Inferring relationships between pairs of individuals from locus heterozygosities

We show that these probabilities (zi) depend on locus heterozygosity (H), and are scarcely affected by variation of the distribution of allele frequencies. This allows us to obtain empirical curves relating zi's to H for a series of common relationships, so that the likelihood ratio of a pair of relationships between any two individuals, given their genotypes at a locus, is a function of a single parameter, H. 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 from special cases, so that the likelihood ratio of the more common relationships between pairs of individuals may be obtained by looking at tabulated zi values.A simple method is provided, which may be used by any scientist with the help of a calculator or a spreadsheet to compute the likelihood ratios of common alternative relationships between pairs of individuals.The usual, long-established method of inferring relationships between individuals in forensic genetics is based on the population frequencies of the observed alleles and on the conditional probabilities of the observed genotypes, given two alternative hypothesized relationships [1]. In the more frequent instances, such as paternity testing of trios, or similar cases with deficiencies, well-known formulas have come into common use [2,3]. However, in more complex cases where, for example, the relationship between pairs of individuals from large samples is under investigation, or where the DNA profile of a number of related individuals is known and we want to know the most likely relationships among them, these calculations become exceedingly complex. Each particular problem requires the development of specific formulas, necessitating either the expertise of highly specialized profe