Genetic laboratories may be asked to determine if 2 persons are sibs for whom the parents are not available for testing. Tzeng et al describe a cumulative sibship index (CSI) to help determine if 2 persons were sibs, using a panel of highly polymorphic short tandem repeats (STR). It is calculated by multiplying the sibling indices (SI) for the individual alleles together.


cumulative sibship index =

= PRODUCT(sibship indices for alleles tested)


Panel used for testing: 15 polymorphic, autosomal, codominant uplinked STR loci.



• Using the 15-loci panel, sibs usually show a CSI >= 3.0. Random unrelated pairs usually had a CSI < 1.0 and none were > 3.0.

• Rarely true siblings may share 0 alleles. As opposed to parentage analysis, there are no obligatory parental alleles per locus to be shared at the odds of 0.25.

• A few known sibs (about 4%) showed a CSI < 3.0. In this group complementary mitochondrial DNA sequence analysis with or without Y-chromosome-specific STRs may be necessary to demonstrate the sibship if parents are not available.

• Occasionally random, unrelated pairs may show a CSI >= 3.0 if 9-12 loci are examined. This is because there are no obligatory parental alleles from which logical deduction can be made to exclude sibhsip. The full panel of 15 loci are needed to identify the sibship in these patients correctly.



• Using 15 loci and a cutoff of 3.0, the CSI was 96% sensitive (4% false negative) and 100% specific (0% false positive).


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