has conducted both mtDNA and Y chromosome analysis. Sadly the tests revealed nothing romantic, no exotic ancestry. I really am, as I feared, largely the product of generic Scots-Irish stock. I cannot even blame my more brutish attributes on ancient Viking incursions into my bloodline.

  CHAPTER TEN

  GENETIC FINGERPRINTING:

  DNA'S DAY IN COURT

  In 1998 Marvin Lamont Anderson, thirty-four years old, was released from the Virginia State Penitentiary. He'd been there for fifteen years, almost all his adult life, convicted of a horrific crime: the brutal rape of a young woman in July 1982. The prosecution had presented an unambiguous case: the victim recognized Anderson from a photograph; she picked him out in a lineup; and she identified him in court. Found guilty on all counts, he was given consecutive sentences totaling over two hundred years.

  A clear-cut case. A better defense attorney, however, might have been more effective in countering the prosecution's efforts to stack the deck against the defendant. Anderson was picked up based exclusively on the (white) victim's report to the police that her (black) assailant had boasted of "having a white woman"; so far as the authorities knew, Anderson was the only local black man with a white girlfriend.

  Among the mug shots the victim looked at, only Anderson's was a color photograph. And of the men whose pictures she was shown, he alone was placed in the lineup. And although another man, John Otis Lincoln, was shown to have stolen, about thirty minutes before the attack took place, the bicycle used by the assailant, Anderson's attorney failed to call Lincoln as a witness.

  Five years after Anderson's trial, Lincoln confessed under oath to the crime, but the trial judge declared him a liar and refused to act. Anderson meanwhile continued to protest his innocence and requested that DNA analysis be done on the physical evidence from the crime scene. But he was told that it had all been destroyed in accordance with standard procedure. It was then that A
nderson contacted the lawyers of the Innocence Project, a group that had gained national attention using DNA analysis to establish definitive evidence of guilt or innocence in criminal proceedings. While the Innocence Project worked on Anderson's request, he was released on parole; assuming no violations, he would remain a parolee until 2088, easily the rest of his life.

  In the end, Anderson's salvation was the sloppiness of the police technician who had performed the inconclusive blood group analysis on the crime scene material in 1982. She had failed to return the samples to the proper authorities for routine destruction, and so they still existed when Anderson asked for a reexamination. The director of the Virginia Department of Criminal Justice, however, refused the request, arguing it might establish an "unwelcome precedent." But under a new statute, the Innocence Project attorneys won a court order calling for the tests to be performed, and, in December 2001, the results proved categorically that Anderson could not have been the assailant. The DNA "fingerprint" matched Lincoln's. Lincoln has since been indicted and Anderson pardoned by Governor Mark Warner of Virginia.

  DNA fingerprinting – the technique that rescued Marvin Anderson from an undeserved life sentence – was discovered by accident by a British geneticist, Alec Jeffreys (see Plate 52). From the earliest days of the recombinant DNA revolution, Jeffreys had been interested in genetic differences among species. His research at Leicester University focused on the myoglobin gene, which produces a protein similar to hemoglobin, found mainly in muscle. It was in the course of this "molecular dissection" that Jeffreys found something very strange: a short piece of DNA that repeated over and over again. A similar phenomenon had been observed in 1980 by Ray White and Arlene Wyman, who, looking at a different gene, had shown that such repeats varied in number from individual to individual. Jeffreys determined that his repeats were junk DNA, not involved in coding for protein, but he was soon to discover that this particular junk could be put to good use.

  Jeffreys found that this short stretch of repeating DNA existed not only in the myoglobin gene but was scattered throughout the genome. And although the stretches varied somewhat from one repetition to the next, all of them shared one short, virtually identical sequence of some fifteen nucleotides. Jeffreys decided to apply this sequence as a "probe": using a purified sample of the sequence tagged with a radio– active molecule, he could hunt for the sequence genomewide. With DNA from the genome laid out on a special nylon sheet, the probe would stick down, by base-pairing, wherever it encountered its complementary sequence. By placing the nylon on a piece of X-ray film, Jeffreys could then record the pattern of radioactive spots. When he developed the film from the experiment, he was astonished by what he saw. The probe had detected many similar sequences across a range of DNA samples. But there was still so much variability from one sample to the next that even among ones taken from members of the same family you could tell the individuals apart. As he wrote in the resulting paper in Nature in 1985, the "profile provides an individual-specific DNA 'fingerprint.' "

  Jeffreys's choice of the term "DNA fingerprint" was quite deliberate. This technology clearly had the power to identify an individual, just like traditional fingerprinting. Jeffreys and his staff obtained DNA samples from their own blood and subjected them to the same procedure. The images on X-ray film, as expected, made it possible to distinguish unambiguously between people. He realized the range of potential uses was extensive:

  In theory, we knew it could be used for forensic identification and for paternity testing. It could also be used to establish whether twins were identical – important information in transplantation operations. It could be applied to bone marrow grafts to see if they'd taken or not. We could also see that the technique [would work] on animals and birds. We could figure out how creatures are related to one another – if you want to understand the natural history of a species, this is basic information. We could also see it being applied to conservation biology. The list of applications seemed endless.

  But the procedure's first practical application was stranger than any Jeffreys had anticipated.

  In the summer of 1985, Christiana Sarbah was at her wits' end. Two years before, her son, Andrew, had returned to England after visiting his father in Ghana. But at Heathrow, British immigration authorities had refused to admit the boy, though he had been born in Britain and was a British subject. Denying that Sarbah was his mother, they alleged that Andrew was, in fact, the son of one of Sarbah's sisters and was trying to enter the country illegally on a forged passport. After reading a newspaper report about Jeffreys's work, a lawyer familiar with the case asked the geneticist for help. Could this new DNA test prove that Andrew was Mrs. Sarbah's son and not her nephew?

  The analysis was complicated by the fact that neither the father nor Sarbah's sisters were available to give samples. Jeffreys prepared DNA from samples taken from the mother and three of her undisputed children. The analysis showed that Andrew had the same father as the other children, and that Sarbah was his mother. Or more specifically, that chances were less than 1 in 6 million that one of her sisters was his mother. The immigration authorities did not challenge Jeffreys's results but avoided formally admitting the error by simply dropping the case. Andrew was reunited with his mother. Jeffreys saw them afterward: "The look of relief on her face was pure magic!"

  But would the technique work with blood, semen, and hair, the body tissues typically found at crime scenes? Jeffreys was quick to prove that it could indeed, and soon his DNA fingerprints would gain worldwide attention, revolutionizing forensic science.

  On a Tuesday morning in November 1983, the body of a fifteen-year-old schoolgirl named Lynda Mann was found on the Black Pad, a footpath outside the village of Narborough, near Leicester in England. She had been sexually assaulted. Three years passed with no arrest in the case. Then, it happened again: on a Saturday in August 1986 the body of Dawn Ashworth, another fifteen-year-old, was found on Ten Pond Lane, another footpath in Narborough. The police were convinced that the same man had committed both murders and soon accused a seventeen-year-old kitchen assistant. But, while confessing to the Ashworth murder, the suspect denied involvement in the earlier case. So it was that the police consulted Alec Jeffreys to confirm that their suspect had killed both girls.

  Jeffreys's fingerprint analysis contained both good and bad news for the authorities: Comparison of samples from the two victims showed that the same man had indeed carried out both murders, as the police believed. Unfortunately (for the police) the same test also proved that the kitchen worker in custody had not murdered either girl, a result confirmed by other experts the police called in. The suspect was released.

  With their only lead now blown, and worries rising in the local community, the police took an extraordinary step. Confident that DNA fingerprinting would yet prove the key to success, they decided to request DNA samples from all adult males in and around Narborough. They set up stations to collect blood samples and were able to eliminate a great many candidates by the traditional, and cheaper, test for blood type. The remaining samples were sent for DNA fingerprinting. A good Hollywood version of the story would, of course, have Jeffreys identifying the true killer. And it did happen that way, but not without a further plot twist, also worthy of Tinseltown. The culprit initially eluded the genetic dragnet. When faced with providing the mandatory sample, Colin Pitchfork, pleading a terror of needles, persuaded a friend to furnish a sample in his stead. It was only later, when the friend was overheard telling of what he had done, that Pitchfork was picked up and thus gained the dubious honor of being the first criminal ever apprehended on the basis of DNA fingerprints.

  The Narborough case showed law enforcement agencies worldwide that DNA fingerprinting was indeed the future of criminal prosecution. And it would not be long before such evidence was first adduced in an American legal proceeding.

  Perhaps the British are, culturally, more accepting of authority, or perhaps recondite molecular mumbo jumbo was just more likely to ru
b Americans the wrong way, but in any case the introduction of DNA fingerprinting into the United States was highly controversial.