Forensic DNA Analysis and PCR

Identity Matching in Forensics

DNA fingerprinting is utilized for identification, split into two main applications:

• Paternity Analysis: Determines the biological father through gamete analysis.

  • Sperm is haploid ($n$), meaning it carries one set of chromosomes from the father's diploid ($2n$) cells.

  • A father donates one of his two homologous chromosomes to each sperm (e.g., one of his two chromosome 1s).

  • The diversity in gametes is vast, with approximately $2^{23}$ possible combinations of chromosomes due to independent assortment and recombination (crossing over) during meiosis.

  • Statistical analysis is crucial to determine the likelihood of a paternal match.

• CSI (Crime Scene Investigation) / Forensic Identification: Used for direct, exact matching of biological samples.

  • Scenarios include crime scenes, disaster victim identification (wartime, fires, homicides), or kidnappings.

  • The goal is to find an exact match between a sample (e.g., blood at a crime scene) and a known individual's DNA profile.

  • Unlike paternity which deals with haploid gametes, CSI matching involves diploid DNA, requiring markers from both homologous chromosomes to match precisely.

Forensic Disciplines

• Forensic Chemistry: Identifies chemical signatures, such as matching explosive residues to their manufacturing source.

• Forensic Biology: Focuses on biological materials for identification.

Traditional Forensic Tools vs. DNA

Historically, and still in use today, various tools aid forensic investigation:

• DNA: The primary tool for modern forensic biology, used to generate a unique DNA profile.

• Blood:

  • Blood Types (ABO, MNS, Rh, etc.): Based on specific sugars and glycolipids on red blood cell surfaces.

    • The Rh factor (positive/negative) indicates the presence or absence of a specific antigen, critical for blood transfusions and managing Rh incompatibility in pregnancy (e.g., Rh- mother carrying an Rh+ child).

    • There are approximately $20$ known blood group systems.

    • While useful for exclusion, the statistical power of blood typing is limited. Combining all $20$ groups, there's roughly a $1$ in $1000$ chance of matching two random individuals.

    • In a population of $250,000$, this could mean $250$ individuals share the same blood profile, which is insufficient for unique identification.

  • Fingerprints: Unique ridge patterns, requiring periodic updates (e.g., every $5$ years).

    • The concept of