Lecture 5a - Paternity Testing

Introduction to Forensic Statistics and Applied Population Genetics

  • Course Overview

    • Focus on paternity, kinship, match probabilities, DNA mixture analysis.

    • Transition from theoretical aspects to practical applications in forensic science.

    • Modules 6 (parentage testing) and 7 (broader kinship applications).

Module Six: Paternity Testing

Definition of Parentage Testing

  • A series of genetic and statistical assessments aimed at determining or estimating the parentage of an individual.

  • Purpose in forensic science: Include or exclude a tested man/suspect from the population of potential fathers.

  • Applications:

    • Human context: Testing to determine the biological father of a child.

    • Animal context: Determining sires or dams of offspring.

Key Concepts in Parentage Testing

  • The Biological Trio:

    • Typically involves samples from the mother (known), child (known), and a potential father (unknown).

  • Reverse Parentage Analysis:

    • Involves testing remains or non-identifiable samples against known genotypes of possible parents.

  • Example Scenarios:

    • Testing parentage in humans or animals (e.g., pups or kittens).

Mendelian Inheritance in Parentage Testing

Basics of Mendelian Testing

  • Assumptions based on Mendelian inheritance laws.

  • Example scenario:

    • Mother has alleles P and Q; father has alleles R and S. Child inherits one allele from each parent.

  • Illustration of allele inheritance with pedigree examples:

    • Example with mom as genotype 28,30 and dad as 31,32. Results in offspring with alleles inherited from both.

Paternal Obligate Alleles

  • Definition: The alleles that a child must inherit from the biological father.

  • Importance in testing: Must show matching alleles between offspring and suspected father.

Outcomes of Paternity Testing

Possible Results

  • Exclusion: Alleged father can be shown not to be the biological father based on allele mismatch at two or more loci due to high mutation rates in microsatellites.

  • Inclusion: Matches between a child's obligate paternal alleles and those of the alleged father suggest he cannot be excluded as a potential biological father.

Handling Single Locus Mismatches

  • Explanation of possible scenarios:

    • Alleged father is not the biological father.

    • A mutation occurred in the father's allele during germline transmission.

    • Alleged father is not the true father but might be a blood relative of the biological father.

General Approach to Parentage Analysis

Recommended Procedures

  • Follow guidelines from the American Association of Blood Banks for handling mutations and inconsistencies in genetic testing.

  • Calculate the paternity index (PI) for any single inconsistencies and include this in the cumulative calculation of the Combined Paternity Index (CPI).

Step-By-Step Example of Parentage Testing

Initial Analysis

  • Description of a process where known maternal and offspring genotypes are analyzed:

    • Example involves mother having genotype 10,12 and child having genotype 10,19 (implying the paternal allele is 19).

Comparative Analysis

  1. First line evaluation:

    • Child's paternal alleles match the alleged father's categories.

  2. Second line evaluation:

    • Non-match scenario with alleles indicating exclusion potential.

  3. Third line evaluation:

    • Rare allele scenarios suggesting identical maternal alleles allow for possible inclusion.

Using DNA Testing to Answer Specific Relationship Questions

Example Case: Is Mike the Father of Brian?

  • Analyzing three microsatellite markers for relationships.

    • VWA: Brian (13,14); Mother's alleles indicate the 14 must come from her; inclusion of 13 as paternal obligate.

    • D12: Brian (22,23); Matching analysis indicates inclusion for 22.

    • D21: No paternal exclusivity as maternal allele overlaps allowing father inclusion at 31.

Estimating Weight of Evidence in DNA Cases

Statistical Weight Calculation

  • Importance of statistical analysis beyond mere inclusion/exclusion for effective data utilization.

  • Focused question: How probable is it that the alleged father is Brian's true biological father compared to random males in the population?

Competing Hypotheses

  • Two hypotheses analyzed:

    • Hypothesis 1: Alleged father is the true biological father.

    • Hypothesis 2: Another random male in the population is the father.

Bayesian Probability in DNA Evidence

  • Use of Bayes Theorem to calculate the posterior odds based on observed data.

  • Focus on likelihood ratios comparing competing hypotheses:

    • Numerator: Probability based on DNA profiles under the assumption of a true biological trio.

    • Denominator: Probability under the alternative hypothesis of a random male being the biological father.

Transmission Probability Analysis

  • Importance of Mendelian inheritance percentages during allele transmission from father to child.

  • Explanation of factors affecting probability calculations in genetic inheritance.

Paternity Index Calculation

Likelihood Ratio Formula

  • The Paternity Index (PI) defined as the ratio of the probabilities of the two competing hypotheses:

    • Numerator: True biological trio probability.

    • Denominator: Probability that child is unrelated to the alleged father.

  • Example Calculation:

    • Case of mother (genotype AB), child (genotype AC), and alleged father (genotype CD).

Conclusion and Further Steps

  • Next lecture will provide practical steps for Paternity Index calculations using known allele frequencies and genotypes.