Rules for DNA Comparison Analysis

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)

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

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

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

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.

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

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

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)