1. Rules for DNA Comparison Analysis

2. DNA Matches Full Profile: Partial Profile: Partial Match:
All 20 target sequences are present in the sample
Partial Profile:
At least 8 of the 20 target sequences are present
Partial Match:
At least 8 of the 20 target sequences are present AND those 8 sequences match the evidence sample
All 20 target sequences are present in the sample AND 50% match the evidence sample (familial match)

3. Using DNA Matches
Generally, any DNA samples legally collected at the crime scene or from suspects are admissible in court
Exceptions:
Credible concerns about contamination
Sample is too small to allow for repeat testing
The analysis was performed by non-expert personnel

4. Using DNA Matches
Even if the DNA sample is admitted to court, it still has to be evaluated as evidence
Possible problems with DNA analysis:
DNA is considered “certain” by many jurors and lawyers
Large DNA databases are producing more false positives
DNA is easily contaminated or planted
Crime labs are super busy and cause delays/errors
DNA analysis technology is expensive and often not kept up-to-date

5. Matching DNA Evidence – Crime Scene Evidence – Suspect
Evidence – CODIS
Crime Scene – Suspect
Crime Scene – CODIS
Even if we don’t have a suspect, we can compare evidence and crime scene samples to the CODIS national and state databases to find a match to:
Convicted felons
Evidence samples from unsolved cases

6. DNA Match Results
Inclusion: The individual is “included” as a possible source of the evidence sample
We can provide statistical evidence of how likely it is to be a false positive
We should continue investigating the suspect
We still have to convince the jury that the match is correct and proves something
We cannot be sure when the DNA was transferred or what the individual was doing at the time.

7. DNA Match Results
Exclusion: The individual is “excluded” as a possible source of the evidence sample
We know the DNA did not come from the individual
We may or may not exclude the suspect entirely
We only know that the individual did not leave this sample – he/she could still have been there

8. DNA Match Results
Inconclusive: The sample was too small or degraded for comparison
We cannot learn anything from the sample at this time
We can keep the results on file to continue comparing
We can (sometimes) re-test the sample

9. DNA Match Results
When including a sample with a partial match we need to quantify the likelihood that we might be wrong
1 band match = 1 in 10
2 band match = 1 in 100
3 band match = 1 in 1,000
4 band match = 1 in 10,000
5 band match = 1 in 100,000
Even partial matches should match at least 9 bands (1 in a billion chance of a coincidence)