Chapter 3: Forensic Biology

3.1: Common Disciplines of Forensic Laboratory Services

• Forensic laboratories provide scientific analysis, evidence evaluation, and consultations to various criminal justice agencies for the investigation of criminal cases.

  • They provide expert testimony related to the resolution of criminal cases to the courts.

• Forensic biology is a subdiscipline of forensic science.

Common Services Provided by US Forensic Laboratories

3.2: Laboratory Analysis of Biological Evidence

• Laboratory analysis utilizes scientific techniques for the examination of evidence, the reconstruction of a crime scene, the identification of biological fluids, and the comparison of individual characteristics of biological evidence.

Identification of Biological Evidence

• The identification is based on a comparison of class characteristics—a set of characteristics that allows a sample to be placed in a category with similar materials.

• By comparing the class characteristics of a sample with known standards of its class, biological samples can be identified.

Comparison of Individual Characteristics of Biological Evidence

• Individual Characteristics: Refer to the unique characteristics of both the evidence and a reference sample such as fingerprints, which share a common origin to a high degree of certainty.

• DNA Polymorphisms: An example of biological evidence possessing individual characteristics.

  • Current forensic DNA profiling can compare individual characteristics of DNA evidence with a known reference sample.

  • The examination of individual characteristics of evidence can also exclude the possibility of a common origin.

Reporting Results and Expert Testimony

• After the analysis of evidence is completed, a report is prepared based on the results of the analysis, which may include sections discussing the specific evidence analyzed, the method of analysis used, the results obtained, and the conclusions are drawn.

• A forensic scientist often serves as an expert witness whose testimony provides professional opinions about the evidence analyzed.

  • They must also communicate their findings to attorneys, judges, and members of a jury.

• Expert Witness: Qualified based on his or her knowledge, skill, experience, training, or education, and may give an opinion to the court that is relevant to the analyses conducted.

3.3: Forensic Science Services Related to Forensic Biology

Forensic Pathology

• When death is deemed suspicious or unexplained, medical examiners frequently perform autopsies to determine the exact cause.

• The manner of death is classified into one of five categories based on the circumstances: natural, homicide, suicide, accident, or undetermined.

• A medical examiner participating in a criminal investigation is often responsible for estimating the time of death.

Forensic Anthropology

• It is the identification and examination of human skeletal remains.

• An examination of bones may reveal an individual’s origin, sex, approximate age, race, and the presence of a skeletal injury.

• A forensic anthropologist may also assist in creating facial reconstructions to aid in the identification of skeletal remains or may be called on to help collect and organize bone fragments in the course of identifying victims of mass disasters such as plane crashes as well as victims in mass graves discovered after wars or genocides.

Forensic Entomology

• The study of insects in relation to a criminal investigation.

• This forensic discipline is valuable for estimating the time of death when the circumstances surrounding the crime are otherwise unknown.

Forensic Odontology

• Forensic Odontologists participate in the identification of victims whose bodies are left in an unrecognizable state.

  • They can analyze the marks left on a victim and compare them with the tooth structures of a suspect to make a comparison.

• The characteristics of teeth, their alignment, and the overall structure of the mouth provide evidence that can identify a specific person.

• Dental records such as x-rays and dental casts allow a forensic odontologist to compare a set of dental remains with an alleged victim.

3.4: Brief History of the Development of Forensic Biology

Antigen Polymorphism

• The human ABO blood groups were discovered in 1900 by Karl Landsteiner in a study of the causes of blood transfusion reactions.

• Landsteiner’s discovery made blood transfusions feasible, and he received the Nobel Prize in 1930, when he revealed the four groups of human blood cells designated A, B, AB, and O.

• By the 1960s, a dozen more blood group systems had been characterized, and at least 29 systems are currently known.

• Subsequent studies found that the blood types in the ABO system were inherited, and the frequencies with which the four types appeared in specific human populations were found to differ

• Forensic Serology: Primarily responsible for the detection and identification of biological material on physical evidence.

Protein Polymorphism

• A few polymorphisms in serum proteins and erythrocyte enzymes were reported.

• By the 1980s, approximately a hundred protein polymorphisms had been discovered.

• A few systems were commonly used in forensic laboratories, including the polymorphisms of erythrocyte enzymes, serum proteins, and hemoglobin.

• Blood groups and protein polymorphism analysis were combined in forensic investigations to lower the probability of a match between two unrelated individuals.

DNA Polymorphism

• The human genome contains all the necessary biological information for cellular and organ structure and function.

  • It consists of the nuclear genome and the mitochondrial genome.

• The human nuclear genome, a set of 23 chromosomes, contains approximately 3 billion base pairs.

• The Human Genome Project was initiated in 1990 to sequence the entire human nuclear genome.

• The genome contains genes and intergenic noncoding sequences.

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Genes and Related Sequences

• Approximately 20,000–25,000 genes have been identified in the human genome, which encode the information for the synthesis of proteins.

  • Most encode the proteins that are responsible for the maintenance of the genome, the functioning of the cells, the immune response, and the structural proteins of cells.

• The coding regions of genes are called exons and are separated by introns.

• During gene expression, the precursor messenger RNA transcript, consisting of both the exons and introns, is produced.

  • The mRNA is a template for protein synthesis in which the sequence is based on a complementary strand of DNA.

• Through the process of splicing, the introns are removed and the exons are joined, producing the spliced mRNA form, which can be used for protein synthesis via the translation process.

• Other gene-related sequences include those responsible for gene transcription such as promoter sequences; those responsible for gene regulation such as cis-regulatory sequences and untranslated sequences, which are transcribed but do not encode proteins.

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Intergenic Noncoding Sequences

• Tandem Repeats: Repeat units placed next to each other in an array.

  • Satellite DNA can be found at centromeres and telomeres consisting of regions composed of long stretches of tandem repeats.

  • Variable Number Tandem Repeats (VNTRs): Form arrays of tandem repeats with a repeat unit length from several to hundreds of base pairs.

  • Short Tandem Repeats (STRs): Also known as Simple Sequence Repeat, the repeat unit length can be 2–6 bp long.

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• Interspersed Repeats: Randomly located throughout the human genome.

  • Transposition: The mobile elements change their locations.

  • During transposition, DNA transposons are excised from one site and inserted at a new site in the genome.

  • Retrotransposons duplicate themselves during transposition and propagate throughout the genome, which is a copy-and-paste mechanism: a copy of the original retrotransposons is generated at the new site and the original copy is retained.

  • Retrotransposition: The transposition of retrotransposons which requires an RNA intermediate.

  • Long Term Repeat Retroposons

  • Non-LTR Retroposons

    • Long Interspersed Elements

    • Short Interspersed Elements

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  Gene structure. Transcription, which can be regulated by the cis-regulatory sequence, is initiated at the transcription start site (arrow) near the promoter. The exons, noncoding introns, and the untranslated sequences are also shown.

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Human DNA Polymorphic Markers

• DNA polymorphisms: The differences between individual genomes that occur at the DNA level.

  • It forms the basis of forensic DNA profiling.

• Sequence Polymorphisms: A DNA polymorphism with alternative forms of a chromosomal locus that differ in nucleotide sequence.

• Lengthy Polymorphisms: A DNA polymorphism that differs in the numbers of tandem repeat units.

• Many DNA polymorphisms are useful for genetic mapping studies and hence are called DNA markers.

• Alleles: Alternative forms of DNA polymorphisms.

• Homozygous: The same allele is present in both homologous chromosomes.

• Heterozygous: Two different alleles present in homologous chromosomes.

• Genotype: A combination of alleles at a given locus.

• DNA Profile: The genotype for a panel of analyzed loci.

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Forensic DNA Polymorphism Profiling

• In 1984, Sir Alec Jeffreys developed a DNA profiling technique using a VNTR technique involving multi-locus profiling and later followed by single-locus profiling.

  • This technique led to the solving of a double murder that had been committed in Leicestershire in the 1980s.

• The case was the first to apply DNA evidence to a criminal investigation. During the investigation, DNA profiling not only identified the true perpetrator but it also excluded an innocent suspect.

• The most important ability of the technique is to reveal far greater individual variability in DNA than can be revealed by antigen and protein polymorphic markers.

• In the mid-1980s, Kary Mullis and his coworkers developed the polymerase chain reaction (PCR) technique, which amplifies a small quantity of DNA.

  • Mullis’s invention had a powerful impact on molecular biology and earned him a Nobel Prize in 1993.

• The application of PCR-based assays makes forensic DNA analysis possible when only minute quantities of DNA can be recovered from a crime scene, for example, from hairs and cigarette butts.

  • The first forensic application of a PCR-based assay utilizing SNPs at the HLA-DQA1 locus was developed in 1986.

  • Amplified fragment length polymorphism (AFLP) at the D1S80 locus has also been implemented in forensic laboratories

  • D1S80 locus: A small-size VNTR marker that can be amplified by PCR.

  • The HLA-DQA1 and AFLP assays were used for some years until the introduction of STR assays.

  • STRs have a number of advantages compared with VNTRs.

  • STRs can be amplified by PCR because of their smaller size, which greatly increases the sensitivity of the assay.

  • STR markers are as highly variable as VNTRs.

• In 1995, the United Kingdom established the first national DNA database for criminal investigations.

• By the end of 1998, several other countries, including the United States, had created their own national DNA databases.

  • The United States has selected 13 STR loci for the Combined DNA Index System (CODIS). These national DNA databases play important roles in solving criminal cases.

• mtDNA: It is maternally inherited genetic material and is therefore particularly useful for human identification.

  • mtDNA typing can be useful for analysis when nuclear DNA is severely degraded or present in very limited amounts, such as in cases involving decomposed human remains.

• Y-chromosomal markers: These are paternally inherited so they can be used for paternity testing.

  • These markers are also very useful in analyzing DNA from multiple contributors in sexual assault cases.

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