Forensic Serology

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Forensic Serology
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Blood

• A complex mixture of cells, enzymes, proteins
  inorganic substances
• Fluid portion of blood is called the plasma (55
  of blood content)
• primarily water
• red cells (erythrocytes)
• white cells (leukocytes)
• platelets

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Searching for Blood

• Tests are based on reducing capacity of
  hemoglobin in blood
• Kastle-Meyer test
• Phenolphthalein turns pink in presence of blood
  and H2O2
• Luminol
• Glows when mixed with blood

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Erythrocytes

• 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

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Antibodies or Agglutinins

• 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

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Blood Type

• Three types (alleles) of blood type gene
• A, B, O
• 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

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Type O Blood

• Possessed by people whose genotype is OO
• both parents passed on the O gene
• 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

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Type A

• 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

• 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

• Possessed by people with genotype
• AB
• A B are co-dominant
• Possesses no antibody
• can have A, B, AB, or O cells added
• Cant be added to any other blood type without
  being destroyed by an antibody

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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 not antigens so will not
  clot

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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

• 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
• PSA -prostate specific antigen test
• Immunoassay for protein found only in seminal
  plasma

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Secretors vs. Nonsecretors

• Secretors have A and B antigens in non-blood
  fluid
• 80 of population
• Nonsecretors dont have blood type antigens in
  saliva, semen, etc.
• Remaining 20 of population

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DNA evidence
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Chromosomes

• 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
• 23 pairs in humans
• one copy from mother one from father
• Each chromosome is a single polymeric molecule of
  DNA
• if fully extended the molecule would be about 1.7
  meters long

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Genes

• 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

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Functions of DNA

• To transmit information from one generation of
  cells to the next
• Replication the synthesis of new DNA form
  existing DNA
• To provide the information for the synthesis of
  components (proteins) necessary for cellular
  function
• Nuclear vs. mitochondrial
• Nuclear contains all genes and is linear
• Mitochondrial contains only genes inherited from
  mother and is circular

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DNA structure
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Nitrogenous Bases
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DNA Fingerprinting

• Sequencing a procedure used to determine the
  order of the base pairs that comprise DNA
• The basic structure of everyones 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

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Short Tandem Repeats

• DNA strands contain information which directs an
  organisms 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
• All humans have some tandem repeats
• Repeats come from the genetic information donated
  by parents

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STR Analysis

• 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
• Example TH01
• A-A-T-G
• From 5 to 11 copies

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CODIS

• Combined DNA Index System
• National database
• 13 STRs
• Population genetics and product rule allow use to
  determine likelihood of finding another
  individual with same genetic profile