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How DNA Evidence Is Used in Long Island Criminal Trials
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How DNA Evidence Is Used in Long Island Criminal Trials
DNA evidence has become an increasingly important tool in criminal trials on Long Island and across New York State. DNA, or deoxyribonucleic acid, contains the genetic code that makes each person unique (except for identical twins). By analyzing DNA found at a crime scene and comparing it to DNA samples from suspects or victims, investigators can sometimes identify the perpetrator with near certainty. This article provides an overview of how DNA evidence is collected, analyzed, and used in criminal trials in Long Island courts.
Collecting DNA Evidence
DNA can be collected from virtually anywhere – blood, saliva, semen, skin cells, and hair follicles all contain genetic material that can be analyzed. At a crime scene, investigators use precautions to avoid contaminating potential DNA samples. They may wear gloves and face masks, use disposable instruments, and change gloves frequently. Common sources of DNA collected at crime scenes include:
- Blood stains or splatters
- Saliva on cigarette butts, envelopes, bite marks
- Semen from sexual assaults
- Hair follicles or skin cells on clothing, bedding or furniture
- Fingerprints
DNA evidence is carefully packaged to avoid degradation. Wet samples are air dried prior to packaging. All samples are sealed in paper bags or envelopes, not plastic which can cause condensation and damage. They are labeled with information on the source and time of collection. A strict chain of custody is maintained by documenting each person who handles the evidence.Suspects and victims may also be asked to provide DNA samples. Suspects’ DNA is collected by court order, usually via a buccal swab of the inner cheek. Victims often voluntarily provide reference samples, such as from the rape evidence kit in sexual assault cases.
Analyzing DNA in the Lab
Forensic DNA analysis is performed in accredited laboratories, like the Suffolk County Crime Laboratory. The analysis process includes:
- Extraction – The DNA is extracted from samples using chemicals, centrifugation and filtration.
- Quantitation – The amount of extracted DNA is measured. A minimum amount is required for analysis.
- PCR amplification – The DNA is replicated exponentially via polymerase chain reaction (PCR) to create billions of copies for profiling.
- Separation and detection – The amplified DNA fragments are separated by capillary electrophoresis. Lasers detect the separated fragments.
- Interpretation – Computer software analyzes the detected fragments to determine the specific STR (short tandem repeat) alleles present. An analyst reviews the results.
- Statistical analysis – If a suspect’s DNA profile matches the evidence, statisticians calculate the match rarity.
DNA Profiles and CODIS
The specific locations (loci) on DNA analyzed for identification are called short tandem repeats (STRs). These STR sequences are repeated a certain number of times, and the number of repeats varies widely among individuals. By analyzing STRs at 13 specific loci plus a gender marker, the lab can generate a DNA profile unique to each person (except identical twins).DNA profiles from crime scenes and convicted offenders are uploaded to CODIS – the Combined DNA Index System. CODIS allows comparisons between profiles in the Forensic and Offender indexes. Matches link crime scenes together or tie offenders to unsolved crimes.
Discovery and Admissibility
In New York, prosecutors are required to provide the defense with copies of DNA analysis reports and sometimes even the raw electronic data. The defense may request additional DNA testing, either by the prosecution’s lab or an independent lab.For DNA evidence to be admissible in court, the prosecution must show the lab followed established protocols and quality control procedures. A pre-trial Frye hearing may be held for the judge to assess if the DNA analysis method is scientifically valid and generally accepted.
Presenting DNA Evidence at Trial
The prosecutor presents DNA evidence at trial through testimony of the criminalist who analyzed the samples. The analyst explains the laboratory methods, results, and statistical significance of any DNA matches.Defense attorneys sometimes question the validity of the DNA evidence. For example, they may argue:
- Contamination occurred during evidence collection or analysis
- Errors were made in following lab procedures
- The match statistics overstate the significance
Mistakes in DNA testing have led to some wrongful convictions. But when properly collected and analyzed, DNA provides extremely reliable evidence. DNA matches that are rarer than 1 in a trillion are not unusual. Such statistics provide powerful evidence of a defendant’s guilt or innocence.
Post-Conviction DNA Testing
Individuals convicted before routine DNA testing may request post-conviction analysis to attempt to prove their innocence. The Innocence Project has used DNA testing to exonerate hundreds of wrongfully convicted persons.In New York, a defendant can motion for post-conviction DNA testing of evidence so long as it could provide reasonable probability of a more favorable verdict. However, such motions are frequently denied by judges when there is other strong evidence of guilt.
DNA’s Impact on Criminal Justice
The advent of forensic DNA analysis in the late 1980s revolutionized criminal investigations and prosecutions. DNA provides a precise, scientific method for identifying perpetrators and linking crimes. It has helped solve cold cases sometimes decades old. DNA evidence has also exonerated hundreds of wrongfully convicted persons.However, most criminal cases still rely heavily on traditional evidence like eyewitness testimony and fingerprints. Less than 10% of violent crimes get DNA evidence tested. And DNA evidence is useless if the perpetrator’s DNA profile is not already uploaded in CODIS.Nonetheless, the ability of DNA profiling to identify individuals with near certainty makes it the gold standard of forensic evidence. As testing becomes faster and less expensive, DNA evidence will likely continue growing in importance for solving and preventing crimes.