The Sibship Index can’t discriminates Half-Sibs from Full-Sibs; a new algorithm, including maternal genotype, can do it

Abstract

Sibship DNA testing is conducted in order to determine if two subjects share one or both biological parents: Full-sibs have both parents in common, whereas Half-sibs have one parent in common, either the mother or the father. When an alleged father is not available for a paternity test, sibship testing is one way to determine family relationships. Results of a sibship test have also been accepted as proof in Social Security benefit and other inheritance claims. To analyze the possibility that the siblings share two common parents versus only one common parent, the ratio of the full sibship index versus half sibship index is computed: a sibship index less than 1, provides genetic evidence which does not favor a full-sibling relationship. In this paper we will demonstrate that the actually used Sibship Index is unable to distinguish Full-sibs from Half-Sibs and the only way to distinguish them is to use a new Index including the genotype of the Mother, that, usually, is the known common parent.HighlightsUsually used Sibship Index (SI) greatly overestimates likelihood ratios;Rising up the number of investigated STR, all Half-Sibs are declared Full-Sibs;The only way to avoid this overestimate is to use a Sibship Index Corrected by the common parent genotype, that usually is the mother (SICMG); all likelihood ratio formulas can be reconducted to five scenarios;For both SI and SICMG there is a better cut-off than 1 to discriminate Half-Sibs: this cut-off is computed for all commercial available forensic STR kit.