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Forensic Serology

Forensic Serology

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Forensic Serology

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  1. Forensic Serology

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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 • Can’t be added to any other blood type without being destroyed by an antibody

  11. 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

  12. Precipitin test • Test which determines whether blood is human or not • Like immunoassay for drugs except for human blood

  13. 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

  14. Secretors vs. Nonsecretors • Secretors have A and B antigens in non-blood fluid • 80% of population • Nonsecretors don’t have blood type antigens in saliva, semen, etc. • Remaining 20% of population

  15. DNA evidence

  16. 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

  17. 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

  18. 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

  19. DNA structure

  20. Nitrogenous Bases

  21. DNA Fingerprinting • 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

  22. Short Tandem Repeats • 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 • All humans have some tandem repeats • Repeats come from the genetic information donated by parents

  23. 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

  24. 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