Forensic Biology Study Guide

Accreditation

Formal recognition that a laboratory meets established standards and is competent to carry out specific tests.

Laboratory Validation

A process to prove that a test method is reliable, reproducible, and suitable for its intended use in forensic work.

quality assurance (QA)

A system of planned activities to ensure quality in the laboratory, including training, documentation, and oversight.

quality control (QC)

The operational techniques and activities used to fulfill QA requirements—like running known standards with each test

proficiency testing

A way to test analysts’ competence by having them analyze a sample with a known outcome.

certification

The recognition of an individual’s expertise in forensic science, such as through written exams and continuing education.

probative

Evidence that is useful in proving something important in a trial (i.e., relevant and significant).

Locard's exchange principle

“Every contact leaves a trace”—the foundational concept that perpetrators of a crime bring something into and leave something from a crime scene.

polymorphism

A genetic variation among individuals that can be used to distinguish people (e.g., STRs, SNPs).

tandem repeats

DNA sequences where a short pattern is repeated consecutively (includes STRs and VNTRs).

minisatellites (variable number tandem repeat, VNTR)

Longer repeating DNA sequences used in early DNA fingerprinting; less common now.

microsatellites (short tandem repeat, STR)

Short tandem repeats, 2–6 base pairs long, currently used in forensic DNA profiling.

mobile elements (interspersed repeats)

DNA sequences that can move around the genome (e.g., Alu, LINEs), not typically used in forensic ID.

CODIS

Combined DNA Index System—an FBI-managed database that stores DNA profiles from convicted offenders, arrestees, and forensic evidence.

transcellular fluids

Fluids that pass through or are contained within body cavities (e.g., cerebrospinal, pleural, synovial).

erythrocytes

Red blood cells; they carry oxygen and do not contain nuclear DNA.

leucoytes

White blood cells; they contain nuclear DNA and are the source of DNA in blood.

plasma

The liquid component of blood, without cells; contains proteins, nutrients, and extracellular nucleic acids.

extracellular nucleic acids

Fragments of DNA/RNA found outside of cells in body fluids; increasingly studied for forensic value.

extracellular vesicles (exosomes and microvesicles)

Membrane-bound vesicles (e.g., exosomes, microvesicles) that carry nucleic acids and proteins; a novel source of forensic information.

apoptotic bodies

Cell fragments formed during programmed cell death; may contain degraded DNA.

microRNAs

Small RNA molecules that regulate gene expression; stable in fluids and useful in body fluid ID.

touch evidence

DNA left behind when a person touches an object or surface (a form of transfer DNA).

transfer DNA

Genetic material that moves from one object or person to another through contact (direct or indirect).

medulla, cortex, cuticle

The three main layers of a hair shaft.

Medulla: Central core

Cortex: Middle layer with pigment

Cuticle: Outer protective layer

hair follicle

Living part of the hair in the skin; rich in nuclear DNA.

anagen, catagen, telogen

Anagen: Active growth phase (contains most DNA)

Catagen: Transitional phase

Telogen: Resting/shedding phase

osteocytes

Mature bone cells; useful in skeletal remains for DNA extraction.

dental pulp and cementoblasts

Soft tissue inside teeth, rich in DNA

Cells on tooth root surfaces that may also hold DNA

immunogen

A substance capable of provoking an immune response.

antigen

A molecule on a pathogen or foreign body recognized by the immune system.

antibodies (immunoglobulin)

Y-shaped proteins made by B cells that bind specific antigens.

isotypes

Different classes of immunoglobulins (e.g., IgG, IgA).

polyclonal antibody

A mixture of antibodies from different B cells recognizing various epitopes.

monoclonal antibody

Identical antibodies from a single clone of B cells; highly specific.

avidity

The overall strength of binding between an antigen and its antibodies.

affinity

The strength of a single antigen-antibody binding site.

epitope

The specific part of the antigen that is recognized by an antibody.

agglutination

Clumping of particles (e.g., cells) due to antigen-antibody interaction.

precipitation

Formation of insoluble complexes when soluble antigens and antibodies bind.

class vs individual characteristics

Shared by a group (e.g., blood type) vs.

Unique to one source (e.g., DNA profile)

presumptive and confirmatory assays

Indicates possible presence (e.g., Kastle-Meyer) and Proves the substance’s identity (e.g., sperm via microscopy)

ELISA

Enzyme-Linked Immunosorbent Assay; detects specific proteins or antigens.

lateral flow immunoassay

Rapid test (like a pregnancy test); used for PSA or saliva ID.

antibody titration

Measures antibody concentration by serial dilution.

hemoglobin

Oxygen-carrying protein in red blood cells; target of many blood tests.

oxidation-reduction

Electron transfer reactions; basis for some forensic color tests.

colorimetric assays

Tests where color change indicates presence of a substance.

chemiluminescence assay

Detects substances by light emission during chemical reaction.

fluorescence assay

Uses fluorescent tags to identify specific targets.

false positive

Test indicates presence when the substance is not present.

false negative

Test fails to detect a substance that is actually present.

seminal fluid

Fluid containing sperm and various proteins.

acid phosphatase

Enzyme used in presumptive semen tests.

prostate specific antigen

Semen protein detected in immunoassays.

oligospermia

Low sperm count

azoospermia

No sperm present in ejaculate

semenogelin I and II

Proteins in semen; confirmatory markers.

Christmas tree stain

Microscopic sperm stain that colors the head red and tail green

immunochromatographic assays

Antigen-based lateral flow tests for bodily fluids

salivary amylase

Enzyme in saliva; targeted in presumptive saliva tests.

glycogenated and parabasal cells

Vaginal epithelial cells used in source determination.

D-dimer

A blood clot breakdown product; studied in trauma or blood ID.

STR - Y-STR, X-STR

Short tandem repeats on Y or X chromosome; used for male lineage or kinship analysis.

SNP

Single Nucleotide Polymorphism; a variation in one nucleotide; useful in identity and ancestry testing.

core repeat region

The repeated sequence in an STR locus.

flanking regions

DNA surrounding the STR; used for primer binding in PCR.

repeat unit length

Number of base pairs in a single repeat.

repeat unit sequence

The actual DNA sequence repeated at an STR locus.

simple repeat

One repeat motif repeated.

compound repeat

Two or more adjacent repeat motifs.

complex repeat

Repeat with internal variations or interruptions.

microvariants

STR alleles that differ by partial base pairs.

population match probability

Likelihood that a random person matches a DNA profile.

electropherogram

A graph showing DNA fragment sizes and quantities.

genotype

An individual's combination of alleles at a given locus.

inclusion

DNA profile matches a known source.

exclusion

DNA profile does not match a known source.

inconclusive result

Result can't be interpreted definitively.

ampliflication artifacts (stuttering, heterozygote imbalance, allelic dropout)

Errors during PCR, like stutter or dropout.

electrophoretic artifacts (pull up peaks, spikes, dye blobs, air bubbles)

Signal issues during capillary electrophoresis (e.g., spikes, dye blobs).

degraded DNA

Fragmented DNA from environmental exposure.

low copy number (LCN)

Very small quantity of DNA; requires sensitive processing.

mixtures

DNA from more than one contributor.

off-ladder allele

Allele not represented in the standard size ladder.

internal size standard

Known-size fragments used to calibrate electrophoresis.

allelic ladder

A mix of common alleles used for sizing comparison in STR analysis.

haplotype

A group of genes inherited together from one parent, often used in Y-STR analysis.

amelogenin

Gene used to determine biological sex (X and Y chromosome versions differ).

rapidly mutating Y-STRs

STRs on Y-chromosome that change more often; useful in distinguishing closely related males.

homoplasmy

Mitochondrial DNA with only one variant.

heteroplasmy

Mitochondrial DNA with a mixture of variants.

single nucleotide polymorphisms

Single base pair changes in DNA; can be used when STRs are too degraded.

restriction fragment length polymorphisms

Older method using enzyme cuts; replaced by PCR-based methods.

variable number tandem repeats

Longer repeats used in early DNA typing.

next generation sequencing

High-throughput DNA sequencing used to analyze many regions at once.

random match probability

The probability that a random person has the same DNA profile as the evidence.

likelihood ratio

A statistical comparison of the probability of evidence under two competing hypotheses (e.g., suspect vs. unknown source).

qualifying questions

Questions used to establish a witness’s expertise.

direct examination

The initial questioning of a witness by the party that called them.