Unit 23; forensic science
Assignment C/D
Content page
C1 Biological evidence techniques
• Fingerprints (four basic patterns, minutiae and ridge counting).
• Hair and fibre identification and analysis – microscopy.
C2 Chemical evidence techniques
• Instrumental analysis – visual examination.
• Chromatography – separation of mixtures, e.g. paper
• Chemical enhancement – fingerprints (ninhydrin).
• Toxicology – science of poisons (anions, corrosive poisons, gaseous
and volatile poisons, metal and metalloid poisons, pesticides, toxins and drugs of
abuse).
• Specimen collection (ante- and post-mortem).
C3 Physical evidence techniques
• Footwear – oblique lighting, casting, electrostatic lifting, gel lifting, visual
analysis and
comparison.
• Tool marks – casting, comparison microscopy.
• IT, to include mobile phones, computers and CCTV.
C. P5
Biological evidence techniques
Blood group analysis:
Blood is the biological fluid that is most frequently found in crime scenes. It
frequently relates to major offences like murder, sexual assault, or physical
violence. In forensic analysis, blood is utilised to identify suspects in particular
kinds of crimes. Detecting blood kinds (blood typing) is a common technique
used by forensic scientists since it is unaffected by illness, medications,
environment, occupation, living condition, or any other physical scenario.
Additionally, scientists employ blood typing to establish paternity.[1]
,ABO blood grouping:
ABO blood typing is a common technique used to analyse blood groups. This
involves forensic scientists analysing which antibodies and antigens are on the
individual's red blood cells to determine if the suspect is blood type A, B, AB, or
O narrowing down suspects with the same blood type as the one examined by
forensics leading investigators to know further information about the suspect.[1]
Blood group Antigens present Reaction with Reaction with
antibody A antibody B
A Antigen A yes no
B Antigen B no yes
AB Antigen A and B yes yes
,O No antigens found no no
Genetics (DNA sequencing, genetic fingerprints):
Short Tandem Repeat is a technique used to analyse DNA sequencing. This
involves forensic scientists examining repeats of short sequences in the DNA at
different areas of the genome. This can then be used to compare these DNA
samples and can then be used to compare it with the DNA from the suspect at
the crime scene.
Restriction fragment length polymorphism (RFLP) can be used to analyse
genetic fingerprinting.This involves using restriction enzymes to cut down DNA
into smaller fragments in which the remnants are then separated by gel
electrophoresis. The resulting sequences are then compared with the individuals
to view a possible link.
,1. When are they gathered?
2. DNA can be extracted from skin cells, blood, hair roots, saliva, and more.
A lysis solution containing detergent and proteinase K is used to treat the
material . The nuclear envelope and cell membrane 2 are damaged by the
detergent, causing the cells to fragment and release their DNA. Histones
are split apart by Proteinase K, releasing DNA.
3. After target DNA extraction, PCR is used to produce enormous amounts of
DNA from a small beginning sample. The DNA sample is processed using
DNA primers, dNTPs, polymerase enzymes, and the PCR reaction mix.
Short DNA sequences called DNA primers can detect and bind to both
sides of an interest STR. The creation of fresh DNA strands is
4. by comparing unknown crime scene DNA samples to suspect reference
samples [5].
5. kicked off by these primers. dNTPs are DNA nucleotide building
components that allow the DNA molecule to stretch out from the primers
and form a new complementary strand. [5]
6. The size of the amplified DNA segment varies due to differences in STR
repeat region sizes. The DNA fragments are then separated by size using
the gel electrophoresis method, which involves passing them through a gel
medium while being affected by an electrical current [5]. Negatively
charged DNA fragments begin to move through the gel toward the
positively charged anode when the current is introduced. Because the gel
works as a molecular filter, smaller molecules move quicker than bigger
pieces [5]. The PCR method may immediately colour the agarose gel to
detect the pattern of DNA
7. fragments since it processes so much DNA. The dye reacts with the DNA
bands to generate dark blue colours on the gel containing the separated
DNA fragments [5].
8. The STR gel DNA band pattern only includes one or two bands at each
VNTR locus, showing the size of the repeat portions on both VNTR alleles.
When the VNTR locus is heterozygous, there are two bands of differing
sizes; when it is homozygous, there is just one band since both alleles
have the same size [5]. On the other hand, the band would be twice as
thick. This would then be compared to a different DNA sample, such as
one taken from a suspect. The presence of a DNA band match can be
determined
,Fingerprints (four basic patterns, minutiae and ridge counting): [25]
Chemical enhancements can be used to analyse fingerprints left at the crime scene.
This involves forensics adding chemicals and powders on the print which reacts with
the oils, proteins, amino acids and other substances forming ridges on the fingerprints
which the number of ridges can then be compared with the suspect's fingerprint.
A fingerprint bears the imprint of a human finger's friction patterns. A crucial
forensic approach is partially recovering fingerprints from a crime scene ].
Grease and moisture on a finger cause fingerprints to be left on materials like
glass and metal. One can intentionally imprint entire fingerprints on flat surfaces
like paper by applying ink or other materials to the skin's top friction ridges. There
are four distinct categories of fingerprints according to Henry's System of
Classification, which are explained in more detail below:
1. Loops- round 60 to 70 percent of all fingerprints analysed contain loops of
some kind. The ridges in loops rotate in reverse but do not twist. This
backward turn or loop can be distinguished by the way it flows on the hand
as opposed to the card where the impression is made. A loop pattern only
contains one delta. This fingerprint imprint is similar to the mirror's reverse
image that we perceive when we look in the mirror. The diagram below
depicts the appearance of the loop fingerprints.
2. Arches- These prints are seen in around only 5% of all fingerprints
examined. The finger's ridges go in a straight line from one side to the
other, with no backward turns . In most arch patterns, there is no delta, but
if there is, there must be no recurving ridge between the core and delta
points . Below in the diagram it shows how the arch fingerprints looks like:
3. Whorls- These can be identified in roughly about 25 to 35 percent of all
fingerprints examined . Usually one circuit is traversed by some of the
ridges in a whorl. As a result, any pattern including two or more deltas is a
whorl . Below in the diagram it shows how the whorls fingerprints looks
like:
, 4. Composite- these include any type of combination of the two or more
fingerprint patterns which do not fit in any other group. Sometimes they are
even known as the whorls made by accident .
Hair and fibre identification and analysis:
Hairs and fibres are two of the most important materials in forensic science and
are commonly used to identify an attacker or suspect Hair found on the person
who has been assaulted or on their clothing is frequently used to determine
gender and ethnicity. Additionally, extrapolating DNA for comparative reasons is
one option . Even though it is not a living entity and is simply pushed through the
follicles of the scalp, arms, legs, or any other place of the anatomy where hair
may be found, hair contains DNA while being classified as benign dead matter [.
Fibres are another important finding that can reveal what an attacker or murderer
was wearing during the crime's time . Many forensic scientists use fibres to
identify the type of the item worn, and in certain situations they can be as precise
as identifying the manufacturer of the clothing.
Microscopy can be used to analyse hair and fibres. This involves forensic
scientists using light microscopes to examine the texture, colour, and shape of
the hair and fibres. Fibres found at the carpet or the victims or suspect clothing
can be used to trace back the manufacturer. [5]
Bone and skeletal remains can be analysed by DNA analysis. This works by
extracting DNA from the forensic bone. This can tell investigators about the sex
an age of the deceased person.Forensic anthropologists can determine a
person's stature or height by measuring their long bones. Based on statistical
calculations of demographic data and measurements of known-height skeletal
remains, this estimate has been made.
Dental remnants may be very important for identification. To compare dental
features with antemortem dental records, forensic odontologists look at dental
traits including dental records, dental X-rays, and distinctive dental work.
