Download Chapter 15 DNA: The Indispensable Forensic Science Tool

Chapter 15
DNA: The Indispensable Forensic Science Tool
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Portions of the DNA structure are as unique to each individual as fingerprints.
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The gene is the fundamental unit of heredity. Each gene is composed of DNA specifically
designed to control the genetic traits of our cells.
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DNA is constructed as a very large molecule made by linking a series of repeating units
called nucleotides.
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Four types of bases are associated with the DNA structure: adenine (A), guanine (G),
cytosine (C), and thymine (T).
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The bases on each strand of DNA are aligned in a double-helix configuration so that
adenine pairs with thymine and guanine pairs with cytosine. This concept is known as
base pairing. The order of the bases distinguishes different DNA strands.
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Portions of the DNA molecule contain sequences of bases that are repeated numerous
times. These “tandem repeats” offer a means of distinguishing one individual from
another through DNA typing.
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Length differences associated with relatively long repeating DNA strands—called
restriction fragment length polymorphisms (RFLPs)—formed the basis for one of the first
DNA-typing procedures.
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Polymerase chain reaction (PCR) can amplify minute quantities of DNA. The technique
evolved from an understanding of how DNA strands naturally replicate within a cell.
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PCR technology cannot be applied to RFLP DNA typing because RFLP strands are too
long, often numbering in the thousands of bases. PCR is best used with DNA strands that
are no longer than a couple of hundred bases.
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Long RFLP strands tend to readily break apart under the adverse conditions at many
crime scenes. The shorter DNA strands used in PCR are more stable and less subject to
degradation caused by adverse environmental conditions.
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The latest method of DNA typing, short tandem repeat (STR) analysis, is the most
successful and widely used DNA-profiling procedure.
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STRs are locations on the chromosome that contain short sequences that repeat
themselves within the DNA molecule. They serve as useful markers for identification
because they are found in great abundance throughout the human genome.
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The entire strand of an STR is very short, less than 450 bases long. This makes STRs
much less susceptible to degradation, and they may often be recovered from bodies or
stains that have been subjected to extreme decomposition.
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Because of their shortness, STRs are ideal candidates for multiplication by PCR, thus
overcoming the limited-sample-size problem often associated with crime scene evidence.
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Hundreds of different types of STRs are found in human genes.
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The more STRs one can characterize, the smaller the percentage of the population from
which a particular combination of STRs can emanate. This gives rise to the concept of
multiplexing, in which the forensic scientist can simultaneously extract and amplify a
combination of different STRs.
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With STR, as little as 125 picograms of DNA is required for analysis.
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Mitochondrial DNA is located outside the cell’s nucleus and is inherited from the mother.
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Mitochondrial DNA typing does not approach STR analysis in its discrimination power
and thus is best reserved for samples, such as hair, for which STR analysis may not be
possible.
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Packaging of bloodstained evidence in plastic or airtight containers must be avoided
because the accumulation of residual moisture could contribute to the growth of blooddestroying bacteria and fungi. Each stained article should be packaged separately in a
paper bag or in a well-ventilated box.
Questions
1.
What is complementary base pairing? What is the importance of the order in which base
pairs are arranged?
2.
How are proteins formed? What determines the shape and function of a protein?
3.
List three advantages gained from decoding the human genome.
4.
Describe the process of DNA replication. What is the importance of DNA replication?
5.
What is polymerase chain reaction (PCR) and how is it useful to forensic scientists?
6.
What are tandem repeats and how are they useful to forensic scientists?
7.
What are short tandem repeats (STRs) and what is their significance to DNA typing?
8.
List two advantages STRs have over restriction fragment length polymorphisms (RFLP).
9.
What is multiplexing and why is it used in DNA profiling?
10.
What characteristics are revealed by the amelogenin gene and Y-STRs?
11.
What are the three main differences between nuclear DNA and mitochondrial DNA?
12.
Name two advantages and two disadvantages of mitochondrial DNA analysis compared
to nuclear DNA analysis.
13.
How should packages containing DNA evidence be stored? Name one common
exception and explain why it should be handled differently and how it should be stored.
14.
List four steps an investigator should take to prevent contamination of DNA evidence.
Chapter 5
Death Investigation
CHAPTER OVERVIEW
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Forensic pathologists associated with the medical examiner’s or coroner’s office are
responsible for determining the cause of an undetermined or unexpected death.
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Although both the coroner’s office and the medical examiner’s office are charged with
investigating suspicious deaths, only the pathologist is trained to perform an autopsy. The
tasks of examining the body for cause and manner of death and recording the results in
the death certificate are all responsibilities of both offices.
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Protection of the body and the overall scene is of paramount importance, as is the
ultimate removal of the body in a medically acceptable manner.
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An autopsy, in its broadest definition, is simply the examination of a body after death.
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The forensic autopsy consists of an external examination and an internal examination.
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The first steps taken for the external examination include a broad overview of the
condition of the body and the clothing.
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The external examination also consists of classifying the injuries. This includes
distinguishing between different types of wounds, such as a stab wound versus a gunshot
wound.
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The dissection of the human body generally entails the removal of all internal organs
through a Y-shaped incision beginning at the top of each shoulder and extending down to
the pubic bone.
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The internal examination entails weighing, dissecting, and sectioning each organ of the
body.
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Blood is often tested to determine the presence and levels of alcohol and drugs.
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Some drugs redistribute or reenter the blood after death and thus may complicate the
interpretation of postmortem blood levels of these drugs.
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A primary objective of the autopsy is to determine the cause of death. The cause of death
is defined as that which initiates the series of events ending in death.
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The most important determination in a violent death is the character of the injury that
started the chain of events that resulted in death.
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Some of the more common causes of death are blunt force injury, sharp force injuries,
asphyxia, gunshot wounds, and substance abuse.
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A blunt force injury is caused by a non-sharpened object such as a bat or pipe. A blunt
force injury can abrade tissue or can cause a contusion arising from bleeding from tiny
ruptured blood vessels within and beneath the skin.
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Sharp force injuries occur from weapons with sharp edges, such as knives or blades.
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Asphyxia encompasses a variety of conditions that involve interference with the intake of
oxygen. For example, death at a fire scene is caused primarily by the extremely toxic gas
carbon monoxide.
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Gunshot wounds originate from projectiles fired by a firearm. The distance a weapon was
fired from a target is one of the most important factors in characterizing a gunshot
wound.
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Because drug abuse is so common, a forensic pathologist will routinely order
toxicological tests for the presence of drugs in nearly all autopsies.
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The manner in which death occurred is classified in death certifications as one of five
categories: homicide, suicide, accidental, natural, or undetermined.
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Homicide is generally defined as a nonaccidental death resulting from grossly negligent,
reckless, or intentional actions of another person.
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Suicide is the result of an individual taking his or her own life with lethal intention.
Although drug abuse is deliberately committed by a victim, it is not considered a cause of
suicide unless it was clearly intended as a lethal act.
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In all deaths that are ruled accidental, there must not be intent to cause harm through
gross negligence on the part of a perpetrator or the victim. Traffic accidents make up a
large percentage of accidental deaths, followed by drug overdoses and drownings.
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The classification of natural death includes disease and continual environmental abuse.
This abuse can encompass various events, such as chronic drug and alcohol abuse or
longtime exposure to natural toxins or asbestos.
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An undetermined cause of death arises when the cause of death cannot be determined by
a physical finding at the autopsy, or because of the absence of meaningful findings in the
subsequent toxicological and microscopic examinations.
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After death the body undergoes a process known as algor mortis, in which it will
continually adjust to equalize with the environmental temperature.
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Another condition beginning when circulation ceases is livor mortis. When the human
heart stops pumping, the blood begins to settle in the parts of the body closest to the
ground. The skin appears bluish-purple in these areas.
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Immediately following death, a chemical change known as rigor mortis occurs in the
muscles, causing them to become rigid.
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Another approach helpful for estimating the time of death is to measure potassium levels
in the ocular fluid.
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The identification of food items in the stomach may help to determine the location of the
decedent prior to death (i.e., during his or her last meal).
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Forensic anthropology is concerned primarily with the identification and examination of
human skeletal remains.
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The gender of the decedent can be determined by the size and shape of various skeletal
features, especially those in the pelvis and skull, or cranium.
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The height of the victim when alive can be estimated by measuring the long bones of the
skeleton, especially those in the lower limbs.
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Forensic entomologists can approximate how long a body has been left exposed by
examining the stage of development of the fly larvae on the body.
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Information about the arrival of other species of insects may also help determine the
postmortem interval. The sequence of arrival of these groups depends mostly on the
body’s natural decomposition process.
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In general, insects first colonize the body’s naturally moist orifices. However, if open
wounds are present, they will colonize there first.
Questions
1.
List items to be collected and sent to the forensic laboratory from an autopsy.
2.
Describe what’s entailed in an external examination during an autopsy.
3.
Describe what’s entailed in an internal examination during an autopsy.
4.
A primary objective of the autopsy is to determine the cause of death. Define the cause
of death.
5.
Describe the characteristics of a blunt force injury.
6.
Describe the characteristics of a sharp force injury.
7.
Describe important factors in characterizing a gunshot wound.
8.
List the five categories of the manner of death.
9.
Distinguish between a homicide and a suicide.
10.
After death the body undergoes a process known as algor mortis. What is this?
11.
Another condition arising after death is livor mortis. What does this entail?
12.
Immediately following death, a chemical change known as rigor mortis occurs. Describe
this phenomenon.
13.
Describe the approach for estimating the time of death by measuring potassium levels in
the ocular fluid.
14.
What important considerations must be made when determining PMI using forensic
entomology?
15.
List the areas of the skeleton that can be used to determine the gender of skeletal remains.
Chapter 6
Fingerprints
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Fingerprints are a reproduction of friction skin ridges found on the palm side of the
fingers and thumbs.
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The basic principles underlying the use of fingerprints in criminal investigations are as
follows: (1) a fingerprint is an individual characteristic because no two fingers have yet
been found to possess identical ridge characteristics; (2) a fingerprint remains unchanged
during an individual’s lifetime; and (3) fingerprints have general ridge patterns that
permit them to be systematically classified.
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All fingerprints are divided into three classes on the basis of their general pattern: loops,
whorls, and arches.
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The individuality of a fingerprint is determined not by its general shape or pattern, but by
a careful study of its ridge characteristics. The expert must demonstrate a point-by-point
comparison in order to prove the identity of an individual.
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The FBI fingerprint database known as AFIS converts the image of a fingerprint into
digital minutiae that contain data showing ridges at their points of termination (ridge
endings) and their branching into two ridges (bifurcations).
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Visible prints are made when fingers touch a surface after the ridges have been in contact
with a colored material such as blood, paint, grease, or ink.
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Plastic prints are ridge impressions left on a soft material, such as putty, wax, soap, or
dust.
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When the finger touches a surface, perspiration and oils are transferred onto that surface,
leaving a fingerprint. Prints deposited in this manner are invisible to the eye and are
commonly referred to as latent or invisible fingerprints.
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Latent prints deposited on hard and nonabsorbent surfaces (such as glass, mirror, tile, and
painted wood) are usually developed by the application of a powder, whereas prints on
porous surfaces (such as papers and cardboard) generally require treatment with a
chemical.
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Examiners use various chemical methods to visualize latent prints, such as iodine fuming,
ninhydrin, and Physical Developer.
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Superglue fuming develops latent prints on nonporous surfaces, such as metals, electrical
tape, leather, and plastic bags. Development occurs when fumes from the glue adhere to
the print, usually producing a white latent print.
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Latent fingerprints are treated with chemicals that induce fluorescence when exposed to a
high-intensity light or an alternate light source.
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Once a latent print has been visualized, it must be permanently preserved for future
comparison and for possible use as court evidence. A photograph must be taken before
any further attempts at preservation are made.
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If an object containing a fingerprint is small enough to be transported without destroying
the print, it should be preserved in its entirety.
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Prints on large immovable objects that have been developed with a powder can best be
preserved by “lifting” with a broad adhesive tape.
Questions
1.
What aspect of a fingerprint determines its individuality?
2.
What is the dermal papillae and why is it important in fingerprinting?
3.
Why is it almost impossible to obscure one’s fingerprints by surgery or mutilation?
4.
Describe each of the three classes of fingerprints. Which class is the most common in the
population? Which is least common?
5.
What aspect of a fingerprint forms the basis for primary classification under the FBI
system? What is the main drawback of the FBI system?
6.
Briefly describe how the Automated Fingerprint Identification System (AFIS) creates a
fingerprint image. What characteristics of the fingerprint does the AFIS record for
comparison?
7.
What is the final step in fingerprint identification? Why is this step necessary if a
computerized database of fingerprints is available to the forensic scientist?
8.
List and describe the three types of fingerprints that may be found at a crime scene.
9.
List two types of specialized fingerprint powders and name one advantage each has over
traditional fingerprint powder.
10.
What is the most commonly used chemical method to visualize latent fingerprints on
porous materials? If this method is unsuccessful, what other technique typically is used?
11.
What phenomenon underlies many of the new chemical techniques used to visualize
latent fingerprints? Why is this phenomenon so valuable in fingerprint visualization?
12.
When should a fingerprint be “lifted” from a crime scene? Describe how to lift and
preserve a fingerprint using adhesive tape.
13.
What is digital imaging and how is it used in fingerprint analysis? What is the greatest
limitation to digital imaging?