2 Distribuzione del numero di alleli condivisi da coppie di fratelli e di non-parenti tipizzati rispettivamente per 5, 9 e 13 markers
3 Potere di discriminazione di un test basato sull’IBS e una batteria di 13 STR Circa il 93% delle coppie di fratelli sono identificate con P ~ 0.02L’85% con P < 0.01Il 73% con P < 0.002Il 57% con P <
4 Allele sharing in first-degree and unrelated pairs of individuals in the Ge.F.I. AmpflSTR® Profiler Plustm database
5 Inferring relationships between pairs of individuals from locus heterozygosities (H) The probabilities that two individuals share 0, 1 or both alleles at a locus (zi), given their biological relationship, are approximate functions of H, meaning that the likelihood ratio of alternative relationships between any two individuals, given their genotypes at a locus, is a function of a single parameter.Application to large samples of mother-child and full-sib pairs shows that the statistical power of this method to infer the correct relationship is not much lower than the exact method.Analysis of a large database of STR data proves that locus heterozygosity does not vary significantly among Caucasian populations, apart for special cases, so that the likelihood ratio of the more common relationships between pairs of individual may be obtained by looking at tabulated zi values.
7 Probabilities of zi for 18 loci and four common relationships
8 ConclusionsThe IBS method presented here may be conveniently used as a preliminary approach to investigate the relationship existing between any pair of individuals. It can be applied by anybody using a desk calculator or a spreadsheet.Furthermore, the IBS method’s using of estimates of H rather than of allele frequencies makes the IBS method particularly attractive in all those cases where ethnicity pose a problem, since H varies less across ethnicities.In certain circumstances, the results of the IBS method may even be accepted without further analyses, since the LRs are highly correlated with those calculated by the exact method.