![]() Either way, allele frequencies obtained from a population study have to be used to assign allele probabilities in the likelihood ratio method. If Linkage Equilibrium is also assumed, this model is also known as the Product Rule model. For F S T = 0 the Balding-Nichols model reduces to a population genetic model assuming Hardy Weinberg Equilibrium. It is common practice among users of probabilistic genotyping software to employ the Balding-Nichols model with a parameter F S T > 0 (also called θ) chosen sufficiently large to obtain conservative estimates of the evidential weight. A population genetic model is needed to assign a likelihood ratio for a pair of hypotheses. Is used to evaluate support for the POI having contributed to the mixture. When F S T > 0 is used in the likelihood ratio calculations, as is recommended, all compared approaches become conservative on average to varying degrees. Population stratification methods may also be non-conservative in some cases. We demonstrate that the common practise of taking a minimum likelihood ratio across populations is not always conservative when F S T = 0. The likelihood ratios are also compared to the minimum likelihood ratio across populations. A simulation study is used to determine the effect of either assumption on the likelihood ratio. The prior distribution can be constrained such that all sample donors are from the same ethnic group, or all permutations can be considered. In this method, likelihoods are weighted across a prior distribution that assigns sample donors to ethnic groups. We first present a method for evaluating DNA profiling evidence using probabilistic genotyping when the contributors may originate from different ethnic groups. We revisit this issue in the context of potentially low-level and mixed samples where the contributors may originate from different populations and study likelihood ratio behaviour. For high template single source profiles, any difference between the methods is immaterial as far as conclusions are concerned. One option is to present a minimum likelihood ratio across populations another is to present a stratified likelihood ratio that incorporates a weighted average of likelihoods across multiple populations. More accurate estimates can be obtained by explicitly modelling different populations. For two-person mixtures without dropout, it has been reported that conservative estimates can be obtained by using the Person of Interest’s population with a θ value of 3%. When multiple populations may be relevant, a choice has to be made. ![]() It is common practice to evaluate DNA profiling evidence with likelihood ratios using allele frequency estimates from a relevant population. ![]()
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