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 Cast-off

Stains

Blood Stain PatternsBlood Stain Patterns Drip

patterns

Blood Stain PatternsBlood Stain Patterns Swipe

Blood Stain PatternsBlood Stain Patterns Wipe

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

Blood Stain PatternsBlood Stain Patterns Point of Origin

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.