Combined DNA Index System

What is Entered into the System?

It is well known today that genetic information can identify individuals and that a database system can hold this information, but most people cannot explain how the system works. There are actually multiple types of specific genetic information, obtained from different sources and by various methods, that are incorporated into the system. The entire DNA makeup of an individual contains a vast amount of material and is referred to as genomic DNA. Scientists have narrowed down this tremendous amount of information to several specific, short segments that are highly variable and can be used to individualize a sample to a particular individual. These short segments are known as microsatellites or short tandem repeats (STRs) and are taken from nuclear DNA (nuDNA). Nuclear DNA comes from the nucleus. What makes STRs so helpful is that original DNA typing required relatively large, pure (uncontaminated) samples to provide information. Obvious problems, from an investigatory stance, are that samples are rarely large or pure. STRs are very small, short segments of information that can be chemically copied quickly (polymerase chain reaction) to create a larger sample for testing. The FBI system has isolated 13 STRs on the nuDNA that can be used together (multiplexed) to calculate the probability that a sample came from no one but a particular individual. This is not a sample-to-sample direct match system, but is based on frequencies of known populations. Thus the chance of 1 in 67 billion that a sample came from someone other than the offender is a pretty good estimate that the sample and offender are one and the same.

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