forensic DNA Flashcards
serology/DNA unit
analyzes evidence for presence of biological material in order to obtain DNA profiles
testing process involves screening and DNA analysis
screening process allows the analyst to identify biological fluids on evidence through chemical, microscopic, instrumental analysis
if DNA profile obtained , it can be compared to a reference sample or uploaded and searched against a database of offenders
detection and identification of biological fluids from crime scene evidence
whose DNA is it anyway?
case and evidence type
burglary - gloves clothing tools food blood at point of entry
sexual assault - sexual assault evidence kits (SAEKs) clothing bedding
homicide/Assault/robbery - blood at the scene, weapons clothing coroner’s evidence miscellaneous items from the scene
missing persons - unidentified remains DNA from family members or objects
paternity - reference samples from mother, child, alleged father
serology screening
visual exam for staining/body fluids - ALS
presumptive tests - phenolphthalein, AP, p30, amylase
confirmatory test - microscopy
sampling for DNA analysis - cutting to positive stains or swabs, swabbing for ‘touch’ or ‘wearer’s’ DNA
DNA analysis
find biological material on the evidence? then generate a DNA profile via several steps
extraction - remove DNA from cells
quantification - determine amount of DNA present
amplification - produce millions of copies of the DNA
detection - determine a DNA profile
DNA profile obtained?
interpretation - make sense of the results
reporting - put the results in writing
How are stains located?
alternative light source (ALS) helps visualize stains that may not be observed with natural light sources
biological fluids fluoresce under certain wavelengths of light
how are stains identified?
presumptive chemical test for blood-phenolphthalein or Kastle-Meyer
establishes possibility of blood
identification vis DNA testing
Bluestar for latent blood/clean-up blood
acid phosphatase (AP) test for semen
establishes possibility of semen
positive result will need microscopic confirmation
prostate specific antigen (PSA or p30) test for samples lacking spermatozoa
identification via DNA testing
DNA
deoxyribonucleic acid
Watson Crick Franklin X-ray crystallography studies in 1953
double helical structure
a chain of nucleotides consisting of deoxyribose sugar, phosphate group, one of four nitrogenous bases (adenine, thymine, guanine, cytosine)
located in cells with nuclei
Rosalind Franklin
obtained images of DNA using X-ray crystallography an idea first broached by Maurice Wilkins
images allowed Watson and Crick to create their famous, two-strand or double helix model
Watson crick and Wilkins received Nobel price for determining structure
Franklin was not honored, died from cancer at 37
DNA organization
we inherit 23 pairs of homologous chromosomes
target region for typing - double stranded DNA molecule - chromosome - cell nucleus
what type of evidence does the serology DNA unit analyze
evidence that may contain DNA
clothing assault kits guns cigarette butts soda cans knives food etc.
best for DNA evidence
bloodstains saliva semen
medium for DNA evidence
clothing or wearers DNA
head hairs with root attached, near least for DNA
least best for DNA
touch or contact DNA
steps in DNA typing
screening and sample collection
extraction (2hrs-2days)
quantitation (3 hours)
amplification via PCR - (5 hours)
profile determination (5 hours to several days) - sample complications
comparison of resultant profile to other sample profile (one day)
quality control process. (technical and administrative review) - 5 days
if match occurs calculate probability of random match using FBI population database and generate a case report
if no match enter profile into FBI CODIS to identify possible suspect provided the sample meets certain criteria
quantitation
primary purpose to determine art of DNA and what tests can be done
quality: need to determine that DNA recovered is human rather than from bacteria
quantity: need to determine the amount fo DNA in a sample. too much or too little DNA can cause problems with tests
real time PCR
amplification
per solution: salt, primers, Taq polymerase, bases
preparing samples for amp (30-60 min)
set up reaction (30-60 min)
instrument run time (3.5 hours)
cycles through denaturation, annealing, and extension approx 28 times
creates copies of specific DNA target regions
STR amplification process incorporates chemical labels ( fluorescent dyes) for subsequent detection of alleles
amounts needed for DNA
1 hair root
250 skin cells
500 sperm cells
bloodstain size of pinhead
short tandem repeats (STRs)
homozygote - both alleles are the same length
heterozygote - alleles differ and can be resolved from one another
DNA separation / detection
separates by repeat size
characterizes the STR repeat regions
generates peaks on a graph
expected results
reference samples, complete single source profiles (blood or saliva- buccal cells)
evidence samples can be a single source or a mixture of 2 or more people (indicated by 3 or more alleles at more than one locus or uneven peak height ratios)
evidence samples can also be low level samples
problems in DNA interpretation
environmental factors and storage conditions can cause degraded samples - may need to submit a larger sample
inhibitors
soil denim leather blood tissues - sample select a ‘clean’ area
low DNA recovery
small stains or few sperm or nucleated epithelial cells
get permission to consume entire sample
‘touch DNA samples’
concentrate as much potential DNA as possible into one swab
mixtures
1:1 mixtures are almost impossible to interpret without a reference sample
sample selection: do not combine stains, submit multiple distinct stains/swabs
contamination
from environment to sample, analyst to sample. or between samples (cross-contamination)
wear PPE, change gloves often, clean often
CODIS
combined DNA index system
2.8 mil profiles in california
14.5 mil profiles nationwide
what kinds of DNA profiles are entered into CODIS?
known offenders - convicted felon samples all known felon arrestees (1/1/09)
forensic unknowns collected at crime scenes
missing persons
unidentified remains, relatives of the missing, personal references form missing persons
DNA profiles consisting ~30 numbers representing an STR DNA profile
does not tell anything about the race or appearance
profiles and samples are confidential
a felony to use or disclose for any other purpose
case to offender hit
case to case hit
CODIS indices are searched and compared weekly
Y-STR typing
male-specific part of human Y chromosome is widely used in forensic DNA analysis particularly in cases where standard autosomal DNA profiling is not informative
during traditional STR testing male DNA may be masked or in competition with excess amounts of female DNA
can result in partial ro no male STR DNA results
Y-STR explicitly targets STR regions in the male Y chromosome that is passed down through the paternal lineage
Y-chromosome gene fragment infers the biological sex of a crime scene sample donor
haplotypes composed of Y-chromosomal STRs are used to characterize peternal lineages of unknown male profile donors, especially suitable when males and females have contributed to the same evidence sample, such as SA
benefits of Y-STR
target male-only DNA in mixed samples
determine number of male donors in a mixed sample
remove male-male mixtures
provide clarity for inconclusive STR results
aid in power of exclusion
detect male DNA from cases involving azoospermic or vasectomize males, saliva following showering, digital penetration, no ejaculation, aged or improperly stored SA kits where sperm cells might be degraded
extended time intervals between incident and collection
familial searching
growing interest in the use of familial DNA searching (FDS) to aid criminal investigations
FDS is extension of traditional matching of DNA profiles whereby, instead of searching for exact matches between unique, non-coding STR patters at the specified CODIS lock, specialized software identifies similar but not exact matches at the same loci that may be indicative of a family relationship
components of FDS
- software comparison of DNA profile from unknown with known profiles from a DNA database
software uses genetic algorithms to identify patterns in similarity that are likely to occur within close family relationships - lineage testing to further support relatedness
further supports or refutes biological related ness between the unknown evidence sample and candidate samples identified through the database
reduce the presence of false positives from a list of partial matches
primarily involves Y-STR testing
only helpful for conforming relatedness between two males
other lineage test for female family members such as mtDNA
