Forensic genetics (Evidence Collection (reference samples from suspects…
- two main sources of evidence material:
crime scene samples and, from suspects and victims.
- Contamination issue: Any person entering the crime scene should be treated as a source of undesired DNA.
They must be properly dressed.
Change gloves very frequently!
- removing undesired DNA:
use disposable equipment Collecting
instruments should be cleaned with sodium hypochloride (scissors, knives, saws etc.)
10% sodium hypochlorite for 5 minutes totally destroys DNA
- Size and location of stains:
preferred method is to collect the entire item (small objects ex. bloodstained knife).
Larger items impossible to submit entirely may be swabbed (ex. bloodstained door) or cut-out (ex. semen stained car seat).
- Large or Immovable Objects cont.
Wet stains (e.g: pool of blood):
1.Soak up stain with sterile swab
2.Allow to air dry
3.Package in clean paper, envelopes or similar, label
- drying remarkably reduces DNA degradation.
• Hairs mixed with body fluids must be air dried.
• Package hair or group of hairs separately .
- Soft tissues samples:
Samples of defragmentated body as a result:
-explosion (terrorist attacks),
-traffic accidents (run over with a train),
should be placed in DNA free, airtight plastic container without formalin and FREEZED.
Storing samples in formalin can create artifacts.
Ethanol (96%) can be used for DNA preservation.
recommended to collect all available bones.
for identification، In the most cases long bones like fibula or femur are collected.
Do not cut bones outside the laboratory because it increases contamination risk.
degradation process starts very rapidly - freeze (-20°C or -80°C)
enamel well protects DNA present in tooth pulp.
Collect teeth in the following order (preferred are nonrestored teeth):
– nonrestored molar – nonrestored premolar
– nonrestored canine – nonrestored front tooth
If not present, collect restored in the same order
• Place teeth samples in plastic container and freeze.
• packages should be maintained in cool, dry locations because heat, sunlight and moisture have destructive effects on biological evidence.
- Factors that may Contribute to the Degradation of Biological Samples
• Humidity - moisture promotes bacterial and fungi growth
• Temperature - high temperatures facilitates the growth of microorganisms.
• Light - exposure to intensive sunlight (UVB) and UV rays (DNA modification).
- at least one sperm gives a proof of semen presence
- Lack of sperm does not mean that semen is not present !!!
- Time Intervals For Sperm Recovery in Living Sexual Assault Victims*
- the best confirmation of the presence of semen.
- Two common staining methods are: haematoxylin with eosin and the Christmas tree staining (stains heads red and tails green).
- Sperm stability:
depends on the numer of sperm cells in ejaculate,
on body surfaces – stable for many days,
on underwear, bedding and clothing - many months
SAP + PSA
- Prostate Specific Antigen:
protein found in very high levels in seminal fluid
Less sensitive than (SAP) test
fewer false positives than the SAP test
Urine, breast milk and sweat glands contain PSA but below forensic limits of detection.
Present in vasectomised mens’ semen
Antibody-antigen reaction test – immunochromatographic test
• Proteins present in seminal fluid in high concentration .
• Test gives positive results in cases of azoospermia, oligospermia and vasectomy.
- places of saliva presence
2.Cups, glasses, spoons, forks
3.Bottles and cans (direct drinking)
4.Chewing gum, gnawed food
5.Stamps and envelops
6.Kissed, bitted and licked body places,
7.Clothe, robbery masks, gags, scotch tapes
- Presumptive tests involve the detection of amylase activity (Starch test) or detection of amylase (immunochromatographic test with antibodies against alpha-amylase). mRNA profiling with markers: statherin and histatin
- sensitive and not specific.
tests produce visible colour or result in release of light (chemiluminescecne)
both tests rely on catalytic properties of blood (heme)
positive results of presumptive tests do not indicate that blood is present !!!
- take advantage of the peroxidase- like activity of the haem group. Haem catalyses the hydrolysis of hydrogen peroxide, which in turn leads to the oxidation of the target compound, resulting in colour change.
- Producing colour reaction:
– Benzidine (Adler’s Test)
First screening test for blood (1904)
Reaction carried out in ethanol/acetic acid solution
Green color produced which sometimes turns to brown
Benzidine is carcinogen
– Phenolphthalein (Kastle-Meyer Test)
commonly used today
phenolphthalein – acid-base indicator
less sensitive than benzidine test
bright pink color produced
– Tetramethylbenzidine (TMB)
- Using chemiluminescence – Luminol
more value in locating and defining areas of possible blood than specifically identifying it
spray chemical mixture of luminol observe the result in darkness
catalytic activity of heme accelerates oxidation of luminol producing blue-white to yellowish-green light if blood present
extremly sensitive – can detected blood present under wallpapers and areas pained up
- specific and moderately sensitive (100% certainty of blood presence)
- Not as sensitive as presumptive tests but give 100% proof of blood presence.
- **Antibody tests – Immunochromatographic tests:
use monoclonal antibodies against
• anti-glycophorine A antibody
• anti-human haemoglobin A antibody
- Spectrophotometric tests (main abs. peaks at 415nm and 560—570nm)
- Microspectroscopic tests, (Teichmann and Takayama tests)
Epithelium and Epidermis
- cells can be detected and typed even after many years, but it depends on environment conditions, mainly humidity and temperature.
- branch of forensic medicine that deals with genetic knowledge to legal problems and legal proceedings.
- divided into:
paternity testing (kinship analysis) – used in civil cases
identification of biological stains - used in criminal cases
- criminal cases: main aim of forensic genetics is to identify with highest certainty the source of samples collected on a crime scene or obtained from human body.
- steps of stains analysis:
- collection of traces on a crime scene.
- Identification of stain type (blood,semen,saliva,hair, bone etc.) on exhibit.
- Identification if sample is human or not
- Individualization of biological stains (DNA analysis)