Download DNA Forensics - Plant Root Genomics Consortium Project

DNA Forensics
MUPGRET Workshop
“DNA evidence…offers prosecutors
important new tools for the identification
and apprehension of some of the most
violent perpetrators, particularly in cases
of sexual assault. At the same time, DNA
aids the search for truth by exonerating
the innocent. The criminal justice system
is not infallible.”
Janet Reno
Biological Basis
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The same DNA is found in virtually all
cells in our bodies.
The four letters of DNA ATCG spell out
instructions that give each individual
their unique appearance.
The arrangement of the letters is
different in each individual.
~1 letter per 1000 is different.
DNA forensics
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Makes use of the similarities and
differences in our DNA sequences to
determine whether two biological
samples come from the same individual
or not.
VNTR
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Variable number of tandem repeats.
Type of DNA marker that is unique to
each individual.
DNA of about 80 base pairs that is
repeated many times in a row.
Number of repeats differs between
individuals = different size DNA
fragments on a gel.
VNTR
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By comparing across multiple VNTR
genes we can get a more accurate test
of whether two samples are the same
or not.
Probability
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One gene has 20 alleles. The probability
that two individuals have the same
allele is 1/20.
If two genes each have 20 alleles and
two individuals have the same alleles at
both the probability is 1/20 x 1/20 =
1/400.
Probability
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People have two alleles for each gene.
If there are 20 alleles for that gene.
There are 210 possibile genotypes for
that gene.
The number of possible combinations at
2 genes with 20 alleles is 2102, for 4
genes with 20 alleles is 2104, etc.
Marker 1
Related
1&4
2&3
1234
Marker 2
Similar DNA Profiles
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Twins from a single egg have identical
profiles.
Siblings, parents, and other blood
relatives share some alleles in common.
The expected proportion depends on
how closely related the pair of
individuals are.
Uses of DNA Profiling
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Victim identification
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September 11
Srebrenica Massacre
Missing persons
Military personnel
Paternity Testing
Use of DNA Profiling
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Criminal Testing
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Conviction
Exoneration
Excluding suspects
Identifying protected species
Do matches establish guilt?
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Not necessarily
Must have additional evidence
Chain of evidence must be preserved
Person may have had reasonable access
to the crime scene
Advantages of DNA Evidence
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More sensitive than blood typing
More informative
More resilient
How reliable is DNA profiling?
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Generally, highly reliable.
Reliability is affected by methods used
to collect, store and analyze samples.
Most common problem is contamination
or mixture of a sample.
Standards have been established for
forensic analysis.
CODIS
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Convicted Offender database
Contains for 44,000 DNA profiles in the
forensic evidence section.
Over 5 million profiles will be entered in
the next 4 years.
Contributed to matches in more 6000
cases.
CODIS
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Who should be included?
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Currently convicted offenders are required
to be registered in the UK.
CODIS rules say only convicted criminals
after conviction can be entered but not all
states comply with this.
Some states submit only certain types of
criminals.
CODIS
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Some states allow use for all types of
crime investigations.
Others only for sex-related or violent
crimes.
Problems with DNA evidence
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Evidence can be degraded if not
properly handled.
Backlogs in processing (16,000 rape
cases backlogged in NYC).
Contamination
Human error
Other questions
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Should tissue samples be saved or
destroyed after profiling?
Can personal or medical information be
obtained from DNA profiles?
Why are innocent people convicted of
crimes and then exonerated by DNA
evidence?
More questions
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Is it possible for an innocent person to
be convicted based on DNA evidence?
How has DNA evidence changed the
criminal justice system?