Lecture:Lecture:Forensic SerologyForensic 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.
DefinitionsDefinitions 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).
Blood Typing & DistributionBlood Typing & DistributionBlood type Antigens on
Red cellsAntibodies in
Serum
A A Anti-B
B B Anti-A
AB AB Neither anti-A or anti-B
O Neither anti-A or anti-B
Both anti-A and anti-B
O A B AB
43-45% 40-42% 10-12% 3-5%
O+ 39%O- 6%
A+ 35%A- 5%
B+ 8%B- 2%
AB+ 4%AB- 1%
Blood Enzymes and ProteinsBlood 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.
Iso-enzymes and ProbabilityIso-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!
BLOODSTAIN BLOODSTAIN CHARACTERIZATIONCHARACTERIZATION
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.
BLOODSTAIN BLOODSTAIN CHARACTERIZATIONCHARACTERIZATION
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).
BLOODSTAIN BLOODSTAIN CHARACTERIZATIONCHARACTERIZATION
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.
Blood Stain PatternsBlood 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
Blood Stain PatternsBlood Stain Patterns Effects of Surface
Texture Horizontal drop
creates circular pattern
Forces of surface tension
Rougher surface = greater distortion
Blood Stain PatternsBlood Stain Patterns Impact Angles
Defined as the internal angle at which blood strikes a target surface
Greater angle = greater elongation
Determine direction of travel
Blood Stain PatternsBlood Stain Patterns Calculating
Impact Angle Determine L/W
ratio Determine W/L
ratio Use calculator
or standard curve
Principles of HeredityPrinciples 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.
Principles of HeridityPrinciples 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.