Specificity of sibship determination using the ABI Identifiler multiplex system

Fifty known siblings and fifty unrelated pairs were genotyped using the ABI Identifiler STR system and sibship indices computed for each pair. Combined sibship indices (CSIs) for the known siblings ranged from less than 10 to greater than 1 billion. CSIs for the unrelated pairs ranged from 4.5 x 10(-8) to 0.12. In the known sibling group the percentage of loci where both alleles matched was approximately 40%, while the percentage of loci where neither matched was approximately 10%. In the non-sibling group, the percentage of loci where both alleles matched was approximately 6%, while the percentage of loci where neither matched was approximately 45%. Interestingly, the percentage of loci where a single allele matched was the same in both the known siblings and unrelated pairs, approximately 50%.