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Forensic BiologyRichard Li, with additions and edits by Ruth Ballard
Lecture 2: Sources of DNA Evidence
Outline
DNA Biological evidence containing DNA
Blood Semen Saliva Hair Bone Teeth Touch/trace
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DNA
Most cells contain DNA Digital information molecule Information carried in sequence
(A,T,C,G) Mutations change the sequence over
time▪ Different forms = alleles
In animals, two types▪ Nuclear▪ Mitochondrial▪ Forensic biologists use both 3
DNA in Forensic Biology
Nuclear DNA Inherited from both parents Two copies per cell Pristine (full profile):▪ Fresh materials (semen, saliva, blood, bone); rooted
hairs▪ E.g. Semen stain taken 12 hours after rape▪ PD in the quintillions
Compromised (partial profiles) or completely degraded (no profile): ▪ Aged or damaged materials; hair shafts▪ E.g. Saliva on a cigarette butt exposed to rain for 1 week▪ PD reduced based on level of degradation
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DNA in Forensic Biology
Mitochondrial DNA Found inside the cell’s mitochondria▪ Powerhouses that provide the cell’s energy
Thousands of copies/cell Inherited only from the mother Can be obtained from most biological materials
(fresh or old)▪ E.g. Skull found in a field; body dumped 2 years prior▪ Usually possible to get full profile
PD low (1 in a few hundred is typical)▪ All maternally-linked relatives carry the same mtDNA
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DNA in Forensic Biology
Nuclear DNA is preferred due to high PD
BUT… Mitochondrial DNA may be the only
option for some samples And it’s much better than nothing!
The fewer cells and/or the more compromised the evidence, the less likely you can get a nuclear DNA profile
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Biological Evidence containing DNA
Blood Often shed at violent crime scenes Is usually the victim’s blood▪ Look for victim’s blood on suspect’s skin,
shoes, or clothing Consists of erythrocytes (red blood
cells), leucocytes (white blood cells), and platelets
Nuclear DNA obtained from leucocytes; erythrocytes and platelets lack nuclei and DNA 7
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Nucleated leukocyte surrounded by enucleated erythrocytes
Biological Evidence containing DNA
Semen Very common in sexual assaults▪ Recovered from skin, vagina, oral cavity,
rectum of victims▪ Condoms = “perfect” evidence item
Rich source of DNA▪ Sperm are “little bags of DNA” (more on this
later…) Persists inside/on victims for up to 72
hours Persists on inanimate objects up to many
years▪ E.g. Semen stain on carpet
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Biological Evidence containing DNA
Saliva Common in sexual assaults▪ Recovered from skin, labia, peri-oral regions
on victim Other common sources:▪ Cigarette butts▪ Back of envelopes and postage stamps▪ Rims of beer cans, drinking glasses
Contains lots of bacteria▪ If evidence not dried and stored properly, will
quickly degrade10
Biological Evidence containing DNA
Hair Root Shaft
Medulla Cortex Cuticle
Grows at approximately 0.3mm per day Three phases:
Anagen phase- growing phase (nuclear DNA) Catagen phase- follicle becomes inactive (nuclear
DNA) Telogen phase- growth cycle ends (mtDNA only)
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A hair follicle
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Cross section of a hair shaft
Biological Evidence containing DNA
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Telogen hairs lack roots People shed more than 200 telogen hairs
per day Look inside caps or on clothing or
furnitureHairs with rooted must be plucked or
pulled out by force Look in the hands of a victim after a
struggleProcedure:
Microscopy to identify phase Choose appropriate DNA testing method
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Hair morphology: Differences in human hair
color
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Hair morphology: Absence (above) or presence (below) or medulla
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Hair morphology: Distinct cuticle layering patterns
Biological Evidence containing DNA
Bone Spongy▪ Rich source of cells and DNA but…▪ Degrades quickly after death▪ Good source of nuclear DNA
Compact▪ Fewer cells than spongy bone but…▪ Degrades less quickly after death▪ Good source of mitochondrial DNA
Mass fatalities (e.g. 911) Sanding prior to DNA analysis to prevent
comingling of remains18
Biological Evidence containing DNA
Bone Rate of degradation of human remains
varies greatly with environmental conditions▪ Soft tissue is lost▪ Bones are more stable
Mass Fatalities-identify victims
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Biological Evidence containing DNA
Teeth Dentin- no cells Pulp Cavity▪ Contains lots of cells and
DNA if tooth is fresh Root▪ Contains root canal with
cells and DNA if tooth is fresh
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Biological Evidence containing DNA
In addition to DNA analysis, teeth can be used to:
Identify a person▪ Characteristics of teeth▪ Alignment▪ Overall structure▪ X-rays
Compare suspect’s with bite marks on victim▪ What else might be present on a bite mark
that could help identify a criminal? 21
Biological Evidence containing DNA
Touch/Trace DNA First described into 1997: “DNA Fingerprints
from Fingerprints” (Nature article) Sources:▪ Surface skin cells▪ Cells transferred onto hands from eyes and mouth
Low levels; usually partially degraded Often contains DNA from more than one person▪ DNA mixtures▪ Difficult to interpret (is it really related to the crime?)▪ E.g. Cell phones, keyboards, doorknobs, clothing
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SemenSalivaBloodHairBoneTrace/touch
Primary Biological Evidence in Cases Reviewed by Dr. Ballard in 2012
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