DNA Evidence

Question 1

Can DNA collected at a scene provide absolute proof of guilt?

Answer 1

no

Question 2

What are the 3 categories of presumptive tests for blood?

Answer 2

1. colour test
2. chemiluminescence
3. precipitin antibody

Question 3

What is the Kastle-Meyer test?

Answer 3

• phenolphthalein reacts with hemoglobin in the presence of hydrogen peroxide to oxidize
• turns bright pink
• not human specific

Question 4

What are Hemastix?

Answer 4

• 3” plastic strips with a hemoglobin detecting reagent
• tip turns green
• not human specific

Question 5

What are luminol and bluestar?

Answer 5

• Chemicals that react with oxidizing agents (heme) that will luminesce
• helpful in finding minute stains
• not blood specific, may react with other oxidizing agents (metals)

Question 6

What is a precipitin test?

Answer 6

• serological test to detect the presence of a specific antigen
• human blood injected into a rabbit, which is extracted (serum) and mixed with human blood
• antigen and antibodies mix forming visible precipitates (A, B, O)
• can be human specific, less likely to give false positives

Question 7

What is hematrace?

Answer 7

• hemoglobin antigens react with monoclonal antihuman antibodies marked with a pink dye
• if a complex forms, a pink T and C line will indicate a positive result

Question 8

What is an Acid Phosphatase Test?

Answer 8

• presumptive test for semen
• created in large amounts by the prostate
• detected in the presence of monophenolic phosphoric acid
• detects dilutions of up to 1/1000
• purple colour

Question 9

What is the Prostate Specific Antigen (P30) test?

Answer 9

• almost exclusive to semen
• can be detected in cases of vasectomies and with minute amounts
• semen most conclusively IDed by microscopic analysis of spermatozoa

Question 10

What is the presumptive test for saliva?

Answer 10

• alpha-amylase detected with a phadebas test
• a blue dye will be visible with a positive test

Question 11

What are the steps in the analysis of DNA evidence?

Answer 11

• collection
• storage
• extraction
• quantitation
• genotyping
• interpretation of results
• database search

Question 12

What are the characteristics of DNA?

Answer 12

• sugar backbone, phosphate group and 4 bases paired by H-Bonds

Question 13

What is a specific region of DNA?

Answer 13

• A locus
• only 5% of a locus is coding (exons)

Question 14

What are the CODIS loci?

Answer 14

• only look at 13, but trying to expand to 20
• use SNP, VNTR and STR sequences

Question 15

what does autosomal mean?

Answer 15

• a non-sex chromosome

Question 16

What is an allele?

Answer 16

• a genetic variation of a locus

Question 17

what does ploidy refer to?

Answer 17

whether a cells is diploid or haploid, (or more)

Question 18

How is variation observed in DNA?

Answer 18

• SNP - one base pair difference in a locus
• Repetitive sequence regions (VNTR, STR-short sequences with large abundance, microsatellites) where alleles are based on the size of the repeat, ie 4,5

Question 19

When was DNA fingerprinting discovered and by whom?

Answer 19

• 1984
• Sir Alec Jeffery’s

Question 20

What is RMP?

Answer 20

• Random Match Probability
• the probability that two individuals have the same DNA profile by chance
• uses data of frequencies of known alelles in the population

Question 21

Describe the PCR and DNA amplification process

Answer 21

• DNA (STR) and chemicals are placed in a thermocycler
• starting template is denatured into two strands at 95 C
• primers are added through annealing, 13 for each loci at 45-55 C
• copies are made as primers are extended at 72 C
• amplified DNA placed in gel electrophoresis plates for analysis

Question 22

Amplification of sex determining markers

Answer 22

• Amelogenin locus, codes for tooth enamel
• in Males there are two different sizes as X gene has a 6 bp deletion

Question 23

Probability of a genotype

Answer 23

• heterozygous - 2pq = 2 x alelle 1 x allele 2
• homozygous - q^2 = allele 2^2
• At multiple loci (ie A1/A2, B1/B2, C1/C2)
  = p(A1/A2) x p(B1/B2) x p(C1/C2)
• a probability is required to be assigned to every profile generated

Question 24

What are the 2 principle indices of DNA databanks

Answer 24

• Convicted offender index (coi) - DNA collected from offenders of determined primary and secondary offences
• Crime Scene Index (CSI) - Unknown profiles obtained from scenes
• often cross-checked and searched a local and federal levels

Question 25

What are the main uses of DNA profiling?

Answer 25

- Parentage/kinship - disasters (Swiss Air 111) - Y-STR testing (ID of Saddam Hussein, multiple male profiles) - mtDNA testing (Laci Peterson) - shift to develop phenotypic profiles from the genotype

Question 26

Who discovered that blood can be classified by type?

Answer 26

Karl Landsteiner

Question 27

What is the liquid that separates from the blood when a clot (precipitate) is formed?

Answer 27

Serum

Question 28

What are antigens and antibodies?

Answer 28

Antigens: chemicals on the surface of red blood cells that impart blood type characteristics - type O has neither A or B antigen, AB has both Antibodies: react/agglutinates to its specific antigen - type B has anti-A, type AB has neither anti-A or anti-B

Question 29

What is oligospermia?

Answer 29

low sperm count

Question 30

How long might semen remain in the vagina after intercourse?

Answer 30

6 days

Question 31

true or false: the longer the strand of DNA the less susceptible it is to degradation

Answer 31

false

Question 32

How many DNA bearing cells are needed to generate an STR profile?

Answer 32

18

Question 33

What is the best type of package for blood evidence?

Answer 33

non-airtight, usually cardboard cartons or paper bags (for clothes) blood is best submitted air-dried