The process used to extract the DNA varies depending on the organic source of the stain and the material containing the stain. If the stains are on a hard object, such as a knife, some of the dried body fluid is removed from the object with a cotton swab (known as swabbing an item) and the DNA is extracted from the cotton swab. Generally, stains on fabric are cut out of the item and the DNA is extracted from the cuttings. When possible, forensic scientists analyze only a portion of the stains on the evidence and save the remainder in case future testing is necessary. Upon receipt of the evidence, forensic scientists first determine if the evidence might provide DNA by visually examining it for indications of body fluid stains, and then performing testing to determine whether specific body fluids that might contain DNA are present. Law enforcement personnel who submit crime scene evidence for DNA analysis must package and seal the evidence and then arrange for its secure delivery to a DNA laboratory. See Appendix 1 (which contains an example of a complete DNA profile). As a result, except for identical twins, examining the 13 locations produces a DNA profile that is essentially unique to an individual. Scientists have determined that, in general, when DNA profiles consist of the alleles present at these locations the probability that two unrelated individuals will have the same DNA profile purely by chance is less than 1 in 200 billion. Under current DNA standards applicable in the United States, an individual's DNA profile consists of the alleles present at 13 specified chromosomal locations. Rather, scientists need only examine enough locations to virtually eliminate the possibility that two unrelated people have the same DNA profile purely by chance. Examining this DNA allows scientists to determine an individual's unique DNA profile (except for identical twins), without that profile revealing personally identifying characteristics or medical conditions.Įven though forensic DNA scientists focus their analyses on specific chromosomal locations that vary widely between individuals, it is not necessary to examine every one of these locations to develop a unique DNA profile for an individual. Since it varies widely among individuals, scientists examine the DNA located between the genes to determine a person's DNA profile.
#Dna sequences with a high degree of polymorphism are code#
While the function of the DNA between the genes is unknown, scientists currently believe that it does not code for anything. It has been estimated that only 2 to 3 percent of the information in a chromosome is organized into genes. For example, if two people both have blue eyes, then they have the same alleles for their eye-color gene. The characteristics of a specific gene, or of a specific location on a DNA strand, is referred to as an allele. The human traits that result from the variations in this part of the DNA can be obvious, like different eye color or different blood types, but may also be so subtle that only laboratory testing can detect them.Įach chromosome contains many genes, which are the portions of the chromosome that code for personally identifying characteristics, like hair color or eye color. Forensic DNA scientists are only interested in the 0.1 percent of the DNA that varies among people. Source: National Human Genome Research Institute, by artist Darryl Leja atĪpproximately 99.9 percent of human DNA is the same. One copy of each chromosome is inherited from an individual's mother and one copy is inherited from an individual's father, giving a child DNA characteristics of both its mother and father. The nucleus contains 46 chromosomes, two copies of each of the 23 different human chromosomes. Chromosomes store information in the chemical structure of DNA much like a book or a compact disk. In human beings, the nucleus contains chromosomes composed of DNA that encode all of the information necessary to produce a complete human body. We describe below the physical structure of DNA, testing methods, and the standards that govern DNA analysis.Īll living things are composed of cells, which typically have a nucleus that regulates metabolism, growth and/or reproduction.
In order to understand the nature of Blake's misconduct and the deficiencies this review identified in the FBI Laboratory's DNA protocols and practices, we first describe in this Chapter the basic characteristics of DNA and the work of forensic DNA scientists.
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