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DNA and biological evidence — collection, preservation and interpretation

Biological evidence is only as good as its collection: the right sample from the right place, air-drying, paper packaging, refrigeration and a documented chain of custody — and then nuclear DNA profiling (STR), Y-chromosome DNA or mitochondrial DNA, according to the question and the type of sample.

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Why it matters in the exam

The exam asks which test suits which situation. A nuclear STR profile is the basis for comparison and databases. In a sample with many female cells and few male cells, such as a vaginal swab, Y-STR testing detects the male component; it works even without sperm cells, for example in a man after vasectomy. Mitochondrial DNA suits hair shafts without roots, old bones and teeth and degraded samples, and passes from a mother to all her children — so it identifies a maternal line, not a single person.

Detecting sperm: sperm cells are sought under the microscope; the tail is lost first, so a late sample shows mainly heads. In the vagina sperm are usually found up to about 3 days, and in the cervix sometimes longer; in the mouth and anus for a shorter time. When there are no sperm cells, PSA (p30) testing identifies seminal fluid. Saliva is detected by amylase testing and by collecting DNA from a bite or kiss site with the double-swab technique — a moist swab followed by a dry swab.

Preventing contamination is essential: gloves and mask, changing gloves between areas, single-use equipment. Moist samples are air-dried and packed in paper, not plastic, because moisture encourages mould and degradation. Each sample is labelled with the case name, date, time and collector's name and handed over sealed according to the chain of custody. In a suspect arrested shortly after the act, even if he has showered, a penile swab (especially from the coronal sulcus) is worth taking to detect the victim's cells.

Key points

• STR for identification; Y-STR for mixtures; mitochondria for degraded samples.
• The sperm tail is lost first; heads remain longer.
• PSA identifies seminal fluid without sperm cells.
• Double swab — moist then dry — for stains and bites.
• Drying, paper, refrigeration and chain of custody.

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Sources

• Butler JM. Advanced Topics in Forensic DNA Typing: Methodology, 2012 — STR, Y-STR and mitochondrial DNA
• WHO: Guidelines for medico-legal care for victims of sexual violence, 2003 — Collection of forensic specimens
• Knight's Forensic Pathology, 4th edition (2016) — Forensic science aspects

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.