In May of 2015, the FBI Crime Laboratory notified all state CODIS (Combined DNA Index System) administrators in the United States that it found errors in population data published by the FBI in 1999 and 2001. This data has been used by almost every DNA laboratory in the United States to calculate allele frequencies for the last 15 years. In other words, the FBI found errors in the DNA matching database that was used to calculate the probability that the DNA found at a crime scene matched that of a suspect.
DNA evidence has long been heralded as the forensic gold standard, but advancements in DNA testing technology now allow for the analysis of DNA beyond the search for perfect matches. Specifically, the advent of mitochondrial DNA testing allows law enforcement agencies to analyze DNA to see if a person can be excluded as a suspect, and to also see what percentages of the population can be excluded as possible donors. The ability to calculate these Combined Probabilities of Inclusion or Exclusion depend up the accuracy of a database maintained by the FBI. This CODIS database contains over 13 million DNA profiles. The FBI also publishes probability tables based on frequencies of alleles appearing in various segments of the population.
The first notice from the FBI attributed the errors in the database to technological limitations that existed in the past as well as to human error. The FBI retested 1,100 of the 13 million profiles using 23 loci instead of the original 13 loci and discovered errors in 33, or approximately 3% of those profiles.
When the FBI notice of errors was released, it was generally believed that the errors were not significant. The FBI Crime Laboratory released a statement that stated, “We are of the view that these discrepancies are unlikely to materially affect any assessment of evidential value.” Similarly, the Texas Department of Public Safety Crime Laboratory Service stated, “The database corrections have no impact on the inclusion or exclusion of victims or defendants in any result.”
Yet, a few months later it came to the attention of the scientific community that in cases where labs were testing small or trace amounts of DNA on objects, situations where DNA mixtures were being analyzed, the probabilities changed significantly. A DNA mixture is a DNA profile contributed by multiple people. DNA mixtures are common when DNA samples are taken from objects or surfaces that have been touched by more than a single person. In cases where mixtures were analyzed, the United States Department of Forensic Sciences (DFS) found that some of the variation did not fall within the range of scientifically acceptable interpretation. (DFS Final Report) The change in magnitude could be as dramatic as going from one in millions to one in tens, and in some cases CPI/CPE could not be calculated at all.
Since the issue has come to light prosecuting agencies and forensic laboratories are preparing for the thousands of cases where DNA evidence must be retested. To its credit, the United States Attorney’s Office first approached the Department of Forensic Sciences to review the interpretation and statistical evaluation of DNA mixtures based on concerns raised in specific cases. Additionally, many local prosecuting agencies in Texas including the Dallas County District Attorney’s Office, Tarrant County District Attorney’s Office, and Harris County District Attorney’s Office have sent out notices to defense attorneys advising them of potential concerns with analyses of DNA mixtures. (Notices: Dallas County District Attorney’s Office, Tarrant County District Attorney’s Office, Harris County District Attorney’s Office)
First, for cases where DNA mixture analysis played a significant role in the conviction of a defendant, a defense attorney may request reinterpretation using a currently acceptable protocol for calculation of CPI.
Second, a defense attorney may request recalculation or re-analysis of CPI based on the new protocols to eliminate the possibility of human error during the initial testing.
Third, members of the criminal justice community should support the move from 13-loci testing to 20-loci testing that the FBI is currently contemplating adopting by 2017. This would significantly improve the accuracy of results for all involved.
Deoxyribonucleic acid (DNA) is a long molecule that has encoded within it the genetic code for most living beings. DNA is made up of 1) a phosphate group, 2) a sugar group, and 3) one of four nitrogen bases. The four nitrogen bases are Adenine, Cytosine, Guanine, and Thymine. If you have seen the double helix structure – what looks like a twisted ladder – the rungs of the ladder are made up of two nitrogen bases. These nitrogen molecules will always be adenine with thymine and cytosine with guanine.
A person gets half their DNA from their mothers and half their DNA from their fathers, but they don’t get the same amount of DNA in all respects. While both parents provide nuclear DNA, only the other provides mitochondrial DNA. This is because only the egg cells, and not the sperm cells, keep their mitochondria during fertilization.
DNA is found in the nucleus of a cell and is often referred to as nuclear DNA. Nuclear DNA can be collected from skin cells, blood, semen, bone etc.
Outside of the nucleus, DNA also exists in the mitochondria of the cell. The mitochondria is the powerhouse of the cell. Multiple individuals can have the same mtDNA; mtDNA is generally extracted when nuclear DNA is not available for testing due to degradation of the sample. While nuclear DNA is present only twice in each cell, mitochondrial DNA is present hundreds to thousands of times in each cell.
Sweat, skin, blood, tissue, hair, dandruff, mucus, semen, ear wax, and saliva are just some of the sources of DNA evidence in criminal cases. DNA can be collected from a crime scene or from an instrumentality of the crime, such as a weapon or tool. Law enforcement agents will do their best to maintain the integrity of a crime scene to avoid contamination. They should also maintain a proper chain of custody to preserve the integrity of any DNA evidence that is collected.
Nuclear DNA is analyzed to look for Short Tandem Repeats or STRs. Y-STR testing looks for only DNA passed from the father to the son.
Mitochondrial DNA traces a person’s lineage through his or her mother’s side. A mitochondrial DNA match is not a unique identifier and can only be used to supplement other evidence.
Combined Probability of Exclusion (CPE): The probability that a randomly chosen, unrelated person from a given population would be excluded as a potential contributor to the observed DNA mixture.
Combined Probability of Inclusion (CPI): The probability that a randomly chosen, unrelated person from a given population would be included as a potential contributor to the observed DNA mixture.
One of 13 or more specific locations on a chromosome.
An allele is a stretch of DNA that is found at a fixed spot on a chromosome. An allele refers to a specific variation of a gene.
The allele frequency represents how often an allele or gene variant exists in a population.
These tables contain information compiled by the FBI and are used to represent how rare certain alleles are in a given population.
A Short Tandem Repeat is a micro-satellite or tract of repetitive DNA where a certain pattern is repeated, generally 5-50 times.
Combined DNA Index System (CODIS) is a database and software maintained by the FBI. CODIS is designed so that law enforcement agents can compare a target DNA against the database of DNA samples.