Forensic Serology and DNA
Introduction
• Prior to the development of DNA typing,
forensic serology was the primary technique
of crime labs.
• Most labs still use basic serological testing
procedures.
• Some do not have a DNA typing facility owing
to either a lack of resources or a lack of
enough cases to warrant the investment.
Blood
• Plasma: the liquid portion of blood
– Accounts for 55% of the total blood volume
– Consists of 90% water and 10% dissolved materials
Blood
• Cellular components of blood
– Account for 45% of the total blood volume
– Include three major types
• Erythrocytes: red blood cells
• Leucocytes: white blood cells
• Thrombocytes: platelets
Tests for the Presence of Blood
• Police want to answer three questions:
– Is this blood?
– Is it from a human?
– How closely does it match the blood of the victim
or the suspect?
Tests for the Presence of Blood
• Presumptive tests for blood
– Luminol: sprayed directly on bloodstained object;
produces a glow when it contacts blood
– Color tests
Tests for the Presence of Blood
• Serological tests for blood: precipitin serological
test
– Determines if blood is of human origin
– Can be used with antiserum prepared for other
animals if it is negative for human blood
– Requires only a small blood sample
– May produce a positive result even if bloodstains
were washed down to a tiny sample remaining
– Is highly sensitive even when bloodstains are odd
Tests for the Presence of Blood
• Serological blood typing
– ABO system: separates human blood into four broad
classifications based on the presence or absence of
the antigen A or antigen B on the surface of red
blood cells
– Rh factor: expressed as positive or negative
Tests for the Presence of Blood
• Because blood types are inherited from a
person’s parents, blood types may become
concentrated among certain ethnic groups.
• 80% of the population are secretors—they
have significant concentrations of antigens in
other body fluids.
Tests for the Presence of Blood
• Other blood typing systems: based on the
presence of proteins in red blood cells
– Polymorphic proteins occur in multiple forms.
– Different forms can be identified and their statistical
occurrence in the population calculated.
– The more independent factors that can be identified
in a blood sample, the smaller the percentage of the
population possessing that combination of blood
traits.
Forensic Characterization of Saliva
• Characteristics of saliva
– Consists of more than 99% water
– pH range = 6.8–7.0
– Contains salivary amylase (a digestive enzyme)
– Produced in three main pairs of salivary glands:
parotid, submaxillary, sublingual
– Cleanses mouth and provides lubrication
Forensic Characterization of Saliva
• Saliva is always present at the crime scene if
there are bit marks on the victim.
• It can be used to identify an individual
through DNA profiling.
Forensic Characterization of Semen
• Characteristics of semen
– Consists of more than 90% water
– pH range = 7.2–7.4
• A crime scene may include a large number of
items stained by semen (e.g., garments, bed
clothing, rugs, drapes, solid surfaces).
Forensic Characterization of Semen
• Identification of semen
– At the crime scene: UV light
– Presumptive tests: acid phosphatase test, p30 test
– In the laboratory: direct observation of sperm
under a microscope
Rape Evidence Collection
• Conviction often hinges on the ability to link the
perpetrator to the victim and the victim’s
injuries.
– Gather evidence from both the physical surroundings
and the victim
– Collect the victim’s clothing if the victim is still at the
scene
– Ensure that the victim is examined by a physician
immediately
Rape Evidence Collection
• Physical evidence collected from the victim:
Blood sample
Combings from pubic hair
Pubic hair reference samples
Vaginal swab and smear
Rectal swab and smear
Head hair
Fingernail scrapings
Oral swab
All clothing
Urine specimen
Rape Evidence Collection
• Saliva residues
– Collect saliva from the victim’s skin if the assailant
bit, sucked, or licked an area of the victim’s body
Rape Evidence Collection
• Physical evidence collected from the suspect:
– All clothing
– Combings of pubic hair
– Head hair and pubic hair standards
– Penile swab
– Blood sample
Principles of Paternity
• Nucleus: largest structure in a human cell;
controls heredity
• Ribosomes: site of protein synthesis
• Mitochrondria: site of energy production
Principles of Paternity
• Chromosomes: hereditary material found in
the nucleus
– Egg cell: contains an X chromosome
– Sperm: contains either an X or a Y chromosome;
determines the sex of the offspring
Principles of Paternity
• Chromosomes are made of nucleic acids.
– Deoxyribonucleic acid (DNA) is the primary
hereditary material.
– DNA consists of a series of coding regions and
noncoding regions that are arranged along the
chromosomes.
– Genes are sections of the DNA molecule.
• Pairs of chromosomes are considered
homologous because they are the same size
and contain the same information.
Principles of Paternity
• When fertilization occurs, one chromosome is
inherited from the mother and one is inherited
from the father.
• Sons inherit their Y chromosome from their
father, so paternity can often be determined by
comparison of the Y chromosomes from father
and son.
Introduction to DNA
• Functions of nucleic acids
– DNA and RNA are responsible for storage and
transmission of genetic information.
– They determine how genetic information is
transferred from one cell to another and how genetic
traits are transferred from parents to offspring.
– The major function of DNA is control and direction or
protein synthesis in body cells.
Introduction to DNA
• Nucleic acids are composed of long chains of
repeating units (nucleotides).
• Each unit includes three components:
– Sugar
– Nitrogen-containing heterocyclic base
– Phosphoric acid unit
Introduction to DNA
• A nucleic acid can contain any of five bases:
– Adenine: double-ring base (a purine)
– Guanine: double-ring base (a purine)
– Cytosine: single-ring base (a pyrimidine)
– Thymine: single-ring base (a pyrimidine)
– Uracil: single-ring base (a pyrimidine)
Introduction to DNA
• Structure of nucleic acids
– Adenine, guanine, and cytosine: found in both DNA
and RNA
– Thymine: found in DNA
– Uracil: found in RNA
Introduction to DNA
• Primary structure of a nucleic acid: sequence of
the four bases
• Secondary structure: the double helix
Introduction to DNA
• The double helix: each DNA molecule has two
polynucleotide chains wound around each
other like a spiral staircase
– The phosphate–sugar backbone represents the
handrails
– Pairs of bases linked together by hydrogen bonds
represent the steps
– Hydrogen bonds hold the two chains together
under normal physiological conditions
Introduction to DNA
• DNA
– Carries the information needed for making and maintaining
the different parts of an organism
• Chromosomes
– Consist of DNA in the nuclei of cells coiled around proteins
(histone molecules)
– Humans have 46 chromosomes, 23 from each parent
• Genes
– Are segments of DNA molecules that control the production
of different proteins in an organism
– Vary in terms of the number and sequence of base pairs they
contain
Introduction to DNA
• Cell replication
– Before a cell divides, the double helix strand begins
to unwind.
– Each unwinding strand serves as a template for the
formation of a new complementary strand.
– Nucleotides are attracted to the exposed bases and
become hydrogen-bonded to them: A to T, T to A, C
to G, G to C.
Introduction to DNA
• Ribonucleic acid (RNA)
– Primary structure is similar to DNA
– Ribose–phosphoric acid units form backbone
– Each ribose unit is bonded to one of the four bases
• Protein synthesis: a series of complex steps
involving RNA
– Transcription
– Translation
Introduction to DNA
• Protein synthesis: transcription
– A single strand of RNA is synthesized inside the cell
nucleus.
– A segment of the DNA double helix separates into single
strands.
– The exposed bases of one strand act as the template for
the synthesis of an RNA molecule.
– The base sequence (messenger RNA) complements the
base sequence on the DNA strand with one exception:
RNA transcribes a uracil instead of a thymine to adenine.
Introduction to DNA
• Protein synthesis: translation
– The code that has been copied to the new protein is
interpreted.
– mRNA leaves the nucleus and takes its chemical
message to the cytoplasm of the cell, where it binds
with ribosomes.
– Guided by the first codon on the mRNA strand, a
transfer RNA molecule with an anticodon that is
complementary to this codon transports a specific
amino acid to the mRNA codon
Introduction to DNA
• Protein synthesis
– The actual protein synthesis occurs in the ribosomes,
which move along the mRNA one codon at a time as
the amino acid chain grows.
– The mRNA is read codon by codon and the protein is
built up one amino acid at a time in the correct
sequence.
Introduction to DNA
• The genetic code
– Each three-base sequence in mRNA codes for a
specific amino acid
– 64 three-base codons can be formed from the
four bases in mRNA
Nuclear DNA and the Law
• 1985: routine analysis of the structure of human
genes led to the discovery that portions of the
DNA structure are as unique to each individual
as fingerprints
• 1987: individuals were first convicted of rape
based on DNA evidence (in both the United
States and the United Kingdom)
Nuclear DNA and the Law
• State v. Woodall: the court accepted the results of
DNA testing, but ruled that the inconclusive results
failed to exculpate Woodall
• Spencer v. Commonwealth: admission of DNA
evidence led to guilty verdicts resulting in the death
penalty for the defendant
• People v. Castro: the court required laboratories and
personnel to follow appropriate practices and prove
the validity of their procedures before DNA evidence
would be accepted in court
• DNA testing is generally accepted as admissible under
Frye or Daubert standards
Mitochondrial DNA
• Mitochondria
– Provide 90% of the body’s energy
– Contain DNA that can be used for testing purposes
• Mitochondrial DNA (mtDNA)
– Is circular and much smaller than nuclear DNA
– Is inherited from the mother
• State of Tennessee v. Ware (1996): first use of
mtDNA in court to match hair samples at the
crime scene to the suspect