Genetic Fingerprinting
Fingerprints are unique to each individual. Methods of recording and matching fingerprints have allowed police to correctly identify many criminals. Genetic scientists have recently developed another tool for identification based on the uniqueness of each person's genes. Genetic differences account for the large variations we see among individuals. This genetic variability is expressed in obvious traits like hair color and genetic disorders such as hemophilia. However, more genetic variability is hidden from view and can only be detected by directly studying the deoxyribonucleic acid (DNA). Each human has approximately 100,000 genes in the chemical form of DNA. The genetic information coded in the genes varies greatly between individuals. Thus, no two humans, except for identical twins, have exactly the same genetic code. A description of a person's DNA that is detailed enough to distinguish it from another person's DNA is called a DNA or genetic "fingerprint." In 1985, an English researcher named Alec Jeffreys developed a technique to visualize a person's genetic code. This direct DNA analysis revealed so much variation in the genetic code between different people that even a small section of the entire genetic code could identify an individual's special combination of traits. Jeffreys knew that human DNA had many multi-repeated segments called minisatellites, and that the number and length of minisatellite DNA varied widely from person to person. He used a special detergent to break open the human cells and release the DNA code into solution. Then a restriction enzyme called HinfI broke the chain of DNA codes at sites close to each minisatellite DNA. The fragments of DNA were then attached to a membrane and allowed to combine with a radioactive minisatellite probe. After several hours, these probe molecules located and attached to certain predefined areas of the DNA fragments.
X-rays were taken of the membrane to show where the radioactive probes attached. These pictures were then used to compare bands of DNA just as fingerprints are compared. Three years later, Henry Erlich developed a method of DNA fingerprinting so sensitive that it could be used to identify an individual from an extremely small sample of hair, blood, semen, or skin. Erlich's technique used Jeffreys' traditional method and combined it with a technique called polymerase chain reaction (PCR). First discovered by Kary Mullis, PCR was used to duplicate DNA and thus copy the genetic code. Erlich was able to duplicate and heat-separate the DNA fragments from a single human hair root many times using PCR. Ultimately, PCR multiplied the DNA from one single hair to an amount equivalent to that found in a million identical strands of hair. The amplified DNA was then used to obtain a DNA fingerprint. Genetic fingerprinting has already proved to be a very useful tool. Initially, it was used exclusively in forensic science and law. This technique has helped to link suspects to crimes where a single drop of blood was the only clue. Maternity and paternity matters have also been settled using genetic fingerprinting. This technology's impact in the study of genetic disorders and evolutionary relationships between different animal groups is also extremely important. While genetic fingerprinting has attained a high public profile through several sensational criminal trials, the process is not accepted by all as definitive proof of guilt. Critics argue that DNA experts may overestimate the odds of having matching DNA fingerprints; they also say that DNA samples can be contaminated when collected and that poor lab procedures can result in mistakes. These problems have been used successfully by defense attorneys to negate some DNA fingerprinting evidence presented in criminal trials. However, most courts in the U.S. allow DNA evidence, and some states have passed specific laws allowing its use.
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