Paternity Testing

PATHCHAT Edition No. 67
May 2020
Please contact your local Ampath pathologist for more information.

Author:

• Dr. Irma Ferreira (PhD in Biochemistry)

Introduction

✅ What is Paternity Testing?

• Paternity testing (also known as parental or human identity testing) determines biological parent-child relationships.
• DNA testing is the most accurate method for paternity determination.
• Older methods included:
  • ABO blood group typing.
  • Analysis of proteins and enzymes.
  • Human leukocyte antigen (HLA) typing.

✅ Accuracy of DNA Testing:

• Probability of paternity is 0% if the alleged father is not the biological parent.
• Probability of paternity exceeds 99.8% if the alleged father is biologically related.
• Current technologies used:
  • Polymerase Chain Reaction (PCR).
  • Restriction Fragment Length Polymorphism (RFLP).

📌 DNA-based testing provides definitive paternity results with high reliability.

Indications for Paternity Testing

✅ Common Scenarios Requiring Testing:

• Disputed paternity cases (e.g., maintenance claims).
• Uncertainty due to multiple sexual partners.
• Investigations into switched-at-birth incidents.
• Inheritance and legacy disputes.
• Fraudulent claims (e.g., Road Accident Fund, immigration cases).
• Identification of human remains after disasters (e.g., earthquakes, Twin Towers attack).
• Criminal investigations.

📌 Paternity testing has forensic, legal, and personal applications.

DNA Paternity Testing Process

✅ Steps in Paternity Testing:

1. Sample Collection:
   • Buccal swab (preferred method).
   • Saliva or blood samples.
2. DNA Extraction:
   • PCR amplification of specific DNA regions.
3. Genetic Profiling:
   • Capillary electrophoresis separates DNA fragments.
   • Allele sizes are compared between child and alleged parent(s).
4. Comparison & Statistical Analysis:
   • If DNA profiles do not match → Exclusion of paternity.
   • If DNA profiles match → Probability of paternity calculation.

📌 DNA paternity testing is simple, highly accurate, and legally admissible when proper chain-of-custody procedures are followed.

How DNA Determines Paternity

✅ Basic Principles of Genetic Inheritance:

• Each individual inherits one chromosome from each parent.
• Paternity testing examines markers on autosomal chromosomes.
• Sex chromosomes determine gender and are not used for paternity assessment.

✅ Short Tandem Repeats (STRs) in DNA Testing:

• STRs are non-coding regions containing repeated DNA sequences.
• Each marker (locus) has a unique number of repeats.
• More markers = higher accuracy.

📌 STR analysis is the gold standard for paternity and forensic DNA testing.

Interpreting Paternity Test Results

✅ Key Steps in Interpretation:

• Each person has two copies of an allele at each STR marker—one from the mother and one from the father.
• Child’s alleles must match those inherited from the biological parents.

✅ Exclusion of Paternity:

• If two or more loci do not match between the alleged father and child, paternity is excluded.

✅ Paternity Index & Probability of Paternity:

• Each locus (STR marker) is assigned a Paternity Index (PI), which measures how strong a genetic match is.
• The Combined Paternity Index (CPI) is calculated by multiplying all individual PIs.
• Probability of paternity is then determined as:

Probability=CPICPI+1\text{Probability} = \frac{\text{CPI}}{\text{CPI} + 1}

✅ Legal Standard for Paternity:

• South African National Accreditation System (SANAS) requires a probability of paternity >99.8% for legal proof.

📌 A high paternity probability (>99.99%) strongly supports biological relatedness.

Specimen Collection & Chain-of-Custody

✅ Sample Requirements:

• At least 1 mL of whole blood (EDTA tube), blood spot on filter card, or buccal swab.
• No special requirements for blood samples.
• Buccal swab collection requires fasting for 30 minutes before sampling.
• Samples transported at ambient temperature.

✅ Chain-of-Custody for Legal Paternity Testing:

• Ensures the integrity of results for legal use.
• Sample collection must be performed by an independent professional.
• All tested parties must provide valid identification.
• Strict documentation is maintained throughout the process.

📌 Legal paternity tests require rigorous documentation to be admissible in court.

Quality Control & Accreditation

✅ Why Choose Ampath for Paternity Testing?

• SANAS-accredited laboratory ensures high accuracy and reliability.
• Internationally validated testing kits used.
• Automated genetic analyzers minimize errors.

📌 Ampath follows globally recognized standards for paternity testing.

Key Takeaways for Clinicians & Legal Practitioners

✅ Paternity testing uses STR DNA analysis, which is >99.99% accurate.
✅ Buccal swabs are the preferred sample type due to ease of collection and stability.
✅ A probability of paternity ≥99.8% is required for legal confirmation.
✅ Chain-of-custody documentation is necessary for legally valid results.
✅ Paternity tests have applications in legal disputes, inheritance claims, forensics, and personal verification.

📌 DNA testing is the definitive method for establishing biological parentage.