Basic DNA Testing Steps

DNA is the abbreviation for deoxyribonucleic acid, which is the genetic material present in the nucleus of cells in all living organisms. DNA has been called the "blueprint of life," since it contains all of the information required to make an organism grow and develop. It encodes all of the information that gives each of us our physical characteristics and allows us to function and be recognized as human. The majority of the DNA is identical from one human to another, but there are locations in the DNA that have been found to differ from one individual to another, with the exception of identical twins. These are the regions of DNA that are analyzed and used to compare the DNA obtained from an unknown evidence sample to the DNA of a known individual in DNA identification testing. Because each individual inherited half of his or her DNA from each parent, DNA testing can be used to determine if individuals are genetically related to each other. DNA is found in all cells with a nucleus and is the same throughout the body, so virtually every fluid or tissue from a human contains some DNA and can be analyzed by DNA identification testing. DNA also is stable and does not change over time,so samples collected years ago may be compared to samples collected recently.

When DNA testing is done, several basic steps are performed regardless of the type of test. The general procedure includes: 1) the isolation of the DNA from an evidence sample containing DNA of unknown origin and, generally at a later time, the isolation of DNA from a sample (e.g., blood) from a known individual; 2) the processing of the DNA so that test results may be obtained; 3) the determination of the DNA test results (or types), from specific regions of the DNA; and 4) the comparison and interpretation of the test results from the unknown and known samples to determine whether the known individual is excluded as (is not) the source of the DNA or is included as a possible source of the DNA.

Each additional test at a previously untested locus (location or site) in the DNA provides another opportunity for the result of "exclusion" if the known individual being used for comparison is not the source of the DNA from an evidence sample of unknown origin. If, however, the known individual is the source of the DNA on the evidence sample, additional testing will continue only to include that individual as a possible source of the DNA. When a sufficient number of tests have been performed in which an individual cannot be excluded as the source of the DNA by any of the tests, a point is reached at which the tests have excluded virtually the world's population and the unique identification of that individual as the source of the DNA has been achieved.