DNA Evidence DNA = DeoxyriboNucleic Acid
DNA evidence is still a relatively new field in forensic sciences.
Developments over the last 15 years have made DNA evidence as important
as fingerprint evidence was 100 years ago.
It would be impossible to compare every genetic sequence for every DNA
strand submitted as evidence. It took scientists over 15 years to map the
human genome. The law simply does not have that kind of time. DNA
evidence relies on probability.
The CODIS (COmbined DNA Index System) uses 13 regions (with 2
sequences each) of the human genome for comparison. If even 1 sequence of
the 26 do not match, then we cannot declare a DNA match. This database
allows all levels of government in the U.S. to share DNA evidence and
samples to assist in solving cases.
If we only used 3 CODIS sites, the possibility that someone would match a
RANDOM sample is about 1 / 1000. Using all 13 sites lowers that possibility
to 1 / 10,000,000,000,000. Again, we are not saying that this is a 100%
match. We are saying that the probability of another match is so close to 0
it is not worth considering!
DNA and CODIS-
http://www.koshlandsciencemuseum.org/exhibitdna/crim01.jsp
Try the activity below and answer the following questions.
1. What does DNA stand for?
2. How many ‘sites’ on our DNA does CODIS use? How many total comparisons
are used?
3. What is the probability of CODIS falsely identifying two samples of DNA as a
match?
4. What is the probability of CODIS falsely identifying a match if only 3 CODIS
‘sites’ are used from our DNA?
5.
How many comparisons must be different for CODIS to declare no match?
6. List at least five sources of DNA evidence.
7. Under the ‘Catch a Criminal Activity, which suspect do you believe committed
the crime?
8. Is it possible that the suspect’s parents or sibling would match the same
number of CODIS ‘sites’?