Distinguishing full siblings from half-siblings in limited pedigrees

BACKGROUND: Full siblings were compared with half-siblings to observe how well the two relationships could be distinguished by informative tests. STUDY AND DESIGN METHODS: Parentage analysis ascertained 25 pairs of full siblings and 25 pairs of half-siblings. The pairs were then examined for the sharing of alleles at three independent variable number of tandem repeat (VNTR) loci. A sibling index (SI) and a half-sibling index (HSI) were calculated for each pair, and an SI:HSI ratio was determined. RESULTS: The SI:HSI ratio favored full siblings in 18 of 25 full sibling pairs. The SI:HSI ratio exceeded 100 in 8 of those 25 pairs. Although the ratio favored half-siblings in 23 of 25 half-sibling pairs, as was expected with the use of only 3 loci, it exceeded 0.1 in all 25 pairs. CONCLUSION: Study of more than three highly informative loci is required to improve the identification of full siblings and might well permit the identification of half-siblings.