Reckoning with Risk by Gerd Gigerenzer

Author:Gerd Gigerenzer
Language: eng
Format: epub
ISBN: 9780141933085
Publisher: Penguin Books Ltd
Published: 2009-03-23T04:00:00+00:00

DNA Evidence

Sir Francis Galton (1822–1911), a first cousin of Charles Darwin's, was a romantic figure in the history of statistics and perhaps the last gentleman scientist.2 He was a man of ideas and a pioneer in diverse fields, including meteorology, the study of heredity, and the measurement of intelligence and the efficacy of prayer. For instance, he collected data to test whether prayer affords any advantage to its intended beneficiaries, and found none: sovereigns for whom whole nations prayed lived no longer than other members of the prosperous classes, nor did clergymen. He also pioneered the use of fingerprints for personal identification. The Galton-Henry system of fingerprint classification was introduced at Scotland Yard in 1901 and quickly adopted in English-speaking countries. In 1924, two large fingerprint collections were consolidated to form the basis of the FBI Identification Division's current database. By the late twentieth century, the division's files contained the fingerprints of more than 90 million people.

In the early 1950s, the chemical basis of heredity was discovered: a sequence of four bases (represented by the letters A, C, G, and T) called DNA (deoxyribonucleic acid). In the 1980s, a new “fingerprinting” technique—DNA fingerprinting, or somewhat less fashionably, DNA profiling—was developed by Sir Alec Jeffreys of the University of Leicester in England. Jeffreys developed a method for viewing fragments of DNA extracted from blood or other biological samples. The fragments that interested Jeffreys were noncoding regions of the human genome, that is, DNA that has no known function in the cellular production of protein.3 Because noncoding DNA is subject to less selection pressure than coding DNA, it shows a higher variability among individuals. This higher variability makes it a better tool for discriminating among people, leading to the development of a tool with enormous potential. The fingerprint analogy is not perfect; for example, identical twins have identical DNA, but not identical fingerprints. However, unlike fingerprints, DNA can be extracted from all kinds of biological traces, such as hair, saliva, and blood, and it lasts much longer.

DNA fingerprinting was widely hailed as the greatest breakthrough in forensic science in the twentieth century because of its evident applicability to criminal cases and paternity suits. Because it is not possible to observe DNA directly using instruments such as high-powered microscopes, DNA fingerprinting typically involves the following indirect method. Forensic laboratories break the long DNA strands into fragments by adding an enzyme to the sample; the fragments are then separated by electrophoresis, plotted onto a nylon membrane, and exposed to X-ray film. This process produces a series of fuzzy lines or bands called an autoradiogram, which resembles a supermarket bar code. This pattern is the DNA profile. If the bands in the DNA profile line up within a specified error range with those of a particular sample, a “match” is declared.4 Computer-based data banks with DNA fingerprints have been, or are in the process of being, installed all over the world. For instance, one month after the murder of Christina, the German Federal Crime Bureau established a DNA data bank to facilitate the capture of sexual offenders.

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