Lecture: Forensic Serology

1. 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.

2. 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).

3. Blood Typing & Distribution

5. 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.

6. 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!

7. 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.

8. 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).

9. 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.

10. 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

11. Blood Stain Patterns • Cast-off Stains

12. Blood Stain Patterns • Drip patterns

13. Blood Stain Patterns • Swipe

14. Blood Stain Patterns • Wipe

15. Blood Stain Patterns • Effects of Surface Texture • Horizontal drop creates circular pattern • Forces of surface tension • Rougher surface = greater distortion

16. 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

17. Blood Stain Patterns • Calculating Impact Angle • Determine L/W ratio • Determine W/L ratio • Use calculator or standard curve

18. Blood Stain Patterns Point of Origin

22. 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.

23. 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.