Download Forensic Serology

Forensic Serology
Chapter 12
Topics this Chapter:
• Blood typing
– Class evidence (presumptive)
• DNA profiling (later chapter)
– Individual evidence (confirmatory)
• Blood Spatter
– Helps reconstruct crime scene
The Nature of Blood
• Blood – made up of cells, enzymes,
proteins, inorganic substances
– Plasma – fluid portion of unclotted blood
• Dissolved proteins, antibodies, hormones, clotting
factors, nutrients (glucose, amino acids, salts,
minerals)
• Comprises 55% of blood
– Solid portion – blood cells
• Comprises 45% of blood
Types of Blood Cells
• Red blood cells –
erythrocytes
• White blood cells –
leukocytes
• Platelets – clotting
factor
Erythrocytes
• Possess surface proteins
called antigens
• Antigens impart blood
type characteristics
–
–
–
–
A protein = Type A
B protein = Type B
AB proteins = Type AB
No proteins = Type O
Genetics
• Chromosomes – threadlike bodies that appear in
nucleus of every cell; made up of DNA
– Human body cells contain 46 chromosomes
– Human reproductive cells contain only 23
chromosomes
• Genes – segments of DNA on chromosomes
that are the basic unit of heredity
– Control the development of specific characteristics
• Allele – alternative forms of genes located at the
same point on a particular chromosome
– A and B are possible alleles for the blood type gene
• Locus – physical location of a gene on a
chromosome
Chromosome
Genotype vs Phenotype
• Homozygous – two identical alleles
– AA for blood type
• Heterozygous – two different alleles
– AB for blood type
• Genotype – combination of alleles present in the
DNA (chromosomes)
– AA, BB, AO, etc
• Phenotype – physical manifestation of a genetic
trait
– Shape, color, blood type
How do we inherit traits?
• Mother’s genotype
vs. father’s
genotype
• Punnet Square
• Dominant traits
• Recessive traits
Blood typing History
• 1901 – Karl Landsteiner
– Noticed a huge number of
deaths in blood transfusions
– Developed the A-B-O system
of classification
– Still most important system for
matching a donor and recipient
for transfusion
Blood Typing
• Three types (alleles) of blood type gene
– A, B, O
– Code for the antigen present on the surface of
red blood cells
• 6 possible combinations (genotypes)
– AA, BB, OO, AB, AO, and BO
– Genotype determines blood type
• Rh factor – D antigen
– D antigen present: Rh positive
– D antigen absent: Rh negative
Type O Blood
• Possessed by people whose genotype is OO
– both parents passed on the O gene
– Have no antigens
Type A
• Possessed by people with genotype
– AA
– AO
• A is dominant to O
• Have A antigens
Type B
• Possessed by people with genotype
– BB
– BO
• B is dominant to O
• Have B antigens
Type AB
• Possessed by people with genotype
– AB
– A & B are co-dominant
• Have both A and B antigens
Antigens and Antibodies
• Antigen - A substance on the surface of a cell
that stimulates body to produce antibodies
against it
– Over 15 different types on red blood cells
– A-B-O and Rh are most important
• Antibodies – a protein that destroys or
inactivates a specific antigen
– Antibodies develop about 2-8 months after birth
– Found in blood serum
– Responsible for ensuring that the only blood cells that
can survive in a person are cells of the correct blood
type
Antigens and Antibodies
• Type A blood has A antigens
– Produces B antibodies
(called Anti-B)
• Type B blood has B antigens
– Produces A antibodies
(called Anti-A)
• Type AB blood has A & B
antigens
– Produces no antibodies
• Type O blood has no
antigens
– Produces A & B
antibodies (Anti-A and
Anti-B)
Antigen-Antibody Response
• Foreign invader is recognized by the
immune system, and the immune system
launches an attack against the invader
– Immune system “recognizes” invader as
foreign by the foreign antigens on the surface
of the cell
– Viruses, bacteria, red blood cells from
different blood type have foreign antigens
Clotting - Agglutination
• An antigen and an
antibody of the same type
react to clump RBC (called
agglutination)
• So, if antigen A is exposed
to anti-A, agglutination will
occur
Blood Typing
• Blood typing is done by reacting whole blood with
antibody A and antibody B
• Antibody A will cause A and AB blood to clot
• Antibody B will cause B and AB blood to clot
• Type O blood contains no antigens so will not clot
Donors
Probability
• Typical Distribution in
U.S.
– O: 43%
– A: 42%
– B: 12%
– AB: 3%
Additional Blood Proteins
1.
2.
3.
4.
5.
6.
M and N proteins (MM,MN,NN)
Phosphoglucomutase (PGM)
Adenylate kinase (AK)
Adenosine deaminase (ADA)
Esterase D (EsD)
Glucose-6-phosphate dehydrogenase (G6-PD)
7. Polymorphic Proteins: Group-specific
Components (Gc) and haptoglobins (Hp)
Forensic Characterizations of
Bloodstains
1. Is it blood?
2. From what species did the blood
originate? (Usually human or animal)
3. If the blood is human, how closely can it
be associated to a particular individual?
Is it blood?
• Benzidine color test– discontinued
– Contains known carcinogen
• Kastle-Meyer color test – took place of
benzidine color test
– Bloodstain, phenolphthalein reagent and
hydrogen peroxide mixed: hemoglobin will
form a deep pink color
Is it blood?
• Hemastix® moistened with
distilled water and
placed in contact with
bloodstain
– Green color is positive
for blood
Is it blood?
• Luminol test – produces light rather than color
– Large areas covered quickly
– Objects must be in darkened area while being viewed
for luminescence
– Extremely sensitive
• Can detect blood diluted up to 300,000 times
– Will not interfere with further DNA testing
• Takayama and Teichmann tests
– Microcrystalline tests for blood
– Specific chemicals added to form crystals with
hemoglobin
Luminol Test
Human or animal blood?
• Precipitin Test
– determines whether blood
is human or not
– Like immunoassay for
drugs except for human
blood
– Uses antiserums of known
original to react with blood
sample
• Precipitin band is formed if
a match
– Can be used on
bloodstains 10 to 15 yrs old
or older
Precipitin Test
Human or animal blood?
• Gel Diffusion
– antigens and
antibodies will move
toward one another on
an agar-coated plate
• Electrophoretic
method
– Electrical potential is
applied to the gel
Can we narrow it down to a
particular individual?
• Enzymes – another substance in blood
used to try to individualize bloodstains
• This is more historical in use – before
1990’s
• DNA is primarily used now
Paternity Testing
• Encountered in civil courts, usually not
criminal courts
• The more blood group systems tested, the
better the chances of excluding an
innocent male
• Routine testing involves other blood
groups than A-B-O
– HLA (human luekocyte antigen)
• White blood cells
• Chances better than 90% suspect is father if he
cannot be excluded after this test