8-1 PRENTICE HALL ©2008 Pearson Education, Inc. Upper Saddle River, NJ 07458 FORENSIC SCIENCE An...

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PRENTICE HALL ©2008 Pearson Education, Inc. Upper Saddle River, NJ 07458

FORENSIC SCIENCE An IntroductionBy Richard Saferstein

FORENSIC SEROLOGY

Chapter 8

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Stain Patterns of Blood• The location, distribution, and

appearance of bloodstains and spatters give useful information for reconstructing the events that produced the blood.

• Surface texture and the stain’s shape, size, and location must be considered when determining the direction, dropping distance, and angle of impact of a bloodstain.

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Stain Patterns of Blood• Surface texture is important. The harder and less porous

the surface, the less spatter.• The direction of travel of blood striking an object may be

determined because the pointed end of a bloodstain always faces its direction of its travel.

• The impact angle of blood on a flat surface can be determined by measuring the degree of circular distortion. At right angles the blood drop is circular, as the angle decreases, the stain becomes elongated.

• The origin of a blood spatter in a two-dimensional projection can be established by drawing straight lines through the long axis of several individual bloodstains. The intersection or point of convergence of the lines represents the origin point.

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Projection of BloodForensic investigators can

determine howblood was projected from the

body by examining factors such as: • Type of injuries• The order in which the

wounds were received• Whose blood is present• The type of weapon that

caused the injuries• Whether the victim was in

motion or lying still when the injury was inflicted

• Whether the victim was moved after the injury was inflicted

• How far the blood drops fell before hitting the surface where they were found.

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Blood & Crime Scene:Categories of Blood Patterns

Pools of blood have evidentiary value in

collecting a wet sample. Drops of blood can

reveal the height and angle from which the

blood fell onto the surface. According to

forensic scientists, the blood spatter

analysis claims that blood which falls

perpendicular to the floor from a distance

of zero to two feet would create a circular

drop with slightly frayed edges. Drops from

a higher distance would have more distinct

tendrils extending off the edges.

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Blood & Crime Scene:Categories of Blood

PatternsA blood smear on the wall or floor can indicate the direction of force of the blow. The direction of force is always

in the direction towards the tail, or smaller end, of the smear. In other words, the largest area

of the smear is the point of origin. Blood

crusts must be tested with crystalline techniques to verify that they are

actually blood. Refrigerated red blood cells

have ashelf life of about forty-two days, and

the serum containing white blood cells

can berefrigerated much longer, almost up

to a year. DNA can be extracted from

blood (if white blood cells which always

contain a nucleus are present), and also from

sperm,bone marrow, tooth pulp, and hair

roots.

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Blood & Crime Scene

Regardless of what type of analysis is used

on the blood at the crime scene, care must

be taken to handle it properly and to

prevent putrefaction. Photographs and

notes should be taken before any blood is

lifted. Samples should not be exposed to

heat, moisture, or bacterial contamination,

because these factors can shorten the

survival time of proteins, enzymes, and

antigens. Delays in bringing samples to the

lab must be avoided at all cost, because it

can diminish evidential value.

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Court SignificanceExperts in bloodstain examination

are usually law enforcement personnel.

In certain jurisdictions, a police

investigatoror blood specialist may testify on the

coreissue because blood evidence is

usually avital aspect of the crime scene.

An expert in bloodstain examination has:

•Completed specialized training•Conducted a sufficient number ofexaminations•Accumulated enough reference

patterns to reinforce an argument

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Heredity and Paternity• The transfer of hereditary material is

accomplished by means of units called genes, located on chromosomes.

• Alleles: Alternative forms of genes that influence a given characteristic (such as eye color or blood type).

• Paternity testing has historically involved the A-B-O blood typing system, along with blood factors such as Rh (D).

• DNA test procedures raise the odds of establishing paternity beyond 99%.

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Principles of Heredity• Transmission of Traits

– Accomplished by genes, which is the basic unit of heredity.

– 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.

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Principles of Heredity• 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.

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Blood & DNA TestingBlood is used in DNA testing, as

shown by the following steps:1. Blood samples are collected from

thevictim, defendant, and crime scene.2. White blood cells are separated

from red blood cells.3. DNA is extracted from the nuclei

of white blood cells.4. A restrictive enzyme is used to cut fragments of the DNA strand.5. DNA fragments are put into a bed

of gel with electrodes at either end.6. Electric current sorts DNA

fragments by length.7. An absorbent blotter soaks up the

imprint; it is radioactively treated, and an X-

ray photograph, called an

autoradiograph, is produced.

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Forensic Characteristics of Semen

• Normal male can ejaculate 2.5-6 ml of seminal fluid

• Each ml contains 100 million or more spermatozoa

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Testing for Semen• Stain must be located and collected

• Acid Phosphatase Color Test– Purple color indicates the presence of semen

• Spermatozoa Test– Semen is diluted with water and dried on

filter paper– Microscopic examination looks for

spermatozoa

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Testing for Seminal Stains• Many of the cases sent to a forensic laboratory involve

sexual offenses, making it necessary to examine exhibits for the presence of seminal stains.

• The best way to locate and at the same time characterize a seminal stain is to perform the acid phosphatase (an enzyme found in seminal fluid) color test.– A purple color indicates acid phosphatase enzyme.

• Semen are identified by either the presence of spermatozoa or of p30, a protein unique to seminal plasma.

• Forensic scientists can link seminal material to an individual by DNA typing.

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Rape Evidence• Presence of seminal fluid

• Physical injuries such as bruising or bleeding confirms a violent sexual assault took place

• Transfer of physical evidence—blood, semen, fibers, and hair—are usually present

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Rape Evidence• The rape victim must undergo a medical

examination as soon as possible after the assault.

• At that time the appropriate items of physical evidence including clothing, hairs, and swabs can be collected for subsequent laboratory examination.

• All outer and undergarments should be carefully removed and packaged separately in paper (not plastic) bags.

• Bedding, or any objects upon which the assault took place, may also be carefully collected.

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An antibody–antigen–antibody sandwich or complex is seen as a colored band. This signifies the presence of PSA in the extract of a stain and positively identifies human semen.

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PSA testing by electrophoresis.

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Collection of Rape Evidence• All outer and undergarments are

collected and packaged separately in paper bags

• Trace evidence is collected by standing on a clean sheet while removing clothing

• Bedding may be recovered if seminal stains are present

• Medical Examination of the victim

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Rape Evidence• If a suspect is apprehended within 24 hours of

the assault, it may be possible to detect the victim’s DNA on the male’s underwear or on a penile swab of the suspect.

• Items routinely collected from the suspect include all clothing, pubic hair, head hair, penile swab, and a blood sample or buccal swab for DNA typing.

• The forceful physical contact between victim and assailant may result in a transfer of such physical evidence of blood, semen, saliva, hairs, and fibers.

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Medical Examination of Victim• Pubic Combings• Pubic Reference Samples from victim• Vaginal swabs and smear• Rectal swabs• Oral swabs• Head hairs• Blood sample• Fingernail scrapings• All clothing• Urine specimen

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• All clothing

• Pubic hair combings

• Pulled head and pubic hairs for reference samples

• Penile swab

• Blood sample or buccal swab

Medical Examination of Suspect

8-1 PRENTICE HALL ©2008 Pearson Education, Inc. Upper Saddle River, NJ 07458 FORENSIC SCIENCE An...

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Transcript of 8-1 PRENTICE HALL ©2008 Pearson Education, Inc. Upper Saddle River, NJ 07458 FORENSIC SCIENCE An...