LECTURE 2: FORENSIC BIOLOGY Flashcards
Biology of a crime scene
A huge part of the evidence identified at a crime scene is biological by nature
- Touch contact
o Fingerprints- sebaceous and eccrine secretions
o “Touch” or “trace” DNA- epithelial cells
o Sweat- Metabolites? (2021 research with ANPC)
- Blood
- Hair
- Semen
- Body Fluids
o Saliva, Faeces, Urine, Vomit
- Tissue
o Soft tissue, brain, bone, teeth
how many ng of DNA is required for a full profile
0.5ng
number of cells required for a full DNA prolfile
~76 cells
factors of skin cells left
- Sloughing of cells
- High vs low shedder
o Varies day to day - Physiological condition
o Eczema - more
o Dry skin - more
o Rough hands (manual labour) - more - Hand washing
- Habits
o Face touching, nail biting, hair - Contact type
o Friction, pressure - Surface type
o Rough vs smooth - Perspiration
Screening for “touches”:
- Alternative light sources (ALS)
- Fingerprint treatments
- Common sense
- Context/ statements
Blood
- Complex suspension
o Dissolved substances
o Suspended cells and particles - Two important components
o Liquid = Plasma
o “Solid” = Cells 45%
Erythrocytes, Leucocytes, Platelets - Weight: RBC > WBC > Plasma
o Lividity - Where is the DNA? WBC
ABCs of Blood:
- Appearance
o The more blood you see, the better you can recognize it.
o Colour, Hue, Tint, Saturation, Shine, Sheen, Reflectivity - Behaviour
o Clotting
o Separate
o Particulates
o Drying pattern - Context
Non-destructive processes: blood
- Alternative Light Sources
o Blood on black/red shirt (IR)
Beams pulses of infrared light onto a surface and camera detects the infrared that is reflected back off it using filter
Where there is blood, it is absorbed and we can see it
Destructive processes: blood
- Presumptive Tests:
o Phenophthalein (KM reagent) – pink colour
Relatively specific- Oxidants x-react
Very sensitive- 1x10E-7
Excellent for screening (most common reagent used), low cost & good shelf life
o Hemastix/TMB
o Luminol - Confirmatory tests:
- RSID Blood/Hematrace
o Immunochromatograpy
o Method of choice
o No false positives (exc ferret)
o Very sensitive, rapid
o Does suffer from high dose hook effect (sample too conc.)
3 main structure on shaft of hair
the medulla, cortex and cuticle
Cuticle
- Series of overlapping scales that form a protective covering (resistent
- Always point to tip
- Human scale pattern called imbricate
- Specialised cells- keratinised
o Species ID- Scale patterns
SEM
Casting
Cortex
- Regular arrayed cortical cells
- Impart colour
o Embedded pigment granules
Medulla
- Appears as central canal
- Medullary Index
o Medulla diameter/total
o Humans < 1/3 - Different person to person, origin to origin
- Continuous, interrupted, fragmented, absent
- Speciation based on pattern
where is the nuclear material of hair
Root
3 phases of hair growth
1) Anagen
2) Catagen
3) Telogen
Anagen
Initial growth phase, actively producing hair, best phase to get DNA
Catagen
Transition phase, hit or miss to get DNA
Telogen
Final phase, hair falls out, Follicular tag, the ones falling out of hair, unlikely to give profile, mitochondrial can be found there
Semen
- Exocrine secretion
o Semen- carrier fluid
Typical ejaculate 1.5-5mL
Contains 40-250 million sperm cells
o Contains Acid Phosphatase and PSA P30
Aspermic
no sperm cells
Oligospermic
low sperm cells
DNA source: Semen
o Sperm head
o Sloughed UT lining
o Cell free DNA – just DNA free in the fluid cell
ABCs of semen:
- Appearance
o White, crusting
o Easily mistaken for female secretions - Behaviour
o Spread liquid - Context
o “Are the circumstances consistent with semen?”
Non destructiver processes: semen
- Alternative Light Sources
o ~450nm wavelength light
Blue light/Orange glasses
Fluorescence due to the presence of molecules such as Flavin and Choline-conjugated proteins
Fluorescence can also be masked by certain types of fabrics and fabric treatments
