Page 39 - Academic Press Encyclopedia of Physical Science and Technology 3rd Molecular Biology

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Encyclopedia of Physical Science and Technology EN004L-956 June 9, 2001 21:7

590 DNA Testing in Forensic Science

first tyrosine hydroxylase locus. Some identified in- forensic scientists], esterase D (ESD), phosphoglucomu-
herited traits do not code for proteins, but are regions tase (PGM1) and some transport and functional proteins
that show variation (polymorphism) in length or se- such as group specific component [GC, now known as
quence of DNA. These areas are referred to as “DNA Vitamin D binding globulin [VDBG]], haptoglobin (HP),
loci,” such as D1S7, which represents and DNA inher- the immunoglobulin allotypes (GM and KM), and trans-
ited trait (D), located on chromosome 1, “S” indicates ferrin (TF) were used forensically. These markers were
it is a single copy region and “7” indicates it was the used in the late 1970s and early 1980s by forensic science
seventh polymorphism found on that chromosome. in the United States and abroad to individualize blood
Phenotype The observed results of a genetic test. If a per- stains. Although, some of these markers did not last long
son has two different alleles (e.g., is heterozygous), the in bloodstains they made it possible to often individual-
phenotype and the genotype are the same. However, if ize bloodstains with greater than a 99% certainty. Semen
a person only has one allele detected, without testing stains, saliva, and urine had relatively few markers that
the parents we cannot be certain that the person has could be detected, making it difficult to provide informa-
two alleles the same (i.e., homozygous) since the per- tion in cases with these types of evidence.
son could have two alleles the same, or one detected In the mid-1970s two independent areas of research
allele and one undetected allele, for what ever reason. would change the future of forensic science. Research on
In the case of RFLP loci this is usually referred to as bacterial enzymes that cut DNA at specific places led to
a single band pattern. For AFLP- and sequence-based the development of restriction fragment length polymor-
PCR-based systems this is referred to as a homozygous phisms (RFLPs). Initially these were only useful for the
phenotype. In general since we do not know the genetic diagnosis of genetic diseases such as Sickle Cell Anemia
type of homozygous individuals it is better to refer to caused by a mutation in the hemoglobin gene. In 1980
homozygous and heterozygous phenotypes, unless the with the identification of the first hypervariable DNA poly-
genetic type of the individual is determined by pedigree morphism (D14S1), detected by restriction length poly-
analysis. morphism technology (RFLP), the door was opened to
Polymorphism Genetically inherited variation with two the possibility that DNA technology could be applied to
or more forms, the least common of which occurs at a forensic evidence. In the next several years, the search for
frequency of greater than 1%. new markers lead to the identification of many forensi-
Restriction fragment length polymorphism (RFLP) cally useful markers, detected by RFLP, some of which are
Polymorphism in length of DNA segments detected us- still used routinely in forensic DNA testing. In the mean-
ing a restriction enzyme, followed by separation using time, hypervariable minisatellite regions which identified
electrophoresis. many genetic regions at one time (multilocus probes) were
Variable number tandem repeat (VNTR) regions of in- found. The term “DNA fingerprinting” was used to de-
herited variation that consist of alleles which contain scribe these bar code-like patterns. Although these regions
different numbers of repeating segments. It is easiest to proved to be highly informative for parentage testing, they
think of them as freight trains with different numbers did not have the sensitivity needed for forensic testing.
of box cars. Different loci will have different numbers Though multilocus probes were used for paternity testing
of repeats. Though RFLP loci are also VNTR loci the and some forensic applications, they are rarely used at the
number of repeats is generally not known because of present time. Using a battery of five to seven RFLP loci
the detection technology. made it possible to individualize samples into the 100s of
millions and billions. This means that the chance of any
two unrelated individuals matching was very unlikely.
FORENSIC SCIENCE is applied science. That is to say At the same time the revolution in RFLP was beginning
that the scientific methodologies used in forensic science in the mid 1970s, an early version of copying DNA using
were developed by biologists, chemists, and geneticists repair enzymes called polymerases was being explored.
and then taken over by forensic scientists to help them It would not be until the early 1980s when the modern
solve problems. The first inherited trait to be used in foren- Polymerase Chain Reaction (PCR) tests was developed.
sic testing was the ABO blood group on red blood cells The role of the polymerase enzymes in copying DNA had
and secreted blood group substance found in saliva and been known since the 1970s. The use of high temperature
other body fluids of individuals called “Secretors.” Re- Taq polymerase allowed for the automation of thermal
search in the 1950s, 1960s, and 1970s identified proteins cycling and the introduction of modern PCR. Tests were
that had genetic variation or were “polymorphic.” Some developedtoidentifyhumanleukocyteantigens(HLA)for
of these were enzymes such as acid phosphatase [ACP, the transplantation community. The first marker that they
referred to as erythrocyte acid phosphatase (EAP) by developed was to the HLA region called DQα (now called