Genetics and Society: Impacts of Dna Fingerprinting
DNA fingerprinting, also called DNA profiling, applies a test to determine the unique DNA sequence that each person has for the purpose of identification. In the 1930s, police detectives found each person has different patterns on the tips of the fingers, and these fingerprints became the standard for identification. However, the finger patterns can be altered by surgery or other means. DNA within living cells is difficult to alter.
Sir Alec Jeffreys at the University of Leicester invented genetic printing in the mid 1980s. The DNA profile is similar to a fingerprint, existing only for that person. Jeffreys coined the term DNA fingerprint and envisioned its powerful use. A single hair, a drop of blood, semen, or other body fluid can reveal the identity of a person. DNA fingerprinting is used for identifying people, studying populations, and forensic investigations. It is in the area of forensics and crime detection that DNA use is the most promising-- as well as the most controversial.
The technology of DNA fingerprinting is based on a single assumption that no two people have the same DNA. The 3-billion-base sequence is made of four biochemical blocks or nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C). The base pairs produce more combinations or variations than there are humans. In DNA fingerprinting, scientists focus on a segment where sequences vary a great deal from one individual to another.
Five to ten percent of the genome has tandem repeats (meaning side-by-side). Jeffreys used long repeats called variable number of tandem repeats (VNTRs). Using a type of chemical scissors, called restriction enzyme, he cut the DNA at specific sites. He determined the fragment size by using a procedure known as electrophoresis.
Criminals almost always leave evidence of their identity—hair, blood, semen, and saliva. Materials are carefully collected from the crime scene. Three sources of materials are prepared for analysis: the samples from the crime scene, a hair from the prime suspect, and a control specimen whose DNA is known.
The procedure is in five steps: DNA is recovered from the sample. Purified DNA is treated with the restriction enzyme that cuts the helix wherever there is a particular sequence. This creates a virtual soup of base pairs of different lengths. A gel from an agarose solution made from seaweed is prepared, and samples to be tested are put into small wells in the gel bed. An electric current causes the fragments to move through the gel. The pieces move according to their size with the smaller ones traveling faster. The DNA of each person has different restriction fragment sizes, called restriction fragment length polymorphisms (RFLPs). Today, technicians use short tandem repeats (STRs). STRs are analyzed using polymerase chain reaction or PCR, a technique for mass-producing the sequence. PCR works on degraded DNA that requires only a few cells. A thin film of nylon is laid across the gel, and the nylon is bathed with a radioactive probe. When developed, the patterns on the nylon screen look like the barcodes in the grocery store. Experts read the prints to determine if the DNA came from the same suspect.
Jeffreys was given the opportunity to demonstrate the powerful technique in March of 1985 when he proved a boy was the son of a British citizen and should be allowed to enter the country.
In 1986, DNA was first used in forensics. In a village near Jeffreys' home, a teenage girl was assaulted and strangled. No suspect was found, although body fluids were recovered at the crime scene. When another girl was strangled in the same way, a 19-year-old caterer confessed to one murder but not the other. However, DNA analysis showed that the same person committed both murders, and the caterer had falsely confessed. Blood samples of 4582 village men were taken, and eventually the killer was revealed when he attempted to bribe someone to take the test for him.
Population statistics are essential to interpreting DNA fingerprints for court presentations. These can be extremely confusing for a lay jury. Each VNTR or STR size is given a probability value. The value is determined by a formula relating the combination of sequences occurring in the population. The alleles in the population are in or near the Hardy-Weinberg equilibrium- meaning the frequencies should not change dramatically from one generation to the next.
The first case to be tried in the US using DNA fingerprinting evidence was of African-American Tommie Lee Edwards. In November 1987, a judge did not permit population genetics statistics that compared Edwards to a representative population. The judge feared the jury would be overwhelmed by the technical information. The trial ended in a mistrial. Three months later, Andrews was on trial for the assault of another woman. This time the judge did permit the population evidence. The prosecutor showed that the probability that the chance that Edwards DNA would not match the crime evidence was one in 10 billion. Edwards was convicted.
Other applications of DNA analysis emerged. In Cardiff, Wales, Skeletal remains of a young woman were found, and a medical artist was able to make a model of the girl's face. She was recognized by a social worker as a local run-away. Comparing the DNA of the femur of the girl with samples from the presumptive parents, Jeffreys was able to declare a match and caused bone DNA analysis to be recognized in British courts. Rumors from Brazil were circulating that Wolfgang Gerhard, who had drowned in a boating accident, was the notorious Nazi of Auschwitz, Josef Mengele. Disinterring the bones, Jeffreys and his team found through DNA that the man actually was the missing Mengele.
In addition to forensics, DNA has been used to unite families. Mitochondrial DNA is inherited through the mother in a different way from nuclear DNA. In 1976, a military junta in a South American country killed over 9000 young people, and the orphaned children were given to military couples. After the regime was overthrown in 1983, Las Abuelas (The Grandmothers) determined to bring these children to their biological families. Using DNA fingerprinting, they found the families of over 200 children.
DNA has been used to solve several historical mysteries. On July 16, 1918, the czar of Russia and his family were shot, doused with sulfuric acid, and buried in a mass grave. In 1989, the site of burial was uncovered, and bone fragments of nine skeletons were assembled. DNA fingerprinting experts from all over the world pieced together the puzzle that ended in a proper burial to the Romanov royal family in Saint Petersburg in 1998.
The O.J. Simpson trial was the most famous for sensationalizing DNA analysis. The power of statistics is only effective if put in terms that the jury can understand. Evidence showed 45 bloodstained samples, and the prosecution contended that the chances that the blood sample at the crime scene could come from no one but the defendant were one in 57 billion (ten times more than people on earth). Yet, the defense was able to use arguments to cast doubt and win the case.
One of the oldest debates in American history stemmed from an 1802 story that President Thomas Jefferson had fathered a child by a slave, Sally Hemings, the half-sister of his wife. Detective work by experts showed the complete match in the polymorphisms between the male descendants of Field Jefferson, the president's paternal uncle, and a male line descendant of Eston Hemings, a descendant of Sally.
During an investigation regarding United States President Bill Clinton, the FBI informed independent prosecutor Kenneth Starr that two semen stains of specimen Q3245, a blue Gap dress owned by White House intern Monica Lewinsky, were found. A blood sample labeled K39 taken from President Bill Clinton revealed an exact match to Q3245. Applying population genetics, the probability that the DNA on the dress could have been from any Caucasian other than Bill Clinton were one in 7,820,000,000,000 7.8 trillion).
DNA fingerprinting has been a powerful tool for convicting criminals as well as proving the innocence of others. The tool will become even more powerful as world-wide databases of profiles are collected and refined.
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