STRs and individuals

Using STRs to identify individuals

What can you tell about a person from STR analysis? STRs aren't genes for physical traits, like eye color or height, so you can't tell what a person looks like or acts like from the STR pattern. The only exception is gender: DNA labs use the amelogenin STR, which has different alleles for X and Y chromosomes, to identify samples as male or female. So, aside from gender, the only information you can get from STR analysis is whether a sample taken from a crime scene matches a particular suspect or not. But STRs are extremely good at matching.

Looking at a single STR can narrow down the possibilities in matching a sample to an individual, but won't give a positive identification. For instance, 20 different alleles have been found for the HUMTH01 STR. The number of tandem repeats varies from 3 to 14, and there are some small variations in the repeat sequence which increase the total number of alleles. So, if the alleles were randomly distributed over the population you would expect to see a particular allele 5% (1/20) of the time.

1 copy of 1 STR: 1 in 20

But remember that everyone gets two copies of each gene, one from each of his or her parents. So you might get allele 4 from your mother and allele 12 from your father. Looking at two copies of the STR narrows down the probabilities.

2 copies of 1 STR: 1/20 X 1/20 = 1 in 400

This helps, but it's not enough, so DNA labs look at many STRs at once for each sample. The FBI uses 13 different STRs for identification.

13 STRs: less than 1 in 1,000,000,000,000,000

Compare this to the population of the world, which is a million times less at 5,000,000,000 and you can see that STR analysis can be used to positively match an individual with a sample.

DNA EVIDENCE

Using STRs to identify individuals