Hair Examination and Microscopy

Begin with the exhibit’s condition, length, colour, curvature, ends and any adhering material. Low-power examination can locate roots, damage and foreign material before more detailed preparation. Distinguish an intact hair from a fragment or hair-like fibre, and preserve the recovery context.

Transmitted-light examination can reveal pigment, cortex and medulla features. A comparison microscope permits side-by-side observation of questioned and reference hairs. Scale casts reveal impressions of the cuticle surface; cross-sections or surface imaging address different questions and may consume or alter material.

The cuticle has overlapping scales; broad descriptions include imbricate, coronal and spinous arrangements. Such patterns vary between species, body sites and hair types and may overlap. Imbricate scales occur in humans and many other mammals, so that observation alone does not establish human origin.

The cortex contains pigment and internal features such as cortical fusi. Describe distribution, density, colour and changes along the shaft. The medulla can be absent, interrupted, fragmented or continuous. Diameter, tip condition and cosmetic changes can provide further observations, but none is automatically individual-specific.

Preparation matters. Mounting medium, optical conditions, orientation and magnification affect what is seen. Coordinate microscopy with DNA, toxicology or other work before using a medium, adhesive or cleaning treatment that could interfere with later analysis.

The medullary index is the medulla width divided by shaft width at the same position and orientation. Calibrate the imaging or measuring system and record where measurements were made. Variation along the shaft, focus and an irregular cross-section can affect the result.

For a teaching calculation, a medulla 18 micrometres wide within a shaft 75 micrometres wide gives 18/75 = 0.24. This is a dimensionless ratio. It describes that measured location, not a probability that the hair is human or a unique feature of its donor.

Rules of thumb such as “human below one-third, animal above one-half” describe tendencies encountered in teaching, not exhaustive diagnostic boundaries. Species, body site, guard versus underhair, damage and preparation matter. Avoid a table assigning universal numerical ranges to every dog, cat, rabbit or deer.

Use several observed features and a suitable reference collection. Where the species question needs stronger resolution, consider appropriate genetic or specialist testing. Do not treat a medullary ratio as equivalent to a validated species sequence comparison.

Hair length, curvature, diameter variation, tips and root form can inform a qualified body-site assessment. Scalp, facial, axillary, pubic and other hairs have overlapping appearances. Trimming, breakage and cosmetic treatment can remove useful context or make a fragment unsuitable for a confident assessment.

A cut-looking tip may be consistent with cutting, but does not establish that it was recently trimmed. An attached root and tissue can inform examination planning, but do not prove the hair was pulled during violence. Rootless material does not disclose a growth phase that cannot be observed.

Historical racial classifications relied on broad morphological descriptions and often overstated what those features could establish. Hair form and pigmentation vary within and across populations. Mixed ancestry and treatment are not the only problems: the categories themselves do not supply a validated individual classification system.

Record the observed morphology rather than assigning a person to a racial or geographic group from the hair. “Consistent with” does not cure an unsupported ancestry conclusion. A different molecular assay for ancestry would require its own validation, reference data and qualified interpretation.

A reference collection should represent the relevant variation in the possible source. Hair from one person differs by body region, follicle, time and treatment. A small or unrepresentative collection can limit both an association and an exclusion; its adequacy needs to be considered before interpreting differences.

Microscopic similarity means that observed features overlap within the comparison performed. It does not establish that the questioned hair came from the reference donor. Differences may support exclusion or a more limited conclusion, depending on their nature, the material available and reference quality. An inconclusive result is also legitimate.

The FBI/DOJ historical review identified testimony that exceeded scientific limits, including unique-source claims, unsupported numerical weight and reliance on an examiner’s personal experience as a population-frequency estimate. Those errors should be explained without treating the reviewed-case percentage as a universal error rate for every microscopic task.

A qualified morphological finding can be reported even when DNA testing is unavailable; it does not become scientifically meaningless merely because no DNA result exists. Conversely, DNA agreement does not supply a numerical probability for the observed microscopic resemblance. State the two findings separately and avoid double-counting information.

Reports and testimony follow applicable laboratory and jurisdictional requirements. US Department of Justice reporting language is one scoped example, not a worldwide law requiring a DNA result before any hair examination can be described.

Adhering nucleated tissue can make conventional STR testing more promising. A complete root does not guarantee a complete profile, and a damaged or contaminated specimen may remain difficult. Preserve the distinction between an observed tissue tag and a predicted molecular yield.

Rootless shafts can contain fragmented nuclear DNA. Appropriate sequencing approaches may recover information even when conventional nuclear STR testing fails. Mitochondrial testing remains useful for many shafts, with the selected method reflecting available material, laboratory capability and the comparison question.

Mitochondrial agreement is not limited to people from one known family; unrelated people may share the typed sequence. Maternal relatives often share results, but heteroplasmy and mutation can matter. Interpretation of differences and agreement follows the typed range, quality and relevant guidance rather than a rule that every difference excludes and every relative must match exactly.

A nuclear donor statistic is calculated from the actual data and appropriate model. It cannot be promised from a full profile’s marker count alone. A genetic donor association also does not establish when or how a hair reached the item. Microscopy, DNA and activity interpretation need separate explanations.

Appropriate clean forceps, tape recovery or other validated methods can recover hairs while retaining location information. A paper fold inside a suitable outer envelope is a common small-trace package. The essential functions are preventing loss, contamination and cross-transfer and preserving condition; a rigid plastic tube is not automatically proof that an exhibit was compromised.

Keep different locations or item groups identifiable. Record collection method, packaging, seals and transfers. Adhesives and mounting media must be considered before later tests. Do not automatically wash or cut every hair before the analytical plan is agreed.

Reference collection must have proper authority or consent and follow the receiving laboratory’s requirements. A representative collection may require multiple hairs or body sites, but “25–50 hairs plus compulsory pulling” is not a universal rule. Buccal reference DNA may address a genetic comparison without an invasive hair collection where appropriate.

The case record should preserve observations, photographs, measurement calibration, comparison material, methods, portions consumed, results and review. A continuity discrepancy requires assessment of its significance; it does not automatically prove contamination or determine admissibility in every jurisdiction.

Where a historical report used “match,” unique identification or a rarity estimate, review the underlying work and wording rather than merely modernising the title. Correct communication is part of the scientific reassessment, especially when new genetic testing is considered.