Unit 1: Dermatoglyphics

Dermatoglyphics (Greek derma = skin, glyphe = carving) is the scientific study of the epidermal ridge patterns on the fingers, palms and soles. The friction ridge skin forms in the foetus between the 10th and 16th week of gestation and remains unchanged throughout life except in size. This permanence, together with individuality, is the foundation of fingerprint identification in forensic science.

Defining properties that later sections rely on:

• Permanence: Ridge detail is fixed for life and reappears identically after superficial injury; only deep dermal scarring alters it.
• Individuality: No two ridge patterns, including those of identical twins, have ever been found identical — the basis of positive identification.
• Immutability: The pattern configuration is set before birth and cannot be naturally changed.
• Friction ridge features: Ridges bear sweat pores; the intervening depressions are furrows.
• Minutiae (Galton details): Ridge endings, bifurcations, dots, islands, enclosures (lakes), spurs and crossovers — the fine points used for comparison.
• Pattern reference points: The core (approximate centre of a pattern) and the delta (a triangulation point where three ridge flows meet) govern classification.

II. Recording Plain and Rolled Impressions

Capturing controlled reference prints for the ten-print card.

A. Purpose and Principle

Reference impressions are deliberately deposited under controlled conditions so that all ridge detail is legible for classification and comparison.

• Medium: Printer's ink, sensitised paper, or livescan optical scanning capture the ridge–furrow contrast.
• Standard form: The ten-print card records all ten digits in fixed boxes plus plain (slap) impressions for verification.

B. To Record Plain and Rolled Impressions

Two complementary techniques appear on every ten-print card.

1. Rolled impressions: The finger is inked and rolled from nail edge to nail edge (one continuous motion, side to side).
   • Reason: Captures the full pattern area including both deltas, essential for accurate Henry classification.
2. Plain (dab) impressions: The four fingers of each hand and the thumbs are pressed straight down without rolling.
   • Reason: Verifies the sequence and correct placement of the rolled prints; detects transposed digits.

• Technique controls: Light, even ink film; single firm motion; clean between prints to avoid smudging.
• Common defects: Over-inking fills furrows; under-inking loses ridges; slipping doubles the pattern.

III. Henry's Classification of Fingerprints

A numerical filing system for the ten-print card.

A. Statement and Basis

Sir Edward Henry's system (introduced c. 1897) classifies a set of ten prints by the presence and position of whorls, converting patterns into a filing fraction.

• Whorl-based: Only whorls carry numerical value; loops, arches and tented arches count as zero.
• Filing fraction: Produces a primary classification expressed as a fraction from 1/1 to 32/32.

B. Perform Henry's Classification of Fingerprints

The primary classification derives from ordered digit values.

• Digit pairing: The ten fingers are grouped into five pairs in fixed sequence:

Pair 1: R.Thumb  , R.Index
Pair 2: R.Middle , R.Ring
Pair 3: R.Little , L.Thumb
Pair 4: L.Index  , L.Middle
Pair 5: L.Ring   , L.Little

• Assigned values: A whorl scores by pair position: 16, 16, 8, 8, 4, 4, 2, 2, 1, 1 (first two fingers 16, next two 8, and so on).
• Fraction rule:
  • Numerator: Sum of whorl values on the even-numbered fingers (2nd, 4th, 6th, 8th, 10th) + 1.
  • Denominator: Sum of whorl values on the odd-numbered fingers (1st, 3rd, 5th, 7th, 9th) + 1.
• The added 1: Prevents a zero denominator when no whorls are present, giving the baseline 1/1.

Worked example: Whorls on R.Index (value 16, even), R.Ring (8, even) and L.Thumb (8, odd):

Numerator   = 16 + 8 + 1 = 25
Denominator = 8 + 1      = 9
Primary classification = 25/9

IV. Identification of Various Fingerprint Patterns

The three fundamental pattern types and their subgroups.

A. Definition and Ridge-Counting Basis

Patterns are grouped by ridge flow and by the number of deltas present; ridge counting and ridge tracing subdivide them.

• Delta count: Arches have none, loops have one, whorls have two.

B. Identification of Various Fingerprint Patterns

Each type is recognised by delta number, ridge flow and defined measurement.

1. Arches (~5% of patterns):
   • Plain arch: Ridges enter one side and exit the other with a slight rise; no delta, no core.
   • Tented arch: A central ridge thrusts up sharply, forming an angle or an upthrust; steep rise distinguishes it from the plain arch.
2. Loops (~60–65%): Ridges curve back toward the side of entry; one delta, one core.
   • Ulnar loop: Ridges open toward the little (ulnar) finger.
   • Radial loop: Ridges open toward the thumb (radial) side.
   • Ridge count: Number of ridges crossing the line from delta to core (delta and core excluded).
3. Whorls (~30–35%): Circular or spiral flow; two deltas.
   • Plain whorl: At least one ridge makes a complete circuit; an imaginary line between the deltas touches or crosses a recurving ridge.
   • Central pocket loop whorl: Mostly loop-like with a small central whorl; the line between deltas cuts no recurving ridge.
   • Double loop whorl: Two distinct loop formations with two separate shoulders.
   • Accidental whorl: Combination of two pattern types or one conforming to none.
   • Ridge tracing: Traces the lower ridge of the left delta to the right delta, classed Inner, Meeting or Outer.

V. Physical Methods of Latent Fingerprint Development

Visualising sweat/oil residues through mechanical adhesion.

A. Purpose and Principle

Latent (invisible) prints are made visible when fine particles physically adhere to the moisture, fats and amino-acid residue of the deposit.

• Deposit composition: ~98% water plus salts, amino acids, urea and sebaceous oils.
• Surface dependence: Best on smooth, non-porous surfaces (glass, metal, plastic).

B. Investigate and Perform Physical Methods of Latent Fingerprint Development

Powders and vapour condensation are the principal physical routes.

1. Powder dusting: A soft camel-hair or fibreglass brush lifts loose powder onto the residue.
   • Regular powders: Black (carbon-based) for light surfaces; grey/white (aluminium, titanium dioxide) for dark surfaces — contrast is chosen against the background.
   • Magnetic powder: Applied with a magnetic wand (no bristles touch the print), ideal on rough or textured surfaces.
   • Fluorescent powder: Viewed under UV or forensic light to isolate prints on patterned surfaces.
2. Small Particle Reagent (SPR): A suspension of molybdenum disulphide adheres to sebaceous components — used on wet, non-porous surfaces where powders fail.

• Lifting: Developed prints are recovered with transparent adhesive tape onto a contrasting backing card, or photographed in situ first.

VI. Chemical Methods of Latent Fingerprint Development

Reacting reagents with specific residue constituents.

A. Purpose and Principle

Chemical development produces visible or fluorescent products by reacting selectively with amino acids, sweat salts or sebaceous fats, and is favoured on porous surfaces.

• Selectivity: Each reagent targets a defined residue component.
• Surface dependence: Porous surfaces (paper, cardboard, raw wood) absorb residue, suiting chemical treatment.

B. Investigate and Perform Chemical Methods of Latent Fingerprint Development

The main reagents differ by target constituent and product colour.

1. Ninhydrin:
   • Target: Reacts with amino acids in eccrine sweat.
   • Product: Purple-blue coloration known as Ruhemann's purple; developed on paper with gentle heat and humidity.
2. Cyanoacrylate (superglue) fuming:
   • Target: Polymerises on residue moisture on non-porous surfaces.
   • Product: A hard white ridge deposit, often dyed or dusted afterward for contrast.

• Silver nitrate: Reacts with chloride (salt) in sweat to form light-sensitive silver chloride; ridges darken on exposure to light.
• Iodine fuming: Sublimed iodine vapour is absorbed by sebaceous oils, giving a transient brown image that must be photographed or fixed quickly.
• DFO and 1,2-Indandione: Amino-acid reagents giving fluorescent results under forensic light, more sensitive than ninhydrin.
• Sequential processing: On porous items the recommended order is iodine → DFO/ninhydrin → silver nitrate, so that each reagent does not destroy the residue targeted by the next.

Physical methods act by mechanical adhesion and suit non-porous surfaces, while chemical methods exploit specific molecular reactions and dominate on porous surfaces — the surface type therefore dictates the developmental route chosen at a scene.