28 • Chemistry Matters
28 • Chemistry Matters
The invention of DNA sequencing has affected society in many ways, few more dramatic than those stemming from the development of DNA fingerprinting. DNA fingerprinting arose from the discovery in 1984 that human genes contain short, repeating sequences of noncoding DNA, called short tandem repeat (STR) loci. Furthermore, the STR loci are slightly different for everyone except identical twins. By sequencing these loci, a pattern unique to each person can be obtained.
Perhaps the most common and well-publicized use of DNA fingerprinting is that carried out by crime laboratories to link suspects to biological evidence—blood, hair follicles, skin, or semen—found at a crime scene. Many thousands of court cases have been decided based on DNA evidence.
For use in criminal cases, forensic laboratories in the United States have agreed on 13 core STR loci that are most accurate for the identification of an individual. Based on these 13 loci, a Combined DNA Index System (CODIS) has been established to serve as a registry of convicted offenders. When a DNA sample is obtained from a crime scene, the sample is subjected to cleavage with restriction endonucleases to cut out fragments containing the STR loci, the fragments are amplified using the polymerase chain reaction, and the sequences of the fragments are determined.
If the profile of sequences from a known individual and the profile from DNA obtained at a crime scene match, the probability is approximately 82 billion to 1 that the DNA is from the same individual. In paternity cases, where the DNA of male parent and offspring are related but not fully identical, the identity of the male parent can be established with a probability of around 100,000 to 1. Even after several generations, paternity can still be inferred from DNA analysis of the Y chromosome of direct male-line descendants. The most well-known such case is that of Thomas Jefferson, one of the nation's founders, who likely impregnated enslaved person Sally Hemings. Although Jefferson himself has no male-line descendants, DNA analysis of the male-line descendants of Jefferson’s paternal uncle contained the same Y chromosome as a male-line descendant of Eston Hemings, the youngest son of Sally Hemings. Thus, a mixing of the two genomes is clear, although the male individual responsible for that mixing can’t be identified with 100% certainty.
Among its many other applications, DNA fingerprinting is widely used for the diagnosis of genetic disorders, both prenatally and in newborns. Cystic fibrosis, hemophilia, Huntington’s disease, Tay–Sachs disease, sickle cell anemia, and thalassemia are among the many diseases that can be detected, enabling early treatment of an affected child. Furthermore, by studying the DNA fingerprints of relatives with a history of a particular disorder, it’s possible to identify DNA patterns associated with the disease and perhaps obtain clues for an eventual cure. In addition, the U.S. Department of Defense requires blood and saliva samples from all military personnel. The samples are stored, and DNA is extracted if the need for identification of a casualty arises.