Dna

Presumptive testing is cheap, rapid and often portable, and gives the first indication of

whether target substances might be present in a sample. The process can be heavily

automated, often using an immunoresponse in combination with fluorescence for

detection. It is not legally defensible, but is used to screen out negative results before

going on to confirmatory testing, which definitively identifies and measures the amount

of specific chemicals. Confirmatory testing is legally defensible but is usually time-

consuming, requires expert operation, and is not portable and also is kind of expensive.

CODIS generates investigative leads in crimes where biological evidence is recovered

from the crime scene using two indexes: the forensic and offender indexes. The

Forensic Index contains DNA profiles from crime scene evidence. The Offender Index

contains DNA profiles of individuals convicted of sex offenses (and other violent

crimes) with many states now expanding legislation to include other felonies. Matches

made among profiles in the Forensic Index can link crime scenes together; possibly

identifying serial offenders. Based on a match, police in multiple jurisdictions can

coordinate their respective investigations, and share the leads they developed

independently. Matches made between the Forensic and Offender indexes provide

investigators with the identity of the perpetrator(s). After CODIS identifies a potential

match, qualified DNA analysts in the laboratories contact each other to validate or

refute the match. DNA analysis methods allow much smaller trace samples to be used.