20 September 2008

DNA Testing vs. CODIS, the Criminal Database

CODIS vs. Genealogy

As I have previously stated, DNA testing for genealogy is NOT useful to insurance companies or government agencies. The portions of the genome used for genealogy cannot pin-point a certain person, and they do NOT contain your health issues.

Law enforcement agencies obtain DNA at a crime scene and test only a specific set of markers to compare against their database.


Both SMG And CODIS are databases for the criminal justice system. SGM is an acronym for Second Generation Multiplex for a commercial multiplex kit (multiple markers can be tested in the same reaction) and is widely used in European countries. CODIS is an acronym for Combined DNA Index System and is used in the U.S. It combines several databases, with DNA profiles for convicted criminals, biological evidence in unsolved cases, and missing persons.

The SGM Plus test uses 10 autosomal Short Tandem Repeat (STR) markers where as CODIS uses 13 markers. Eight markers overlap.

NOTE: Short Tandem Repeat (STR) is a short pattern of the four chemical bases (Adenine, Cytosine, Thymine and Guanine) in the DNA that repeats itself, side by side for each marker. The number of times they repeat is the test results (called the allele) for that particular marker. That is, if you test your Ydna and on marker DYS 393 and you have the result of 13, then a particular pattern of the four basic chemicals in your DNA has repeated this pattern 13 times. This type of results helps to compare different testers.

Profiling with SMG and CODIS:

The purpose of both SMG and CODIS is to identify an individual unambiguously based on probabilities. The odds are billions against finding that any two individuals would have exactly the same DNA. The odds are exactly the same CODIS or SGM profile.

A single mismatch on any of the compared markers means you do not have the same person represented in your reference sample.

Mixed DNA Crimes:

In some cases, it is impossible to do conventional PCR (Polymerase Chain Reaction, a process to amplify the DNA in order to obtain results.) analysis where there is a mixture of male and female DNA in sexual assault crimes. The only DNA that can be uniquely identified and separated is the Y-chromosome. In such cases, STR markers are used, and it is not possible to uniquely identify a person, but it is possible to narrow down the population of potential suspects.


General Background:

Interactive Map on Profiles

List of CODIS markers:

CODIS Testing vs. Genealogical Testing:

The DNA is chemically chopped up into chemically-defined fragments using the PCR techniques. In analogy, suppose you had a long piece of tickertape with random numbers on it and you cut the tape every time you saw the number 43. In the end you'd wind up with a pile of scraps of tickertape of different lengths.

Each DNA fragment length you obtain from chopping up the DNA upon analysis appears as a dark line in the long sheet of X-ray film (autorad) you see in detective stories on TV.

The position and intensity of the lines created by your unknown DNA are compared to the position and intensity of lines created by the DNA in the known sample. If the two sets of lines match, the two sets of fragments match, the two kinds of DNA match, and you have your criminal.

In Genetic Genealogy Testing…

The DNA analysis used for genetic genealogy uses only very small segments (short tandem repeat or STR markers) occurring on the Y-chromosome only. They are the same type but not necessarily the same markers as those used by CODIS. In either case you wind up analyzing only a few hundred or, at the most, a couple of thousand bits in the genome.

The analysis is based on the lengths of those markers again, but defined differently. They are defined not according to where "43" might appear on the tickertape. They are defined according to where certain chemical motifs are repeated X number of times.

In analogy, these markers would be defined by the tickertape again, but you would only be interested in how many times 43 is repeated at a certain position on the tape.

Government Fingerprinting Databases:

Ysearch is only one of the public DNA databases for genetic genealogy. Although the criminal justice system can access it or any of the others, doing surveillance on a surname group to see which one of the Smiths or Joneses committed the crime would be so time consuming and expensive that it would not be worth it.

By the time law enforcement gets this far in an investigation, they probably have a list of suspects. If one of them is named Smith or Jones, they will be under surveillance and they might be arrested…but not the whole group.

If you are planning to rob a bank or commit some other crime, DNA is only one way you can get caught. More likely you will be caught because your fingerprints match those in the FBI database. But no one worries about that.

Colleen Fitzpatrick was instrumental in providing the above analogies.

E. Aulicino
© 2007

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