Forensic Science - Biological Evidence

Why is DNA valuable in forensic science?

It’s unique to each person, making it excellent for identification.

Where are DNA profiles stored?

In computer databases containing profiles from offenders and crime scenes.

What happened in the Colin Pitchfork case?

• 2 women were raped and murdered (Lynda Mann and Dawn Ashworth).

• Over 5,500 local men were tested in a mass DNA screening.

• DNA cleared an innocent suspect (Richard Buckland) and identified the true killer Colin Pitchfork.

• Pitchfork was convicted in 1988, this was the first conviction based on DNA evidence.

What does gel electrophoresis do?

Separates DNA fragments by weight as they pass through a gel.

What is blood composed of?

Plasma and cellular components.

What is the first stage of blood analysis?

Centrifugation (separating components).

What can be tested for in blood?

• Poisons.

• Toxins.

• Drugs.

What is saliva and where does it come from?

It is a liquid secreted by the salivary glands.

It’s rich in digestive enzymes like amylase.

How is saliva detected at a crime scene?

It fluoresces under UV light.

Name 3 saliva tests.

• Radial diffusion.

• Starch-Iodine.

• Phadebas.

What does a positive Phadebas test show?

Blue colour → presence of amylase → possible saliva stain.

What is semen made up of?

• Cells.

• Amino acids.

• Sugars.

• Salts.

How is semen detected under light?

It fluoresces under UV light.

What enzyme is detected in the presumptive test for semen?

Acid Phosphatase (ACP).

What colour shows a positive ACP test?

Deep purple.

Name 3 confirmatory semen tests.

• Microscopic exam.

• Christmas Tree Test.

• Prostate-Specific Antigen (PSA) test.

What is urine composed of?

95% water and inorganic ions, plus urea and creatinine.

What is urine useful for?

Detecting metabolites (signs of drug or toxin use).

How is urea detected?

Urease breaks down urea into ammonia and carbon dioxide. The ammonia produced is detected using acid-base indicators. This changes the colour on the paper meaning urea is present.

How is creatinine detected?

Jaffe reaction (bright red colour with picric acid).

How are amines detected?

DMAC reagent → dark pink/red colour.

What can vomit analysis show?

pH, amylase, food identification and time since last meal.

Why is vomit important in forensic investigations?

It helps determine time since death, poisoning or drug use.

What does forensic toxicology investigate?

Poisons, drugs and their effects on the body.

Name the 4 stages of toxin processing in the body.

• Absorption.

• Distribution.

• Metabolism.

• Excretion.

What is first pass metabolism?

When a drug is mostly broken down before reaching the bloodstream.

Give an example of a drug that becomes stronger after metabolism.

THC → converted into 11-hydroxy-THC (more potent).

What is PMA and why is it dangerous?

Para-Methoxyamphetamine.

It is slow-acting, people don’t feel the high so they take more and this can cause severe hyperthermia.

What is forensic odontology?

The study of teeth and bite marks.

What can teeth reveal?

Identity and age.

Name a case study that famously used bite mark evidence and what happened.

• Ted Bundy was an American serial killer in the 1970’s.

• He sexually assaulted and murdered at least 30 women.

• He left bite marks on several victims and forensic experts used these marks as evidence against him.

• Bundy’s dental impressions were compared to the marks left on his victims.

• The bite marks matched Bundy’s teeth perfectly and he was prosecuted.

What does PMI stand for?

Post-Mortem interval - the time between death and body examination.

Why is estimating PMI important?

It helps identify when the crime occurred and to verify alibis.

What is the difference between the cause of death and mechanism of death?

Cause = reason (e.g. heart attack).

Mechanism = body change (e.g. heart stops beating).

What is Algor Mortis?

The cooling of the body after death (only accurate up to 24 hours).

What is Livor Mortis?

Discoloration from blood settling (starts after 2-3 hours, fixed by 8 hours).

What is Rigor Mortis?

Stiffening of muscles (full by 12 hours).

What happens in the later decomposition stages?

Bloating → Putrefaction → Adipocere → Mummification → Dry remains.

What does forensic entomology study?

Insects on decomposing bodies to estimate time and circumstances of death.

What can insects tell investigators?

Time, season, location of death, movements of body, drug use.

What is the main insect studied?

Flies (life stages of a fly helps estimate PMI).

What are diatoms?

Microscopic algae found in water.

How are diatoms used in forensics?

To determine if a person was alive when they entered water.

How do diatoms indicate drowning?

Found in bone marrow → shows they entered bloodstream while alive.

What is acid digestion used for?

Removing organic matter to isolate diatoms for microscopic examination.