Quick Overview
DNA is Present in everyone as well as STRs. CODIS uses 13 core STRs . Upon the FBI-determined STR allele frequencies reveals that the probability of two unrelated Caucasians having identical STR profiles, or so-called “DNA fingerprints,” is approximately 1 in 575 trillion. Because forensics is a science largely rooted in probabilities, even a confirmed “match” does not supply concrete proof of guilt. If for examples only 4 of the STR are matched on a small sample this does not mean is was that exact person
Overview
DNA is present in nearly every cell of our bodies, and we leave cells behind everywhere we go without even realizing it. Flakes of skin, drops of blood, hair, and saliva all contain DNA that can be used to identify us. In fact, the study of forensics, commonly used by police departments and prosecutors around the world, frequently relies upon these small bits of shed DNA to link criminals to the crimes they commit. This fascinating science is often portrayed on popular television shows as a simple, exact, and infallible method of finding a perpetrator and bringing him or her to justice. In truth, however, teasing out a DNA fingerprint and determining the likelihood of a match between a suspect and a crime scene is a complicated process that relies upon probability to a greaterextent than most people realize. Government-administered DNA databases, such as the Combined DNA Index System (CODIS), do help speed the process, but they also bring to light complex ethical issues involving the rights of victims and suspects alike. Thus, understanding the ways in which DNA evidence is obtained and analyzed, what this evidence can tell investigators, and how this evidence is used within the legal system is critical to appreciating the true ethical and legal impact of forensic genetics.How Does DNA Identification Work?
How Does DNA Identification Work?
- Although the overwhelming majority of the human genome is identical across all individuals, there are regions of variation. This variation can occur anywhere in the genome, including areas that are not known to code for proteins. Investigation into these noncoding regions reveals repeated units of DNA that vary in length among individuals. Scientists have found that one particular type of repeat, short tandem repeat (STR), is relatively easily measured and compared between different individuals. In fact, the Federal Bureau of Investigation (FBI) has identified 13 core STR loci that are now routinely used in the identification of individuals in the United States, and Interpol has identified 10 standard loci for the United Kingdom and Europe
As its name implies, an STR contains repeating units of a short (typically three- to four-nucleotide) DNA sequence. Because there 12 different alleles for this STR, there are therefore 78 different possible genotypes, or pairs of alleles. Specifically, there are 12 homozygotes, in which the same allele is received from each parent, as well as 66 heterozygotes, in which the two alleles are different.
The Statistical Strength of a 13-STR Profile
Assuming that all 13 STRs follow the principle of independent assortment (and they should, as they are scattered widely across the genome) and that the population randomly mates, a statistical calculation based upon the FBI-determined STR allele frequencies reveals that the probability of two unrelated Caucasians having identical STR profiles, or so-called “DNA fingerprints,” is approximately 1 in 575 trillion (Reilly, 2001)
Making an STR Match
In order to match, for example, crime scene evidence to a suspect, a lab would determine the allele profile of the 13 core STRs for both the evidence sample and the suspect’s sample. If the STR alleles do not match between the two samples, the individual would be excluded as the source of the crime scene evidence. However, if the two samples have matching alleles at all 13 STRs, a statistical calculation would be made to determine the frequency with which this genotype is observed in the population. Such a probability calculation takes into account the frequency with which each STR allele occurs in the individual’s ethnic group. Given the population frequency of each STR allele, a simple Hardy-Weinberg calculation gives the frequency of the observed genotype for each STR. Multiplying together the frequencies of the individual STR genotypes then gives the overall profile frequency.
Suspect b has the same STR sequences and the evidence.
http://www.nature.com/scitable/topicpage/forensics-dna-fingerprinting-and-codis-736
Reilly, P. Legal and public policy issues in DNA forensics. Nature Reviews Genetics 2, 313–317 (2001) doi:10.1038/35066091
