4 Flashcards
What is the forensic biology workflow? What happens at each step of the
workflow?
Visual/ALS Examination-find and circle biological material on items of evidence
Serology-determine what the biological material is and if it is human ; is it important to the case
DNA Analysis-identify the biological material to an individual
Define forensic serology
The science of body fluid identification and characterization (typically blood, semen, saliva, and urine)
What are the goals of forensic serology?
1) Identify the presence of a body fluid. What type of
body fluid is it?
2) To answer: Is the body fluid human – or what species
is it from?
3) Help identify what biological evidence is important to
a case and what should be sent to DNA analysis.
What is a presumptive test? Why do we use presumptive tests? What issues do they have?
chemical tests using a color change reaction to indicate the possible presence of a particular biological material
They are fast and inexpensive
Susceptible to false positives (substance that gives a positive test
result even though it is not what is being tested for)
What is a confirmatory test?
Identifies or confirms a biological fluid. Confirms it is that biological material.
Do we use presumptive or confirmatory tests first? Why?
Presumptive tests first- because they are fast and inexpensive and indicate if we should do further testing
What are the two components of blood?
Cells and plasma
Where is the DNA in blood?
White blood cells
Why can we not get DNA from red blood cells?
There is no nucleus in a red blood cell
Describe the phenolphthalein test. Is it a presumptive or confirmatory test? Is it susceptible to false positives? Is it human specific?
Phenolphthalein is a presumptive test for blood that indicates the “possible presence” of blood with a pink color change that happens within 30 seconds.
Presumptive=false positive; not human specific
Describe the Hemastix test. Is it a presumptive or confirmatory test? Is it
susceptible to false positives? Is it human specific?
Hemastix test is a presumptive test for blood that indicates the “possible presence” of blood with a blue/green color change within 39 secs. Does have false positives and is not human specific.
As the sensitivity of a presumptive test increases its susceptibility to false positives ________________.
Increases
What is luminol used for?
Look for trace amounts of blood at crime scenes. It causes the blood to fluoresce.
What is a false positive?
Substance that gives a positive test result even though it is not what it’s being tested for
What are some examples of false positives for presumptive blood tests?
Fruits, vegetables, household chemicals, rust, metallic objects, sand, wasabi
What is the RSID blood test? Is it a presumptive or confirmatory test? How does it work? Is it human specific? Why is it not used as often as some other confirmatory methods?
RSID blood test is a “pregnancy test” for blood. Antibody-antigen interactions to identify the presence of blood. It is human specific. Tells you it is blood and it is human-confirmatory. We do not use as often because it is expensive.
What is the Takayama test? Is it a presumptive or confirmatory test? Is it human specific?
Takayama test is confirmatory test for blood that confirms (or identifies) the presence of blood. Results in red feathery crystals that are viewed under the microscope. Is not human specific.
What does the Oucterlony test tell us?
Tells us that blood is human.(pair with takayama)
Which is more discriminating: ABO blood typing or DNA analysis?
DNA analysis
What are the two components of semen?
Spermatozoa and seminal fluid
What are spermatozoa?
Sperm cells
What are the two components of seminal fluid that are tested for in forensic serology?
Acid phosphate and PSA
Where is DNA in semen?
Spermatozoa (sperm cells)
Acid Phosphatase is ________________ times more concentrated in semen than in other body fluids
400X
What is the AP Test? Is it a presumptive or confirmatory test? What does a
positive result look like?
Acid phosphate test. It is a presumptive test for semen. Positive result is a purple color within 30 secs. This is possible semen.
What are some examples of false positives for the presumptive test for semen?
Banana, yeast, bread, mushrooms
What is the p30 or PSA test? Is it a presumptive or confirmatory test? How does it work? What does a positive result look like?
P30/PSA (prostate specific antigen) is a presumptive test for seminal fluid. It works using an antibody -antigen interaction- two pink lines indicate a positive result.
What is the confirmatory test for sperm cells? Describe the test. What does a positive result look like?
Christmas tree staining test. Stains the head red and the tail green. Observe sperm cells microscopically.
Describe the presumptive test for saliva.
Agar/starch gel. Gel is made of agar and starch. Holes are made in the gel. Samples are placed in the holes (wells). Gels are allowed to sit for about 6 hrs. If sample contains amylase (a component of saliva) it will break down the starch and turn into sugar. The gels are dyed with iodine after 6 hrs. Iodine stains starch. If the starch has been broken down, it will not stain and a clear ring will be observed.
Is there a confirmatory test for saliva?
No
Where is DNA in saliva?
Epithelial cells (cheek skin cells)
Do cases ever use plant or animal DNA? Examples?
Yes, they can carry DNA.
What is a positive control? A negative control?
Positive control-is a known sample used to make sure that reagents and equipment are working properly.
Negative control-is used to check for contamination
In forensic biology, what are some steps to prevent contamination?
Wearing PPE; sterilizing equipment and tools; cleaning your workspace with bleach and water; using negative control
If you have limited sample is it more important to conduct serology testing or DNA analysis? Why?
DNA analysis
What types of biological fluids are commonly examined in a forensic biology lab?
Blood, semen, saliva, urine
What type of DNA is unique to individual? What is the only exception?
Nuclear DNA is unique to an individual. Identical twins have the same DNA.
Where in the cell is nuclear DNA found?
Nucleus
Describe the structure of nuclear DNA
Double helix made of a sugar-phosphate backbone and nucleotides in the middle
Why do we use DNA as evidence?
Unique to an individual (can identify a person to the exclusion of all others); validated and widely used
Describe the 4 steps in DNA analysis. What happens during each step?
Extraction- nuclear DNA is extracted from the cell and purified
Quantification-the quality and quantity of the nuclear DNA
Amplification step-PCR is used to make a bunch of copies of the DNA
Genetic Analyzer-DNA profile is generated by the genetic analyzer
What does PCR stand for? What is PCR? How does the PCR process work?
PCR=polymerase chain reaction; amplification step- where millions of copies of the DNA are made
Why was PCR a breakthrough in forensic DNA analysis?
It allows scientists to rapidly amplify tiny amounts of DNA found at a crime scene
Nuclear DNA involves STR Analysis. What are STRs?
Short tandem repeats- determining an individual’s DNA profile by counting the number of times a small DNA sequence is repeated at a specific chromosomal location
What is the database used for DNA?
CODIS-combined DNA index system
How many loci are used in a CODIS search?
20
How is nuclear DNA inherited?
1 copy from mom and 1 copy from dad
If dad is a 14,18 at loci vWA and mom is a 15,17 what are the possible
combinations the baby could have?
15, 18-15,14-17,18-17,14
Be able to analyze an electropherogram (DNA profile) and determine if it belongs to a male or female.
One peak-female, two peaks-male
What process is used to separate sperm cells and epithelial cells in a sexual assault case?
Differential extraction
What are 2 issues that complicate DNA analysis and the interpretation of
electropherograms
Mixtures and degraded DNA
What process is used to analyze mitochondrial DNA?
Sequencing
Where in the cell is mtDNA found?
Mitochondria
Why is mtDNA important in forensic science?
-more than 1000 copies per cell (easier to recover;even in degraded samples)
-single, circular unit(~16000 base pairs) (versus nuclear which is a double helix and 3.2 billions base pairs )
-inherited from mother
-not unique to an individual
How is mtDNA inherited?
Maternally inherited
What types of biological samples are typically used for mtDNA analysis?
Hair, teeth, bones
Hair examination generally involves microscopic examination and mtDNA
analysis. Which is done first and why?
Microscopic examination- inclusions and exclusions; faster than mtDNA
What is the 11% factor?
FBI went and re-examined all of the hair analysis cases that results in a conviction just based on microscopy; re- examined using mtDNA analysis and found that 11% of the hair analysis cases results in wrongful conviction.
Often nuclear DNA cannot be recovered from degraded samples but mtDNA can. Why?
There is only 1 copy of nuclear DNA; hundreds of copies of mtDNA
How are hair samples prepared for mtDNA extraction?
The hair is physically ground using a micro tissue grinder
How are bone samples prepared for mtDNA extraction?
Bone is cleaned and a small piece is removed for testing. The piece is then placed into the freezer mill. The freezer mill pulverizes the bone into fine powder.
What regions of mtDNA are amplified for analysis? Why?
Hyper variable regions (HV1 and HV2)-these regions have the most variability between individuals (who not maternally related)
Describe the 4 steps in mtDNA analysis
Extraction-remove the mtDNA from the cell and purify the mtDNA
Amplification-PCR to make copies of the mtDNA
Sequencing-genetic analyzer (this step is different from nuclear DNA)
Sequence comparisons
Be able to compare a known mtDNA sequence (from a suspect) and a mtDNA sequence from an unknown sample (evidence) and determine if the suspect can be included or excluded as being the source of the unknown sample.
Excluded
If an unknown sample and a known sample have the same mtDNA sequence –what does this tell you?
This indicates that the DNA is from that individual or someone in their material lineage
Which is more discriminating nuclear DNA or mtDNA? Why?
Nuclear DNA-because you can identify a person with nuclear DNA
What are the advantages to mtDNA analysis?
1000s of copies (can recover from degraded samples); useful on hair and bones which can be difficult to recover nuclear DNA; useful in missing persons; maternally inherited; combined with hair comparisons
YOU CANNOT DETERMINE GENDER FROM mtDNA
What are some types of cases where the identity of a relative might be important to a case?
Missing persons; paternity test
Familial DNA associations work best for which types of relationships?
Parent-child; siblings
What is an obligate allele?
Required allele(often in reference to parent-child relationships)
How does partial match or familial DNA assist in an investigation?
Provides investigative leads. Provides partial matches to individuals already in the DNA database-indicating that the perpetrator is likely a family member.
Are familial DNA searches permitted in CODIS at the federal level? State level?
No;maybe
Do familial DNA searches use STRs or sequences? Is this associated with nuclear DNA or mtDNA analysis?
STRs-nuclear DNA
