§24.12 DNA PROFILING

• Jurisdiction: United States

§24.12. DNA PROFILING

DNA profiling was first reported in 1985 by Dr. Alec Jeffreys of the University of Leicester, England. The initial reports on DNA evidence were dramatic. One judge wrote that it was the "single greatest advance in the 'search for truth' . . . since the advent of cross-examination."²⁰³ There is little dispute that the basic science upon which DNA testing rests is valid. Nevertheless, questions have arisen concerning the methods used and the interpretation of the results in some cases.

[A] DNA Technology

DNA (deoxyribonucleic acid) is a chemical messenger of genetic information, a code that gives both common and individual characteristics to people.²⁰⁴ Except for identical twins, no two individuals share the same DNA pattern. DNA is found in every body cell with a nucleus, and with few exceptions, DNA does not vary from cell to cell. Each cell contains the entire genetic code, although each cell reads only the part of the code that it needs to perform its job. Thus, DNA in blood obtained from a suspect can be compared with the DNA in semen or saliva cells from a crime scene.

DNA is composed of a chain of nucleotide bases twisted into a double helix structure, resembling a twisted ladder. Each rung of the helix is a "base pair." The order of the base pairs on the DNA ladder is known as the DNA sequence; it constitutes the "genetic code." In other words, these base pairs provide specific instructions to the cell; a sequence of base pairs that is the source for a particular trait is called a gene.²⁰⁵ It is the area of base pair variation that is used in DNA analysis. These base pairs are called "polymorphisms."

DNA profiling involves two fields: molecular biology and population genetics. The analysis involves two corresponding steps — first, determining whether the genetic characteristics at several loci on the DNA strand "match," and second, assuming a match at each locus, calculating the population frequency for these matches. Any non-match at any locus in the first step means the suspect is excluded as a source of the evidence DNA. In other words, it is much easier to exclude a suspect than to include. In this respect, DNA profiling is similar to the use of forensic blood typing, which was used in the pre-DNA era. Approximately 45% of the population has type O blood, 42% type A, 10% type B and 3% type AB. If the crime scene blood type is O and the suspect is type A, the suspect is excluded as the source of that blood.

A possible "inclusion" will be reported as a random match probability — what are the chances that a person with the same genetic characteristics as in the evidence DNA would be found in the population? Thus...