Download 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.