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By Melissa Kruse 

Sept 15, 2003 | DNA profiling typically involves the analysis of 13 core regions, or loci, of short tandem repeats (STRs). At each STR, the length of the repetitive tract of DNA varies according to the number of repeats. Most of the STRs used in forensic identification are tetranucleotide repeats (e.g., "AATG AATG AATG" repeated several times). Naturally occurring variations in the length of these STRs are inherited and, in aggregate, form a unique genetic fingerprint.

The U.S. Department of Justice (DOJ) set the standard of using 13 STR locations across the human genome where the frequencies in various ethnic populations are known. Each locus is genetically unlinked from the other 12, thereby allowing them to be treated as independent data points for statistical analysis. None are believed to contain information pertinent to medical history.

Since humans have two copies of each chromosome, they carry two versions of each STR. For example, an individual might inherit an STR containing eight repeats from one parent and 12 repeats from another. Or each parent may contribute 10 repeat units, leading to a single value in the assay. The forensic biologist needs to determine only the length of each STR.

A representative STR profile might look like this, with the name of the repeat locus in the center column, and the number of repeats on the right:

Locus STR repeats


D3S1358  15/16


VWA 17/18


FGA 22/24


D8S1179 13/14


D21S11 29/30


D18S51 13/17


D5S818  11/12


D13S317 8/8


D7S820 10/11


D16S539 9/11


THO1 7/9


TPOX  8/11


CSF1PO 10/12

The likelihood of this profile occurring by chance in the population is 9.1 x 1014, or less than one in 900 trillion.

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