Distinguishing full siblings from half-siblings in limited pedigrees

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.

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.

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.