Transcript Forensic Serology

Forensic Serology
Blood
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A complex mixture of cells, enzymes, proteins &
inorganic substances
Fluid portion of blood is called the plasma (55%
of blood content)
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primarily water
red cells (erythrocytes)
white cells (leukocytes)
platelets
Searching for Blood
Tests are based on reducing capacity of
hemoglobin in blood
 Kastle-Meyer test
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– Phenolphthalein turns pink in presence of blood
and H2O2
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Luminol
– Glows when mixed with blood
Erythrocytes
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Transport oxygen from the lungs to the body
tissues
Transport carbon dioxide from the tissues to the
lungs
Red blood cells possess chemical structures on
their surfaces called antigens or agglutinogens
impart blood type characteristics
Antibodies or Agglutinins
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Types of proteins that are present in the serum
responsible for ensuring that the only blood cells
that can survive in a person are cells of the correct
blood type
Antibodies produced by the A alleles remove any
red blood cells containing B antigens by clumping
them together
Antibodies produced by the B alleles remove any
red blood cells possessing A antigens
Blood Type
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Three types (alleles) of blood type gene
– A, B, O
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Each individual inherits one blood type gene from
their mother & one from their father
6 possible combinations (genotypes)
AA, BB, OO, AB, AO, and BO
Genotype determines blood type
Type O Blood
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Possessed by people whose genotype is OO
– both parents passed on the O gene
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have no antigens
these cells can be introduced into a person with
Type A or Type B because these cells are not
attacked by the antibodies these people possess
have both a & b antibodies
can only have other O type cells mixed with this
blood
Type A
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Possessed by people with genotype
– AA
– AO
A is dominant to O
 Possesses antibody b
 will destroy any Type B red cells
 compatible with A or O red cells
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Type B
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Possessed by people with genotype
– BB
– BO
B is dominant to O
 Possesses antibody a
 will destroy any Type A red cells
 compatible with B or O red cells
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Type AB
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Possessed by people with genotype
– AB
– A & B are co-dominant
Possesses no antibody
 can have A, B, AB, or O cells added
 Can’t be added to any other blood type
without being destroyed by an antibody
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Blood Typing
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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 not antigens so will not clot
Precipitin test
Test which determines whether blood is
human or not
 Like immunoassay for drugs except for
human blood
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Testing for semen samples
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Acid phosphatase test
– Enzyme found in seminal fluid
– Moist filter paper is rubbed over garment and
treated with reagents
Purple color means positive for semen
 MUP reagent - UV florescent
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PSA -prostate specific antigen test
– Immunoassay for protein found only in seminal
plasma
Secretors vs. Nonsecretors
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Secretors have A and B antigens in nonblood fluid
– 80% of population
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Nonsecretors don’t have blood type
antigens in saliva, semen, etc.
– Remaining 20% of population
DNA evidence
Chromosomes
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Cells contains all the genetic info the cell
needs to exist & to reproduce
In most types of cells, genetic information is
organized into structures called chromosomes
usually X shaped
– Males have XY, females XX
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23 pairs in humans
– one copy from mother & one from father
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Each chromosome is a single polymeric
molecule of DNA
if fully extended the molecule would be about
1.7 meters long
Genes
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Each chromosome contains hundreds to thousands
information blocks called genes
Each gene is the blueprint for a specific type of
protein in the body
only identical twins will have all identical genes
Functions of DNA
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To transmit information from one generation of
cells to the next
– Replication: the synthesis of new DNA form existing
DNA
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To provide the information for the synthesis of
components (proteins) necessary for cellular
function
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Nuclear vs. mitochondrial
– Nuclear contains all genes and is linear
– Mitochondrial contains only genes inherited from
mother and is circular
DNA structure
Nitrogenous Bases
DNA Fingerprinting
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Sequencing: a procedure used to determine the order of
the base pairs that comprise DNA
The basic structure of everyone’s DNA is the same
the difference between people is the ordering of the base
pairs
Every person can be distinguished by the sequence of their
base pairs
millions of base pairs make this impractical
a shorter method uses repeating patterns that are present in
DNA
Short Tandem Repeats
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DNA strands contain information which directs an
organism’s development called exons
Also contain stretches which appear to provide no relevant
genetic information called introns
– repeated sequences of base pairs
– “junk” DNA
– 30% of genome
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All humans have some tandem repeats
Repeats come from the genetic information donated by
parents
STR Analysis
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Usually an individual will inherit a different variant of the
repeated sequence from each parent
Restriction enzymes can cut of the sequence of repeats
Number of repeats can be determined by electrophoresis
– More repeats: higher MW: moves less
– Fewer repeats: lower MW; Moves farther
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Example: TH01
– A-A-T-G
– From 5 to 11 copies
CODIS
Combined DNA Index System
 National database
 13 STR’s
 Population genetics and product rule allow
use to determine likelihood of finding
another individual with same genetic profile
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