Forensic Science

What was forensic biology originally called?

Forensic serology.

What does forensic biology examine?

Biological evidence such as blood, semen, saliva, urine, and feces.

What is a presumptive test?

A screening test that indicates a substance may be present; it is not definitive.

Which presumptive blood tests are listed?

Phenolphthalein, leuco-malachite green, and luminol.

What does luminol detect?

Possible occult (hidden or cleaned-up) blood through light emission.

Does a positive luminol reaction prove blood?

No; luminol cross-reacts with other compounds.

What does a ring precipitin test determine?

Species origin of blood using species-specific antiserum.

What is the acid phosphatase (ACP) test for?

Chemical screening of suspected semen stains.

What color is the positive ACP reaction?

Purple azo dye.

What two tests are used to confirm semen in the slides?

Microscopic sperm search and prostate-specific antigen (PSA) testing.

What does PSA stand for?

Prostate-specific antigen; a protein from the prostate gland.

What is used to identify saliva?

High concentration of amylase; a human salivary alpha-amylase antibody assay.

What does Edelman’s test detect?

Urobilinogen in fecal material.

What does RSID for Urine detect?

Tamm-Horsfall protein.

Which older urine markers are listed?

Urea and creatinine.

What is the purpose of chain of custody?

Document collection, handling, transfer, and integrity of evidence.

How should clothing with biological stains be packaged?

Individually in paper bags; air-dry wet stains before final packaging.

What does SANE stand for?

Sexual Assault Nurse Examiner.

What are the blood components named in the slides?

Red blood cells, white blood cells, platelets, and plasma.

What is a locus?

Position of a gene or DNA marker on a chromosome.

What is an allele?

An alternative form of a gene or marker at a locus.

Homozygous vs. heterozygous?

Homozygous = identical alleles; heterozygous = different alleles at a locus.

Genotype vs. phenotype?

Genotype = genetic makeup; phenotype = physical result of genotype.

What is a polymorphism?

Presence of more than one allele at a locus.

What does RFLP stand for?

Restriction Fragment Length Polymorphism; first-generation forensic DNA method.

What does PCR stand for?

Polymerase Chain Reaction; amplifies selected DNA regions.

What are the three PCR steps in order?

Denaturation → annealing → extension.

What occurs during denaturation?

Double-stranded DNA separates.

What occurs during annealing?

Primers pair with complementary single-stranded DNA.

What occurs during extension?

Taq polymerase adds dNTPs to extend primers.

What is a thermal cycler used for?

Repeated temperature cycling for PCR amplification.

What is DNA extraction?

Isolation of DNA from the biological sample.

What is DNA quantitation?

Determination of the amount of DNA available for amplification.

What does RT-qPCR measure in these slides?

Amount of amplifiable DNA.

What are the six PCR-method workflow steps?

Extraction → quantitation → amplification → electrophoresis → recording → interpretation.

What does organic extraction use?

Phenol/chloroform; produces high-molecular-weight DNA.

What does Chelex extraction use?

An ionic resin that binds magnesium; produces single-stranded low-molecular-weight DNA.

How does magnetic-particle extraction work?

DNA binds magnetic particles, is washed, and then eluted; can be automated.

What is differential extraction used for?

Separating sperm from non-sperm cellular components.

What does STR stand for?

Short Tandem Repeat.

Why are STRs useful for identification?

Repeat counts vary between individuals; short fragments work with small or degraded samples.

What is a tetranucleotide repeat?

An STR repeat unit of four DNA bases.

What is multiplex PCR?

Simultaneous amplification of multiple STR loci in one reaction.

How are amplified STR fragments separated?

Capillary electrophoresis.

What does a genetic analyzer use to detect STR fragments?

Fluorescent dyes with laser-based detection.

What is an STR stutter peak?

An extra peak, usually one repeat shorter than the true allele, caused by strand slippage.

What is a null allele?

An allele present in the sample but not amplified, often due to a primer-binding-site change.

What is a microvariant STR allele?

An allele not an exact multiple of the basic repeat motif or with sequence variation.

What is non-template addition?

Taq polymerase adds an extra nucleotide, usually A, to a PCR product.

What does CODIS stand for?

Combined DNA Index System.

What is the purpose of CODIS?

Develop investigative leads using computerized DNA profiles.

What are LDIS, SDIS, and NDIS?

Local, State, and National DNA Index Systems.

What is the CODIS forensic index?

Evidence-derived profiles used for case-to-case hits.

What is the CODIS convicted offender index?

Convicted-offender profiles used for case-to-offender hits.

What are three possible DNA comparison outcomes?

Match, exclusion, and inconclusive.

What is the product rule in forensic DNA statistics?

Multiply genotype frequencies at loci to estimate a combined profile frequency.

What is probabilistic genotyping?

Biological and mathematical modeling of complex DNA mixtures.

What does a likelihood ratio compare?

How well the DNA evidence is explained by two competing hypotheses.

How is mitochondrial DNA inherited?

Through the maternal line.

When is mitochondrial DNA useful?

When nuclear DNA is not feasible, including hairs, bones, and decomposed remains.

How are Y-STRs inherited?

Through the paternal line.

Why are Y-STRs useful?

They can help isolate a male DNA component in a mixture.

What is a simple microscope?

A magnifying glass.

What are the two main lenses of a compound microscope?

Objective and ocular (eyepiece) lenses.

What does parfocal mean?

A specimen remains approximately in focus when changing objectives.

What does the illuminator do?

Provides light.

What does the condenser do?

Focuses light.

What is field of view?

Area visible through the eyepiece.

What is depth of focus?

Thickness that can be clearly seen.

How is total magnification calculated?

Ocular magnification × objective magnification.

How does a compound microscope orient the image?

Inverted.

What is a comparison microscope used for?

Viewing two objects side by side using linked compound microscopes.

What view does a stereomicroscope provide?

Three-dimensional and right-side-up.

What does a microspectrophotometer measure?

Color spectrum of microscopic objects, often fibers and paint chips.

What does FTIR help determine?

Chemical composition of fibers, plastics, paint chips, and drugs.