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 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 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 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 By comparing across multiple

VNTR genes we can get a more accurate test of whether two samples are the same or not.

Probability 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 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 Marker 2

1 2 3 4

Related 1 & 4 2 & 3

Similar DNA Profiles 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 Victim identification

September 11 Srebrenica Massacre Missing persons Military personnel

Paternity Testing

Use of DNA Profiling Criminal Testing

Conviction Exoneration Excluding suspects

Identifying protected species

Do matches establish guilt? 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 More sensitive than blood typing More informative More resilient

How reliable is DNA profiling? 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 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 Who should be included?

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 Some states allow use for all types

of crime investigations. Others only for sex-related or

violent crimes.

Problems with DNA evidence Evidence can be degraded if not

properly handled. Backlogs in processing (16,000

rape cases backlogged in NYC). Contamination Human error

Other questions 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 Is it possible for an innocent

person to be convicted based on DNA evidence?

How has DNA evidence changed the criminal justice system?