The Analysis Of Biological Evidence In Sexual Assault Cases

Forensic biological evidence can be found in several cases of assault; in fact, these are often the only way to prove sexual contact occurred and to identify the perpetrator.

These must be carefully managed, as the relevance of any finding in Forensic Genetics is determined primarily by the integrity and quantity of the samples submitted for analysis.

In every crime against people, as in sexual assault, the contact between the perpetrator, the victim and the environment, always leaves biological evidence which is transferred from the perpetrator to the victim, to the scene, and vice versa.

Typically crime scene evidence could be found on any place where a criminal offence was committed, on anything worn or carried by the victim during the time the offence and/or on the body of any person associated with the offence.

The collection of biological evidence for DNA studies is particularly useful to establish the occurrence of sexual contact and to proceed with suspect identification.

Good evidence management must properly ensure procedures in the sequence ranging between selecting and collection, packing, sealing, labeling, and insertion into the kit, its storage, preservation and transportation, and reception by the forensic laboratory, always ensuring the compliance of chain of custody.

In addition, to perform the comparative DNA test, a biological reference sample of the suspect and the victim must be collected, labeling them properly to avoid confusion with or contamination of the evidence.

Depending on the purpose, swabs of different design, shape, and size are commercially available and should be judiciously selected. Generally, the collection should be done by gently (to prevent exfoliation of the victim’s own epithelial cells) rubbing in a circular motion for 15 seconds, a restricted area of the mucosa or skin, from the periphery to the center and rotating the swab.

If oral-genital contact is suspected, blood or hair sample may be preferred to act as reference sample since oral sample might be contaminated with the perpetrator’s DNA.

To collect a hair sample, at least 7 hairs should be pulled out in order to keep the roots intact, where the DNA is concentrated.

Evidence must be properly labeled and packaged, in order to ensure that evidence is not lost, damaged, or contaminated until handled by the laboratory personnel and to guarantee that reliable results and the chain of custody compliance to evidence be admissible in a court of law.

Each evidence must be dried before packaging.

For debris such as hairs, leaves, and fibers, a bindle can be used and then put into a paper package (double packaged). All evidence should be individually sealed.


Magalhães, T., Dinis-Oliveira, R. J., Silva, B., Corte-Real, F., & Nuno Vieira, D. (2015). Biological evidence management for DNA analysis in cases of sexual assault. The Scientific World Journal, 2015.

Also Read: Do Fingerprints Contain DNA?

Authored By

This article is written by Ms. Chiara Lucanto. She is a Forensic Biologist.

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