Detailed Study Notes on DNA Analysis and Forensic Applications

Polymorphism in Genetic Markers

  • Genetic markers exhibit polymorphism, meaning they have multiple forms or alleles present in a population.

  • This polymorphism allows for effective amplification using Polymerase Chain Reaction (PCR).

  • A panel of 16 markers can be used to determine both the sex and identity of an individual with high certainty.

  • More than 10 markers provide a strong basis for calculating random match probabilities accurately.

Probability of Random Matches

  • For a specific marker, the chance of finding a particular allele by random sampling needs to be calculated.

  • Example: If a chosen marker has a probability of hitting an allele of $0.0099 ext{%}$, in a population of 100 individuals, approximately 9 people may potentially have that genotype by chance.

  • This indicates that using only one marker is insufficient for reliable identification.

Importance of Using Multiple Markers

  • Using multiple markers increases the odds against finding two individuals with the same genotype by chance.

  • The desired outcome is to reach a point where the combined probability of matching a particular genotype falls below the world's population, which is significant.

  • If the probability is lower than $10^{-10}$, it strongly suggests that the match is unique to the specific individual unless identity fraud is involved.

Calculation Example

  • Random match probability calculated yielded a figure of $1/5,000,000$ or $5 imes 10^{-6}$.

  • The target analysis population considered was 20,000,000.

  • A common misconception is directly stating the probability that evidence belongs to a suspect; rather, it indicates the rarity of the pattern.

Genetic Markers and Genotype Patterns

  • An example is presented using Amelogenin (Amel), which indicates sex by differentiating X and Y chromosomes (peaks occur for each allele).

  • For given microsatellite genotypes:

    • Person 1: $12, 15$

    • Person 2: $14, 16$.

  • The task involves calculating the likelihood of finding an identical genotype in the global population.

Using Allele Frequency Tables

  • To perform probability calculations effectively, allele frequency tables for specific populations (e.g., Caucasian, Hispanic, African American) are used.

  • For a given suspect type, allele frequency must be referenced from the table corresponding to their demographic background.

  • An example computation demonstrates that using just four markers can yield a random match probability of $10^{-5}$, which is significant.

Ethical Implications of DNA Evidence

  • The power of PCR can lead to misinterpretations of evidence, as in noted cases where DNA evidence wrongfully convicted individuals.

  • Notably, a man was imprisoned due to DNA improperly associated with him because of procedural errors.

  • There is potential for DNA contamination, warranting caution in forensic investigations.

Potential for Misinterpretation and Misuse

  • DNA can inadvertently link innocent individuals to crime scenes through transferred traces, such as paramedic gloves not being changed from a patient to a crime scene.

  • The reliability of DNA as the sole evidence can create false confidence in judicial convictions, thus necessitating robust support for calculations.

Practical Exercise

  • The class is tasked to perform calculations with provided allele frequencies and genetic markers from a hypothetical crime scene.

  • The evidence sample shows markers aligning with those of a primary suspect and involves five specific markers for which probabilities will be determined.

  • Students require access to allele frequency tables and must identify the relevant allele frequencies to carry out the assignment successfully.

Note: The document included mentions of genetic data and PCR's impact on identifying genes that can either clarify or complicate forensic investigations.