Forensic Serology Chapter 12
Download
Report
Transcript Forensic Serology Chapter 12
Lecture:
Forensic Serology
Serology: the medical science dealing with
serums.
Serum: the clear yellowish fluid obtained when
whole blood isseparated into its solid and
liquid components.
Forensic serologists: scientists who examine
physical evidence with the intent of finding,
identifying and individualizing stains of
biological origin.
Definitions
Blood: suspended in the blood are solid
materials, including red blood cells
(erythrocytes), white blood cells (leukocytes)
and platelets.
Antigens: chemical structures attached to
the surfaces of red blood cells.
Antibodies: proteins floating in the serum
that exist because people have allergies or
may have come in contact with a common
disease (TB, smallpox, and hepatitis are
common antibodies).
Blood Typing & Distribution
Blood type
Antigens on Red
cells
Antibodies in Serum
A
A
Anti-B
B
B
Anti-A
AB
AB
Neither anti-A or
anti-B
O
Neither anti-A or
anti-B
Both anti-A and
anti-B
O
A
B
AB
43-45%
40-42%
10-12%
3-5%
O+ 39%
O- 6%
A+ 35%
A- 5%
B+ 8%
B- 2%
AB+ 4%
AB- 1%
Blood Enzymes and Proteins
Enzymes: proteins that have important functions in regulating the
body’s chemistry.
Enzymes have the characteristic of existing in different forms
(polymorphic) and can be separated into protein components called
iso-enzymes.
A common polymorphism is Hb, which causes sickle-cell
anemia.
Iso-enzymes can be separated by electrophoresis.
Forensic serologists have studied these iso-enzymes with the
goal of being able to individualize blood samples found at
crime scenes.
While there are many iso-enzymes in blood, only those that
survive the drying and aging process are useful to the
forensic serologist.
Iso-enzymes and Probability
Each of these protein and enzyme variants, as well as all blood subtypes,
have known distributions in a population.
It's therefore a simple matter to calculate probability estimates
that border on individualized blood typing.
Probability is defined as the frequency of an event occurring.
If there are several event that may occur, the overall probability
will be the product of these probabilities.
Probability is usually expressed in per cent (i.e., 25%), while the
frequency of an event is expressed as a decimal value (i.e., 0.25)
At a crime scene, a blood sample and the suspect’s blood had the
following characteristics: A blood (42%), basic subtype A2 (25%),
protein AK (15%) and enzyme PGM 2 (6.0%). What is the frequency
of occurrence of these blood characteristics in the general
population?
Answer: 0.00094 (0.42 X 0.25 X 0.15 X 0.06) or 0.094% probability!
BLOODSTAIN
CHARACTERIZATION
Bloodstain analysis traditionally follows the following steps:
Is the stain blood?
Is the stain animal or human blood?
If human blood, what type?
Can the sex, age, and race of the source of blood be
determined?
Is the stain blood?
The benzidine test was long used until replaced by the
Kastle-Meyer test.
Both tests are color tests based on the observation that
hemoglobin will oxidize several classes of organic
compounds.
While Kastle-Meyer is not a specific test for blood, the
other materials that will cause a pink color change are
unlikely to be found at crime scenes.
BLOODSTAIN
CHARACTERIZATION
Is the stain blood?
Another test used is a Hemastix® strip,
which will turn green in the presence of
blood.
Luminol is an important presumtive
identification test for blood. The iron in
hemoglobin reacts with the luminol and
causes the emission of light
(luminescence).
BLOODSTAIN
CHARACTERIZATION
Is the stain animal or human blood?
To answer Question 2, forensic serologists
use antiserum tests.
The standard test is called the precipitin
test
This involves forming antibodies to human
blood in an animal.
The animal serum thus treated will cause a
precipitate to form when reacted with human
blood.
Blood Stain Patterns
Information Obtained:
Origin of bloodstains
Distance between point of impact and origin
Type and direction of impact
Object/weapon used
Minimum number of blows
Position of victim, offender, and objects
Movement by victim or offender at scene
Support/contradict witness statements
Indicate staged/secondary scenes
Blood Stain Patterns
Cast-off
Stains
Blood Stain Patterns
Drip patterns
Blood Stain Patterns
Swipe
Blood Stain Patterns
Wipe
Blood Stain Patterns
Effects of Surface
Texture
Horizontal drop creates
circular pattern
Forces of surface tension
Rougher surface =
greater distortion
Blood Stain Patterns
Impact Angles
Defined as the
internal angle at
which blood strikes
a target surface
Greater angle =
greater elongation
Determine direction
of travel
Blood Stain Patterns
Calculating Impact
Angle
Determine L/W
ratio
Determine W/L
ratio
Use calculator or
standard curve
Blood Stain Patterns
Point of Origin
Principles of Heredity
Transmission of Traits
Accomplished by genes, which is the basic unit of heridity.
Genes are on chromosomes (46 in 23 pairs).
The human egg and human sperm contain 23 chromosomes, which
combine during fertilization.
A female has XX chromosome, and male has XY chromosome.
Transmission of Traits
Genes and chromosomes come in pairs.
The position of a gene on the chromosome is called the locus.
Alleles are alternative forms of genes that influence an
inherited characteristic.
An example of allele genes is the A-B-O blood type system.
Principles of Heridity
Transmission of Traits
A-B-O blood types:
When a gene is made of two similar genes, they are said to
be homozygous. Examples include AA or BB.
If the gene is made up of two different genes, it is said to
be heterozygous. An example would be AB.
A and B genes are dominant, while O genes are recessive.
A pair of A-B-O allele genes together are the genotype of an
individual.
The phenotype of an individual is the outward characteristic of
the individual.
There is no lab test to determine a person’s A-B-O genotype.
If the genotypes of both parents are known, a Punnet square
may be constructed to determine potential genotype of
offspring.