Trace Evidence Examination Questions and Answers

Answer: D

Hackle marks are found in glass.

When glass breaks, the edges of the radial and concentric cracks show curved stress lines called rib marks, and fine lines at right angles to them called hackle marks. Their direction tells the examiner which side of the glass the force came from: on a radial crack the rib marks meet the side opposite the impact at right angles (the 3R rule). This helps in reconstructing break-ins and shootings through windows.

Paint, wood and steel fail differently: paint chips and flakes, wood splinters, and steel deforms or shears, so they do not show hackle marks.

Answer: B

The optical property is refractive index.

Refractive index is the ratio of the speed of light in a vacuum to its speed in the material, and it describes how much the material bends light. It is the most important property in comparing glass fragments in forensic work, measured by the immersion method or with a GRIM instrument.

Hardness and brittleness are mechanical properties, describing resistance to scratching and the tendency to fracture. Density is a physical property, mass per unit volume, used for glass and soil comparison too, but it has nothing to do with light.

Answer: A

Many manufactured fibres exhibit double refraction or birefringence.

When a synthetic fibre is drawn during manufacture, its polymer molecules line up along the fibre axis. Light polarised parallel to the axis then travels at a different speed from light polarised across it, so the fibre has two refractive indices. Their difference, the birefringence, is measured with a polarising microscope and is characteristic of the fibre type, distinguishing nylon, polyester, acrylic and others.

Colour, diameter and texture are useful for comparison, but they can be varied freely by the manufacturer and are not a property of the polymer itself, so they do not identify the fibre type as birefringence does.

Answer: A

The common white pigment is titanium dioxide, \(TiO_{2}\).

Titanium dioxide, mainly in its rutile form, is the most important white pigment in paint because of its very high refractive index, which gives excellent opacity and brightness. It largely replaced the older lead white and zinc white. In forensic paint analysis it is one of the inorganic components identified by techniques such as SEM-EDX or XRF.

Aluminium oxide is used as an abrasive and filler, tin oxide as an opacifier in ceramic glazes, and cadmium sulphide is a yellow pigment (cadmium yellow), not a white one.

Answer: C

They are lanugo hair.

Lanugo is the fine, soft, unmedullated and unpigmented hair that covers the foetus from about the fifth month. It is mostly shed in the last months of pregnancy, from about the seventh month, so the paper's 'after six months of gestation' describes it, and its presence helps estimate foetal age.

Vellus hair (printed 'Verlus') is the fine body hair of children and adults, terminal hair is the coarse, pigmented hair of the scalp and body, and pili annulati is a ringed hair condition, not a hair type.

Answer: A

Hair colour comes from melanin pigments principally found in the cortex.

Melanocytes in the hair bulb transfer melanin granules (eumelanin, black-brown, and pheomelanin, red-yellow) into the cortical cells as they form. The size, density and distribution of these granules in the cortex are compared under the microscope in forensic hair examination.

Hair contains no carbon granules. Air spaces in the medulla can make it look dark under transmitted light but are not pigment, and the cuticle is a layer of translucent, unpigmented scales.

Answer: A

The condition is pili annulati.

Pili annulati (ringed hair) shows alternating light and dark bands along the shaft. The light bands are caused by clusters of small, irregular air-filled spaces in the cortex, which scatter light; under transmitted light they look dark. The paper's 'bonding' refers to this banding.

Monilethrix gives beaded hair with periodic narrowings, trichorrhexis invaginata gives 'bamboo hair', where the shaft telescopes into itself, and trichonodosis is knotting of the hair. None involves bands of cortical air spaces.

Answer: A

Sex is shown by the presence of the Barr body.

In females one X chromosome in each cell is inactivated and condensed into sex chromatin, a dark body against the nuclear membrane named after Murray Barr. Cells from a hair root sheath, stained and examined under the microscope, show Barr bodies in a proportion of female cells and not in male cells.

The Davidson body, or drumstick, is the sex chromatin seen in neutrophil leucocytes of blood, not in hair root cells. The Golgi body is an organelle present in both sexes, and the medullary index is a ratio of hair widths used to tell human from animal hair.

Answer: D

The first step is to compare both colour and diameter.

Fibre comparison begins with non-destructive screening under a stereo and comparison microscope, looking at colour, diameter, cross-section and surface features. Colour and diameter together quickly exclude fibres that cannot match, before any destructive chemical tests are done.

Colour alone or diameter alone is too weak a screen, since many fibres share one or the other. Chemical constitution (by FTIR, solubility or melting point) is determined later, only for fibres that survive the microscopic comparison.

Answer: D

The correct matching is (a)-IV, (b)-I, (c)-II, (d)-III.

• Glass (printed 'Gloss') is quite brittle and easily broken (IV), which is why its fragments are such common trace evidence.
• Soil is a complex mixture of a diverse range of particles: minerals, organic matter and debris (I).
• Paint evidence usually comes in the form of chips, as from vehicles in hit-and-run cases (II).
• Tool marks form when two surfaces come in contact with force, the harder tool marking the softer object (III).

The other codes give soil or glass the description of tool marks or paint, which does not fit.