Sorenson Forensics - Tumblr

DNA testing has revolutionized how investigators solve crimes and identify victims, as well as the exoneration of the innocent. As there are various types of forensic DNA testing available, understanding the differences can help law enforcement and legal professionals navigate the labs to contact and the conversations that will take place.

STR Analysis (Short Tandem Repeat)

This is the most widely used method in forensic DNA testing. It examines specific regions of DNA that vary greatly among individuals. This technique is especially effective for human identification in criminal cases due to its high degree of specificity and reliability.

mtDNA Analysis (Mitochondrial DNA)

This analysis is used when nuclear DNA is degraded or unavailable, often in older samples. It examines the maternal lineage and can provide valuable insights in cases involving skeletal remains or hair samples. While less discriminative than STR, mtDNA can still be crucial for historical cases.

Y-STR Analysis

Y-STR targets the Y chromosome, making it ideal for tracing paternal lineage. This method is particularly useful in sexual assault cases where male DNA is mixed with female DNA, allowing forensic analysts to isolate male-specific markers for identification.

SNP Analysis (Single Nucleotide Polymorphism)

This emerging technique is often headlined in the industry and focuses on variations at specific single nucleotide sites in the DNA sequence. It's useful for ancestry testing, specifically forensic genetic genealogy in criminal cases, and can provide additional information when traditional methods are inconclusive.

DNA Phenotyping

Judicial & Legal History of DNA Evidence in US Courts

·             The First State & Federal Court Which Accepted DNA Admissibility

The first state appellate court decision to uphold the admission of DNA evidence was in 1988 (Andrews v. Florida.) In this case, Andrews was suspected of more than 20 sexual assaults. Scientists collected the semen from the crime scene and connected it with the DNA of the perpetrator. At preliminary trials, he was convicted of rape and assault and given a sentence of 22 years but afterward when his DNA matched the crime scenes of other previous victims, his sentence was expanded to 100 years.

The first major federal court decision to uphold the admission of DNA evidence was in 1992 (U.S. v. Jakobetz.) In this case, Randolph Jakobetz, a truck driver, was suspected of kidnapping and raping a Vermont woman in a semi-trailer truck. Police looked through the trailer that Jakobetz had hauled on the night of the crime and found hairs matching those of the victim. After arresting Jakobetz, law enforcement officers sent a sample of his blood to the FBI laboratory in Washington, D.C., for DNA investigation and comparison with DNA taken from semen found in the victim shortly after the crime. At Jakobetz’s trial, an FBI expert affirmed that the blood and semen samples were a “match.” Based on this and other strong evidence, Jakobetz was convicted and sentenced to almost 30 years in jail.

·             Admissibility Standards

In general, two standards are used to judge the admissibility of novel scientific evidence.

Frye standard

Daubert standard

Frye StandardDaubert Standard

Origin→The Frye standard originates from a 1923 case,

Frye v. United States

→ The Daubert standard is more latest, derived from the 1993 case

Daubert v. Merrell Dow Pharmaceuticals

Court Ruling→ The court ruled that to be admissible, scientific evidence must be adequately established to have acquired general acceptance in the particular field in which it belongs.→ The Supreme Court went ahead of Frye to say that scientific evidence must have sufficient validity and reliability to be admitted as relevant “scientific knowledge” that would “assist the trier of fact.”

·             The Emergence of DNA Databases

The Federal Bureau of Investigation (FBI) introduced the “Combined DNA Index System” (CODIS) forensic DNA database mandated by the federal DNA Identification Act of 1994. It not only helps law enforcement identify perpetrators or link serial crimes but also helped affirm a suspect’s innocence.

·             Development of All Felons Databases

In 1990, Virginia became the first state to pass an all felons’ law that entailed DNA databases from anyone convicted of a felony. In 2002, the first state to execute a criminal convicted of murder and rape based on a “cold-hit” was Virginia. By 1991, six states had “all felons’ databases” and today every US state has a database of criminal offenders’ DNA profiles.

·             Post-Conviction DNA Testing

Just as recent legislation has sustained the increased use of DNA for prosecution, legislation to protect the falsely convicted has also been gaining ground in recent years. The Innocence Project was established in 1992 to support the privilege of convicted felons who maintain their innocence and has been a driving force in supporting legislation in this field. Most states have since passed legal provisions for post-conviction DNA testing.

·             Justice for All Act

On October 30, 2004, President George Bush signed the Justice for All Act, which significantly enhanced funding and guidelines for the use of DNA technology in the judicial process.

How Reliable Is DNA Evidence in Court Rooms?

DNA evidence is extremely useful and accurate if it’s properly handled and analyzed. The chances of one person’s DNA profile matching another individual are negligible (about one in a billion by estimate.)

Compared to eyewitness testimony and fingerprinting, which both can have inherent errors and inaccuracies, DNA evidence is a highly valuable approach to matching a suspect to biological samples collected during a criminal investigation. DNA evidence is also highly useful to determine the biological relationship between parent and child (paternity/legitimacy cases.) The results of the paternity DNA test can be used as evidence in legal courtrooms for child custody, inheritance/ancestral property disputes, immigration purposes, child swapping conspiracies, and to prove fidelity between marriages.

Conclusion

Creating DNA profiles is a complex process. About 99.9 percent of information contained in all human DNA is the same. Thus, scientists are only left with 0.1 percent of DNA to work with when developing individual profiles. STR analysis is the most common DNA profiling method, and is the industry standard for creating profiles uploaded to the CODIS (Combined DNA Index System) database, the FBI's DNA index system. STR (short tandem repeat) analysis determines a person’s DNA profile by establishing how many times a DNA sequence, called a short tandem repeat unit, appears in a chromosomal location. The chromosomal location refers to the area in a chromosome where the DNA strand resides. Human DNA contains about 5 percent of the code responsible for traits. The rest is long nucleotide base pair stretches whose function scientists still don’t understand. Short tandem repeats appear as short, repetitive base pairs within these stretches. Since the number of repeats in a stretch is inherited and easily identifiable, they are the perfect genome identification markers. Quantifying STR markers is a technique used by forensic investigators to solve crimes. Scientists also use STR analysis to identify human remains, confirm male descent lines, confirm or reject paternity, and study ancient human migration patterns. The procedure comprises three processes. The first is amplification, where chemical reagents are added to the extracted DNA and then heated. The heat causes the DNA molecule’s two strands to separate. The resulting individual strands can be used as templates to synthesize new DNA molecules with two strands. The chemical reagents have primers, which are markers that identify the new duplicated DNA fragment’s start and endpoints. These primers are tiny DNA pieces matching highly-variable human DNA regions, stimulating the synthesis. The primers contain fluorescent labels that lasers can identify during the testing phase. Additionally, they attach to single-stranded DNA when the chemicals and DNA begin to cool. After they bind to the copied segment’s start and endpoints, individual DNA blocks from the reagents move to fill any remaining empty places. Scientists use a thermal cycler to heat and cool DNA. Inside the machine are cubes that hold the reagents and DNA. Notably, the operator can program this machine to heat and cool the contents at given intervals. After many cycles, the process creates millions of copies of the original sample’s DNA. This process amplifies any DNA present in the tubes. This means that if there was any DNA at the investigation site that didn’t belong to the criminal, it too would be amplified and potentially interfere with the interpretation of the results. Therefore, the standard practice is to use various control samples. Electrophoresis is the second step, where scientists sort the resulting DNA according to length. They begin by adding DNA to a molecular sieve, a gelatinous material with many tiny openings. By applying electric current to the material, DNA fragments start to move. During this process, the smaller fragments move longer distances than the larger ones. The distances these fragments move help establish their sizes. Capillary electrophoresis is similar to general electrophoresis. However, instead of using a gelatinous material, researchers use a capillary tube the size of a human hair containing a sieving material. It is mostly an automated process requiring minimal human input. A capillary electrophoresis machine works with lasers that identify fluorescent markers in the primers. The last step is interpretation. Using proprietary software, scientists can interpret results from the electrophoresis equipment. The software determines the DNA fragment sizes. Consequently, the information collected helps create DNA profiles.

Feedback Provided

Published On:

https://sorensonforensicsllc.wordpress.com/(February 14, 2022)