Written Assignment 2

In another time, Colin Pitchfork, a murderer and rapist, would have walked free. But in 1987, he was captured and convicted, betrayed by his DNA, and is now serving two life sentences in prison. Pitchfork’s downfall began when he raped and murdered two 15-year-old high school girls in a small village in England in the 1980s. The police thought they had their perpetrator when a man confessed, but only to the second murder. He denied any involvement with the first murder, though, which perplexed the police because the details of the two crimes strongly suggested that the same person committed both. At the time, British biologist Alec Jeffreys made the important discovery that there were small pieces of DNA within every person’s chromosomes that were tremendously variable in their base sequences. In much the way each person has a driver’s license or Social Security number that differs from everyone else’s, these DNA fragments are variable enough that it is extremely unlikely that two people would ever have identical sequences at these locations. Thus, a comparison between these regions in a DNA sample from a person and in evidence left at a crime scene would enable police to determine that the evidence came from that person. Jeffreys analyzed DNA left by the murderer/rapist on the victims and found that it did indeed come from a single person, and that that person was not the man who originally confessed. That original suspect was released and has the distinction of being the first person cleared of a crime due to DNA fingerprinting. To track down the criminal, police then requested blood samples of all men in the area who were between 18 and 35 years old, collecting and analyzing more than 5,000 blood samples. This led them to Colin Pitchfork, whose DNA matched perfectly the DNA left on both of the victims, and ultimately was the evidence responsible for his conviction. (He almost slipped through, having persuaded a friend to give a blood sample in his name. But when the friend was overheard telling the story in a pub, police tracked down Pitchfork to get a blood sample.)

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The DNA from different humans is almost completely identical. More than 99.9% of the DNA sequences of two individuals are the same, because we’re all of the same species and thus share a common evolutionary history. Even so, in comparing two individuals’genomes of three billion base pairs each, a one-tenth of a percent difference still translates to about three million base-pair differences. These differences are responsible for the fact that all individuals have their own unique genome. When we are trying to evaluate whether the DNA from a crime scene matches that from a suspect, the analysis focuses on the parts of our DNA that differ. There are thousands of these highly variable regions in the human genome. Among these thousands of variable regions, one particular type is used for the determination of a person’s genetic fingerprint. These regions are called STRs (for short tandem repeats) and are characterized by a short sequence (commonly four or five nucleotides) that repeats over and over within the region. The number of repeats is what varies among individuals. Here’s an example. In Individual A, the number of times the sequence repeats at one STR region (say, on chromosome 2) is 3 times on the maternal copy of chromosome 2, and 14 times on the paternal copy. Individual A is said to have two different alleles for this STR region: 3 and 14. In contrast, in Individual B, in the same STR region on chromosome 2, the sequence repeats 5 times and 11 times. Individual B has alleles 5 and 11. For an STR region within the human genome, there typically are about 10 different alleles that occur within a population, and each is shared by about 10% of all individuals in a population. So the likelihood that two individuals carry the same two alleles is about 1 in 100. This is unlikely, but given enough people, many are likely to carry the same alleles. If two different STR regions are analyzed, the likelihood that two individuals have the same four alleles is 1/10 × 1/10 × 1/10 × 1/10, or 1/10,000. This is much rarer, but still would lead to multiple individuals within the same large city carrying the same four alleles. The real power of DNA fingerprinting comes from simultaneously determining the alleles an individual carries (that is, their genotype) not simply at one or two STR locations but at 13 different STR locations. This is the number used by the FBI in constructing DNA fingerprints in the United States, and it makes the probability that two individuals would have exactly the same genotype extremely low.

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To produce a fingerprint, an individual’s genotype is determined by using PCR to amplify the STR region. Using a technique called electrophoresis, the length of the STR region is measured. The length of the region is then used to determine the number of times the STR is repeated. For a single STR region, an individual’s genotype is expressed by two numbers, as described above, reflecting the number of STR repeats in the copies inherited from the mother and from the father. And a person’s full DNA fingerprint is a string of 26 numbers, consisting of the two numbers for each of 13 STRs. In court, a suspect’s genotype might be compared with the DNA fingerprint obtained from evidence found at the crime scene. DNA samples from different people produce different 26- number fingerprints, whereas different samples of DNA from the same person will have exactly the same genotype. Despite universally accepted methods, DNA fingerprinting is not foolproof. Numerous incidences of human error—accidental as well as intentional—have been documented, ranging from mislabeled test tubes to tissue from a suspect being added to evidence from a crime scene. So we should not blindly draw conclusions solely from this one type of evidence. Nonetheless, DNA fingerprinting is an increasingly valuable tool for law enforcement, particularly because it is generally more reliable than eyewitness accounts. The FBI, for example, has reported that nearly one-third of their suspects are cleared immediately by DNA testing, and because of DNA fingerprinting, many more criminals now plead guilty to the crimes they have committed.

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