29 June 2009

Genetic “fingerprinting”

The invention: Atechnique for using the unique characteristics of each human being’s DNA to identify individuals, establish connections among relatives, and identify criminals. The people behind the invention: Alec Jeffreys (1950- ), an English geneticist Victoria Wilson (1950- ), an English geneticist Swee Lay Thein (1951- ), a biochemical geneticist Microscopic Fingerprints In 1985, Alec Jeffreys, a geneticist at the University of Leicester in England, developed a method of deoxyribonucleic acid (DNA) analysis that provides a visual representation of the human genetic structure. Jeffreys’s discovery had an immediate, revolutionary impact on problems of human identification, especially the identification of criminals. Whereas earlier techniques, such as conventional blood typing, provide evidence that is merely exclusionary (indicating only whether a suspect could or could not be the perpetrator of a crime), DNA fingerprinting provides positive identification. For example, under favorable conditions, the technique can establish with virtual certainty whether a given individual is a murderer or rapist. The applications are not limited to forensic science; DNA fingerprinting can also establish definitive proof of parenthood (paternity or maternity), and it is invaluable in providing markers for mapping disease-causing genes on chromosomes. In addition, the technique is utilized by animal geneticists to establish paternity and to detect genetic relatedness between social groups. DNAfingerprinting (also referred to as “genetic fingerprinting”) is a sophisticated technique that must be executed carefully to produce valid results. The technical difficulties arise partly from the complex nature of DNA. DNA, the genetic material responsible for heredity in all higher forms of life, is an enormously long, doublestranded molecule composed of four different units called “bases.” The bases on one strand of DNApair with complementary bases on the other strand. A human being contains twenty-three pairs of chromosomes; one member of each chromosome pair is inherited fromthe mother, the other fromthe father. The order, or sequence, of bases forms the genetic message, which is called the “genome.” Scientists did not know the sequence of bases in any sizable stretch of DNA prior to the 1970’s because they lacked the molecular tools to split DNA into fragments that could be analyzed. This situation changed with the advent of biotechnology in the mid-1970’s. The door toDNAanalysis was opened with the discovery of bacterial enzymes called “DNA restriction enzymes.” A restriction enzyme binds to DNA whenever it finds a specific short sequence of base pairs (analogous to a code word), and it splits the DNAat a defined site within that sequence. A single enzyme finds millions of cutting sites in human DNA, and the resulting fragments range in size from tens of base pairs to hundreds or thousands. The fragments are exposed to a radioactive DNA probe, which can bind to specific complementary DNA sequences in the fragments. X-ray film detects the radioactive pattern. The developed film, called an “autoradiograph,” shows a pattern of DNA fragments, which is similar to a bar code and can be compared with patterns from known subjects. The Presence of Minisatellites The uniqueness of a DNA fingerprint depends on the fact that, with the exception of identical twins, no two human beings have identical DNA sequences. Of the three billion base pairs in human DNA, many will differ from one person to another. In 1985, Jeffreys and his coworkers, Victoria Wilson at the University of Leicester and Swee Lay Thein at the John Radcliffe Hospital in Oxford, discovered a way to produce a DNA fingerprint. Jeffreys had found previously that human DNA contains many repeated minisequences called “minisatellites.” Minisatellites consist of sequences of base pairs repeated in tandem, and the number of repeated units varies widely from one individual to another. Every person, with the exception of identical twins, has a different number of tandem repeats and, hence, different lengths of minisatellite DNA. By using two labeled DNA probes to detect two different minisatellite sequences, Jeffreys obtained a unique fragment band pattern that was completely specific for an individual. The power of the technique derives from the law of chance, which indicates that the probability (chance) that two or more unrelated events will occur simultaneously is calculated as the multiplication product of the two separate probabilities. As Jeffreys discovered, the likelihood of two unrelated people having completely identical DNAfingerprints is extremely small—less than one in ten trillion. Given the population of the world, it is clear that the technique can distinguish any one person from everyone else. Jeffreys called his band patterns “DNAfingerprints” because of their ability to individualize. As he stated in his landmark research paper, published in the English scientific journal Nature in 1985, probes to minisatellite regions of human DNA produce “DNA ‘fingerprints’ which are completely specific to an individual (or to his or her identical twin) and can be applied directly to problems of human identification, including parenthood testing.” Consequences In addition to being used in human identification, DNA fingerprinting has found applications in medical genetics. In the search for a cause, a diagnostic test for, and ultimately the treatment of an inherited disease, it is necessary to locate the defective gene on a human chromosome. Gene location is accomplished by a technique called “linkage analysis,” in which geneticists use marker sections of DNA as reference points to pinpoint the position of a defective gene on a chromosome. The minisatellite DNA probes developed by Jeffreys provide a potent and valuable set of markers that are of great value in locating disease-causing genes. Soon after its discovery, DNA fingerprinting was used to locate the defective genes responsible for several diseases, including fetal hemoglobin abnormality and Huntington’s disease. Genetic fingerprinting also has had a major impact on genetic studies of higher animals. BecauseDNAsequences are conserved in evolution, humans and other vertebrates have many sequences in common. This commonality enabled Jeffreys to use his probes to human minisatellites to bind to the DNA of many different vertebrates, ranging from mammals to birds, reptiles, amphibians, and fish; this made it possible for him to produce DNA fingerprints of these vertebrates. In addition, the technique has been used to discern the mating behavior of birds, to determine paternity in zoo primates, and to detect inbreeding in imperiled wildlife. DNA fingerprinting can also be applied to animal breeding problems, such as the identification of stolen animals, the verification of semen samples for artificial insemination, and the determination of pedigree. The technique is not foolproof, however, and results may be far from ideal. Especially in the area of forensic science, there was a rush to use the tremendous power of DNA fingerprinting to identify a purported murderer or rapist, and the need for scientific standards was often neglected. Some problems arose because forensic DNA fingerprinting in the United States is generally conducted in private, unregulated laboratories. In the absence of rigorous scientific controls, the DNA fingerprint bands of two completely unknown samples cannot be matched precisely, and the results may be unreliable.