What are STRs, loci, and alleles? How are they involved in DNA analysis?
Short tandem repeats are regions of DNA that contain short segments (usually 2 to 5 bases long) repeated several times in tandem (side-by-side). The majority of STRs used in forensic DNA analysis are named in a manner that indicates which human chromosome the locus (location of the gene) is found on. Upon examination of a human chromosome seen below (for purposes of this explanation this will be considered Chromosome 7), which is where the STR genetic locus D7S280 is located. D7S280 is one of many loci used in forensic STR DNA analysis.
Using STR DNA kits commercially manufactured for use in forensic DNA analysis, allele(s) (one of two genes located on a chromosome) can be determined for an individual.
Location of D7S280
Has a repeat sequence of GATA
Repeats 6-15 times
The following shows an example of how the STR DNA data for all possible genes observed at the D7S280 location would appear on a computer printout after the DNA data has been run through the genetic analyzing software.
However, when comparing the possible genes observed at this location with a suspect/victim DNA profile, it is important to remember that the individual in question can only inherit any two of these genes (with the exception of mutation), one from each parent.
Here are some examples of how these genes could be passed down to an individual from each parent:
Individual 1 Individual 2 Individual 3 Individual 4
S7S280: 8, 10 10, 14 11, 11 9, 12
This is only one particular genetic locus tested in forensic STR DNA analysis. Forensic Labs across the Country typically test for the entire 20 genetic loci that are mandated by the FBI as eligible for upload to CODIS. When looking at an individual's DNA profile in its entirety (all 20 genetic locations), the power of discrimination is quite significant.
A simple analogy is to hand 10 people a phone book and have them choose 20 phone numbers at random. Upon comparison of the numbers chosen by these individuals, what is the possibility that any one of the 10 people would have chosen even one number in common, let alone all 20?