In a paternity dispute, the Combined Paternity Index across tested STR loci is 5,000. Using the conventional prior probability of paternity of 0.5 applied in Indian laboratories, the calculated probability of paternity is closest to:

• A 50.00 percent
• B 95.00 percent
• C 99.00 percent
• D 99.98 percent ✓

Explanation

Probability of paternity W equals CPI divided by CPI plus 1 when the prior odds are 0.5, so W = 5000 divided by 5001, which is approximately 0.9998 or 99.98 percent. Option C would correspond to a CPI near 100, and option B to a CPI of 19. The prior of 0.5 means the man is assumed equally likely before testing, a convention followed in Indian paternity reporting.

Reference: Parikh's Textbook of Medical Jurisprudence, Forensic Medicine and Toxicology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.