Distinguishing kinship from genealogical likelihoods

Abstract

Kinship analyses, besides paternity, are increasingly common in forensic genetics routine. The evaluation of the probability of a disputed kinship between individuals using unlinked autosomal markers employs a likelihood ratio format: the probability of the observations assuming the kinship is true against the probability of the same observations under the alternative hypothesis of a different degree of relationship (usually its absence). However, with very few exceptions (identity, first degree relatedness—paternity or maternity, full-brotherhood or unrelated), the algebraic expressions are not unique for a given kinship, the same probability value being obtained for various genealogical constellations. Then, in a likelihood ratio, what is compared are sets of possible kinship configurations, which are defined by identity-by-descent probabilities known as Jacquard's coefficients. Pedigrees with the same identity-by-descent probabilities are thus undistinguishable by unlinked autosomal markers. In the literature the most common reported undistinguishable kinships, by autosomal transmission, are avuncular, half-sibs and grandparent–grandchild. Therefore, when reporting the results of genetic information upon a specific kinship case a particular caution must be taken in order to avoid misconceptions and fallacious inferences.