In a paternity dispute, the Paternity Index (PI) of a particular STR marker is 15. The Combined Paternity Index (CPI) calculated from 10 independent loci is 2,500,000. What is the correct interpretation?
- A The probability that the alleged father is NOT the biological father is 1 in 2,500,000
- B The alleged father is 2,500,000 times more likely to be the biological father compared to a random unrelated man from the same population ✓
- C The alleged father shares 2,500,000 alleles with the child
- D The match probability of the child's profile in the population is 1 in 2,500,000
Correct answer: B. The alleged father is 2,500,000 times more likely to be the biological father compared to a random unrelated man from the same population
Explanation
The Paternity Index (PI) for each locus expresses the likelihood ratio that the alleged father contributed the allele versus a random man from the same population. The Combined Paternity Index (CPI) is the product of individual PIs and is equivalent to the likelihood ratio for the entire profile. A CPI of 2,500,000 means the alleged father is 2.5 million times more likely to be the biological father than a random man. The Probability of Paternity (W) converts this to a percentage: W = CPI / (CPI + 1).
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.