Lecture 9_Part 4

CODIS Overview

  • Definition: CODIS stands for the Combined DNA Index System.

    • Purpose: It is the core of the national DNA database, established and funded by the Federal Bureau of Investigation (FBI).

    • Functionality: Designed to enable public forensic DNA laboratories to create searchable DNA databases of authorized DNA samples or profiles.

    • Global Reach: Allows laboratories throughout the U.S. and parts of the world to share and compare DNA data.

  • Central Database:

    • Maintains a database of DNA profiles from all user laboratories.

    • Conducts weekly searches of the DNA profiles in the national database, known as the National DNA Index System (NDIS).

    • Matches resulting from searches are automatically returned to the laboratories that submitted the profiles.

Misuse of CODIS Terminology

  • Common Misuse: The term CODIS is often misused by law enforcement, prosecutors, and forensic practitioners when referring to a DNA database in general.

  • Database Levels: The CODIS index system has three levels of operation.

    • Misunderstanding can arise when "DNA database" is used without specific reference to the level.

    • Some profiles allowed in a state's database may not be permitted in the national database.

  • CODIS Lab Misnomer: Referring to a state laboratory as a "CODIS lab" is a misnomer as CODIS includes various profiles, not just those from convicted offenders.

CODIS Functionalities

  • Profile Requirements: Requires either:

    • Greater than four RFLP markers, or

    • 13 core STR markers (now increased to 20).

  • Core STR Loci:

    • Original 13 core loci defined with their chromosomal positions.

    • The 13 core loci include amelogenin for sex discrimination.

  • Standardization Project:

    • The CODIS STR standardization project developed kits primarily, AmpFlSTR and PowerPlex, to include core loci and amelogenin.

    • Collaboration with the forensic science community allowed for beta testing and feedback, benefiting technology development.

CODIS Software Development

  • Initial Creation: CODIS software was created in 1990 as a pilot project by the FBI.

    • Early focus was on analysis and storage of RFLP profiles.

  • Evolution: With advancements in forensic DNA and computer technologies, software was upgraded to address the needs of forensic science.

  • Current Software: The current CODIS software mainly stores and searches STR profiles.

    • A separate version exists for mitochondrial DNA profiles.

Case Examples

  • Example Case: In 1998, the Fort Worth crime lab linked a man in Abilene to five offenses after a cold hit through CODIS.

    • Two females aged ten and eleven had been assaulted; their evidence was used in the case.

DNA Profiles in CODIS

  • Profiles Included:

    • Profiles from violent felons, crime scene samples, and convicted offenders.

  • Global Variances: Laws vary by state and country regarding who is eligible for database entry.

    • Current estimates indicate about 2,500,000 profiles in the database.

  • Effectiveness: CODIS is a powerful tool for linking crimes and generating investigative leads with cold hits.

Challenges and Ethical Concerns

  • Issues with DNA Mixtures: DNA mixtures and degraded profiles can lead to false matches; require careful examination.

  • Ethical Concerns:

    • Database inclusion laws concerning innocent individuals raise ethical considerations.

    • The presumption of innocence is called into question for arrestees.

  • Prosecution Rates: The rate of successful prosecutions based on DNA matches is around 10%, which challenges the perceived effectiveness of the system.

DNA Analysis Technology

  • Key Equipment: Applied Biosystems 310 Genetic Analyzer is a central device in DNA profiling.

    • Features: Robotic control, sample cleaning, and efficient processing capabilities.

  • Data Output: DNA test results can be obtained quickly with high accuracy.

    • Typical probability for a random match can be one in three trillion.

Outcomes of DNA Testing

  • Possible Outcomes:

    • No match (exclusion)

    • Inconclusive result (data insufficient to conclude)

    • Match (statistical interpretation necessary)

  • Statistical Interpretations: Different statistics are debated for understanding the probability of profiles matching.

Hardy-Weinberg Theorem

  • Basis: Utilized to predict genotype frequencies under conditions of random mating.

    • Genotype probability computed via the product of parental allele frequencies.

  • Example: Given frequency values for alleles A1: 0.3, A2: 0.5, A3: 0.2:

    • Probability for homozygous A1A1 = (0.3imes0.3=0.09)(0.3 imes 0.3 = 0.09)

    • Probability for A2A3 = (2imes(0.5imes0.2)=0.2)(2 imes (0.5 imes 0.2) = 0.2)

DNA Profile Match Probability

  • Calculation Example: The matching probability when using 13 CODIS markers and specific allele frequencies.

    • For alleles with frequencies of 0.9 (4 occurrences) and 0.2 (9 occurrences):

    • Match probability becomes (0.94imes0.29)(0.9^4 imes 0.2^9) resulting in approximately one in 29 billion.

  • Inbreeding Adjustments: Inbreeding coefficients can complicate calculations, affecting match probabilities.

    • The typical inbreeding coefficient is less than 0.05 in most human populations.

Population Genetics Data

  • D13 Allele Frequency: Variation across ethnicities is demonstrated through allele frequency data.

    • Different repeat lengths have distinct frequencies.

Conclusion

  • Frequency Calculation Process: DNA profile frequencies calculated based on population samples to determine the likelihood of matches.

    • Minimum allele thresholds are enforced to ensure valid samples are used in analysis.

  • Performance of CODIS: Statistics from the national DNA database indicate the effectiveness in resolving cases.

    • As of 2017, there were over 52,546 hits with various crimes being investigated, showcasing CODIS as a critical tool in forensic science.