Blackett Family DNA Activity 2

What are the 13 core CODIS loci?

A National DNA Databank

The Federal Bureau of Investigation (FBI) of the US has been a leader in developing DNA typing technology for use in the identification of perpetrators of violent crime. In 1997, the FBI announced the selection of 13 STR loci to constitute the core of the United States national database, CODIS. All CODIS STRs are tetrameric repeat sequences. All forensic laboratories that use the CODIS system can contribute to a national database. DNA analysts like Bob Blackett can also attempt to match the DNA profile of crime scene evidence to DNA profiles already in the database.

There are many advantages to the CODIS STR system:

  • The CODIS system has been widely adopted by forensic DNA analysts
  • STR alleles can be rapidly determined using commercially available kits.
  • STR alleles are discrete, and behave according to known principles of population genetics
  • The data are digital, and therefore ideally suited for computer databases
  • Laboratories worldwide are contributing to the analysis of STR allele frequency in different human populations
  • STR profiles can be determined with very small amounts of DNA

A DNA Profile: The 13 CODIS STR loci

As part of his training and proficiency testing for DNA Profile analysis of STR (Short Tandem Repeat) Polymorphisms, Forensic Scientist and DNA Analyst Bob Blackett created a DNA profile on his own DNA. Here is Bob's DNA Profile for the 13 core Genetic Loci of the United States national database, CODIS (Combined DNA Index System):

Locus D3S1358 vWA FGA D8S1179 D21S11 D18S51 D5S818
Genotype 15, 18 16, 16 19, 24 12, 13 29, 31 12, 13 11, 13
Frequency 8.2% 4.4% 1.7% 9.9% 2.3% 4.3% 13%

Locus D13S317 D7S820 D16S539 THO1 TPOX CSF1PO AMEL
Genotype 11, 11 10, 10 11, 11 9, 9.3 8, 8 11, 11 X Y
Frequency 1.2% 6.3% 9.5% 9.6% 3.52% 7.2% (Male)

For each genetic locus, Bob has determined his "genotype", and the expected frequency of his genotype at each locus in a representative population sample. For example, at the genetic locus known as "D3S1358", Bob has the genotype of "15, 18". This genotype is shared by about 8.2% of the population. By combining the frequency information for all 13 CODIS loci, Bob can calculate that the frequency of his profile would be 1 in 7.7 quadrillion Caucasians (1 in 7.7 times 10 to the 15th power!

In Bob's forensic DNA work, he often compares the DNA profile of biological evidence from a crime scene with a known reference sample from a victim or suspect. If any two samples have matching genotypes at all 13 CODIS loci, it is a virtual certainty that the two DNA samples came from the same individual (or an identical twin).

Introduction: Overview | STR P | CODIS | Analysis | Inheritance | Frequency Calc.

Activities : Pedigree | Collect data | Paternity testing | Missing person | RCMP freq. calc.

The Biology Project
University of Arizona
October 27, 2000
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