Familial DNA searching in forensic databases identifies a partial match to a crime scene profile. The database hit belongs to a convicted offender's sibling, not the offender himself. The probability that the crime scene profile belongs to a full sibling of the database entrant compared to an unrelated individual is approximately:

• A Identical — siblings cannot be distinguished from unrelateds by STR matching alone
• B Siblings share on average 50% of alleles, greatly increasing the likelihood ratio ✓
• C Siblings always match all STR loci because they have identical DNA
• D Only monozygotic twins share alleles at multiple STR loci

Explanation

Full siblings share on average 50% of their genome, which means their STR profiles will share more alleles than two unrelated individuals. Familial DNA searching exploits this elevated allele-sharing to generate investigative leads, not identifications. A likelihood ratio is calculated comparing the hypothesis 'these profiles are from full siblings' versus 'these are from unrelated persons', and this ratio is substantially greater than 1. Siblings are not identical — only monozygotic twins have identical DNA. Full STR profiles can still distinguish siblings in most cases.

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.