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Forensics > 4 > Flashcards

4 Flashcards

1
Q

What is the forensic biology workflow? What happens at each step of the
workflow?

A

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

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2
Q

Define forensic serology

A

The science of body fluid identification and characterization (typically blood, semen, saliva, and urine)

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3
Q

What are the goals of forensic serology?

A

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.

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4
Q

What is a presumptive test? Why do we use presumptive tests? What issues do they have?

A

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)

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5
Q

What is a confirmatory test?

A

Identifies or confirms a biological fluid. Confirms it is that biological material.

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6
Q

Do we use presumptive or confirmatory tests first? Why?

A

Presumptive tests first- because they are fast and inexpensive and indicate if we should do further testing

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7
Q

What are the two components of blood?

A

Cells and plasma

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8
Q

Where is the DNA in blood?

A

White blood cells

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9
Q

Why can we not get DNA from red blood cells?

A

There is no nucleus in a red blood cell

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10
Q

Describe the phenolphthalein test. Is it a presumptive or confirmatory test? Is it susceptible to false positives? Is it human specific?

A

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

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11
Q

Describe the Hemastix test. Is it a presumptive or confirmatory test? Is it
susceptible to false positives? Is it human specific?

A

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.

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12
Q

As the sensitivity of a presumptive test increases its susceptibility to false positives ________________.

A

Increases

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13
Q

What is luminol used for?

A

Look for trace amounts of blood at crime scenes. It causes the blood to fluoresce.

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14
Q

What is a false positive?

A

Substance that gives a positive test result even though it is not what it’s being tested for

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15
Q

What are some examples of false positives for presumptive blood tests?

A

Fruits, vegetables, household chemicals, rust, metallic objects, sand, wasabi

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16
Q

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?

A

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.

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17
Q

What is the Takayama test? Is it a presumptive or confirmatory test? Is it human specific?

A

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.

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18
Q

What does the Oucterlony test tell us?

A

Tells us that blood is human.(pair with takayama)

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19
Q

Which is more discriminating: ABO blood typing or DNA analysis?

A

DNA analysis

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20
Q

What are the two components of semen?

A

Spermatozoa and seminal fluid

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21
Q

What are spermatozoa?

A

Sperm cells

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22
Q

What are the two components of seminal fluid that are tested for in forensic serology?

A

Acid phosphate and PSA

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23
Q

Where is DNA in semen?

A

Spermatozoa (sperm cells)

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24
Q

Acid Phosphatase is ________________ times more concentrated in semen than in other body fluids

A

400X

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25
Q

What is the AP Test? Is it a presumptive or confirmatory test? What does a
positive result look like?

A

Acid phosphate test. It is a presumptive test for semen. Positive result is a purple color within 30 secs. This is possible semen.

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26
Q

What are some examples of false positives for the presumptive test for semen?

A

Banana, yeast, bread, mushrooms

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27
Q

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?

A

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.

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28
Q

What is the confirmatory test for sperm cells? Describe the test. What does a positive result look like?

A

Christmas tree staining test. Stains the head red and the tail green. Observe sperm cells microscopically.

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29
Q

Describe the presumptive test for saliva.

A

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.

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30
Q

Is there a confirmatory test for saliva?

A

No

31
Q

Where is DNA in saliva?

A

Epithelial cells (cheek skin cells)

32
Q

Do cases ever use plant or animal DNA? Examples?

A

Yes, they can carry DNA.

33
Q

What is a positive control? A negative control?

A

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

34
Q

In forensic biology, what are some steps to prevent contamination?

A

Wearing PPE; sterilizing equipment and tools; cleaning your workspace with bleach and water; using negative control

35
Q

If you have limited sample is it more important to conduct serology testing or DNA analysis? Why?

A

DNA analysis

36
Q

What types of biological fluids are commonly examined in a forensic biology lab?

A

Blood, semen, saliva, urine

37
Q

What type of DNA is unique to individual? What is the only exception?

A

Nuclear DNA is unique to an individual. Identical twins have the same DNA.

38
Q

Where in the cell is nuclear DNA found?

A

Nucleus

39
Q

Describe the structure of nuclear DNA

A

Double helix made of a sugar-phosphate backbone and nucleotides in the middle

40
Q

Why do we use DNA as evidence?

A

Unique to an individual (can identify a person to the exclusion of all others); validated and widely used

41
Q

Describe the 4 steps in DNA analysis. What happens during each step?

A

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

42
Q

What does PCR stand for? What is PCR? How does the PCR process work?

A

PCR=polymerase chain reaction; amplification step- where millions of copies of the DNA are made

43
Q

Why was PCR a breakthrough in forensic DNA analysis?

A

It allows scientists to rapidly amplify tiny amounts of DNA found at a crime scene

44
Q

Nuclear DNA involves STR Analysis. What are STRs?

A

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

45
Q

What is the database used for DNA?

A

CODIS-combined DNA index system

46
Q

How many loci are used in a CODIS search?

A

20

47
Q

How is nuclear DNA inherited?

A

1 copy from mom and 1 copy from dad

48
Q

If dad is a 14,18 at loci vWA and mom is a 15,17 what are the possible
combinations the baby could have?

A

15, 18-15,14-17,18-17,14

49
Q

Be able to analyze an electropherogram (DNA profile) and determine if it belongs to a male or female.

A

One peak-female, two peaks-male

50
Q

What process is used to separate sperm cells and epithelial cells in a sexual assault case?

A

Differential extraction

51
Q

What are 2 issues that complicate DNA analysis and the interpretation of
electropherograms

A

Mixtures and degraded DNA

52
Q

What process is used to analyze mitochondrial DNA?

A

Sequencing

53
Q

Where in the cell is mtDNA found?

A

Mitochondria

54
Q

Why is mtDNA important in forensic science?

A

-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

55
Q

How is mtDNA inherited?

A

Maternally inherited

56
Q

What types of biological samples are typically used for mtDNA analysis?

A

Hair, teeth, bones

57
Q

Hair examination generally involves microscopic examination and mtDNA
analysis. Which is done first and why?

A

Microscopic examination- inclusions and exclusions; faster than mtDNA

58
Q

What is the 11% factor?

A

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.

59
Q

Often nuclear DNA cannot be recovered from degraded samples but mtDNA can. Why?

A

There is only 1 copy of nuclear DNA; hundreds of copies of mtDNA

60
Q

How are hair samples prepared for mtDNA extraction?

A

The hair is physically ground using a micro tissue grinder

61
Q

How are bone samples prepared for mtDNA extraction?

A

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.

62
Q

What regions of mtDNA are amplified for analysis? Why?

A

Hyper variable regions (HV1 and HV2)-these regions have the most variability between individuals (who not maternally related)

63
Q

Describe the 4 steps in mtDNA analysis

A

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

64
Q

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.

A

Excluded

65
Q

If an unknown sample and a known sample have the same mtDNA sequence –what does this tell you?

A

This indicates that the DNA is from that individual or someone in their material lineage

66
Q

Which is more discriminating nuclear DNA or mtDNA? Why?

A

Nuclear DNA-because you can identify a person with nuclear DNA

67
Q

What are the advantages to mtDNA analysis?

A

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

68
Q

What are some types of cases where the identity of a relative might be important to a case?

A

Missing persons; paternity test

69
Q

Familial DNA associations work best for which types of relationships?

A

Parent-child; siblings

70
Q

What is an obligate allele?

A

Required allele(often in reference to parent-child relationships)

71
Q

How does partial match or familial DNA assist in an investigation?

A

Provides investigative leads. Provides partial matches to individuals already in the DNA database-indicating that the perpetrator is likely a family member.

72
Q

Are familial DNA searches permitted in CODIS at the federal level? State level?

A

No;maybe

73
Q

Do familial DNA searches use STRs or sequences? Is this associated with nuclear DNA or mtDNA analysis?

A

STRs-nuclear DNA

Forensics (4 decks)

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