### Description

The Sibship Index (SI) can help identify the sibling relationship between 2 persons, separating full from half siblings. This method is based on two person phenotypes and Bayes's theorem.

sibship index =

= (probability two persons truly related) / (probability two persons unrelated)

 Genotype Person #1 Genotype Person #2 Formula for Sibship Index AA AA (k2 + (2 * k1 * a) + (k0 * (a^2))) / (a^2) AA AB (k1 + (k0 * a)) / a AA BB k0 AA BC k0 AB AB (k2 + (k1 * a) + (k1 * b)+ (k0 * 2 * a * b)) / (2 * a * b) AB AA (k1 + (k0 * a)) / a AB AC (k1 + (2 * k0 * a)) / (2 * a) AB CD k0

 Sibship full half k0 0.25 0.50 k1 0.25 0.25 k2 0.25 0

where:

• a = frequency of allele A

• b = frequency of allele B

ratio of full to half sibship =

= (SI for full sibling) / (SI for half sibling)

Interpretation:

• If the full SI > half SI (ratio > 1), then the 2 people are probably full siblings. In the series of Wenk (2000), 72% of full siblings and 8% of half siblings had a ratio > 1.0.

• If the full SI < half SI (ratio < 1), then the 2 people are probably half siblings. In the series of Wenk (2000), 28% of full siblings and 92% of half siblings had a ratio < 1.0.

• Examination of several alleles may be necessary to determine the sibship of 2 persons. Full siblings will share 2 alleles at several loci (about 25% of the total), while half siblings will share 2 alleles only rarely or not at all.

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