Department of Healthcare Surveillance and Bioethics, Catholic University, Rome, Italy.
PLoS One. 2021 Oct 15;16(10):e0247344. doi: 10.1371/journal.pone.0247344. eCollection 2021.
This study introduces a methodology for inferring the weight of the evidence (WoE) in the single nucleotide polymorphism (SNP)-typed DNA mixtures of forensic interest. First, we redefined some algebraic formulae to approach the semi-continuous calculation of likelihoods and likelihood ratios (LRs). To address the allelic dropouts, a peak height ratio index ("h," an index of heterozygous state plausibility) was incorporated into semi-continuous formulae to act as a proxy for the "split-drop" model of calculation. Second, the original ratio at which a person of interest (POI) has entered into the mixture was inferred by evaluating the DNA amounts conferred by unique genotypes to any possible permutation of any locus of the typing protocol (unique genotypes are genotypes that appear just once in the relevant permutation). We compared this expected ratio (MRex) to all the mixing ratios emerging at all other permutations of the mixture (MRobs) using several (1 - χ2) tests to evaluate the probability of each permutation to exist in the mixture according to quantitative criteria. At the level of each permutation state, we multiplied the (1 - χ2) value to the genotype frequencies and the h index. All the products of all the permutation states were finally summed to give a likelihood value that accounts for three independent properties of the mixtures. Owing to the (1 - χ2) index and the h index, this approach qualifies as a fully continuous methodology of LR calculation. We compared the MRs and LRs emerging from our methodology to those generated by the EuroForMix software ver. 3.0.3. When the true contributors were tested as POIs, our procedure generated highly discriminant LRs that, unlike EuroForMix, never overcame the corresponding single-source LRs. When false contributors were tested as POIs, we obtained a much lower LR value than that from EuroForMix. These two findings indicate that our computational method is more reliable and realistic than EuroForMix.
本研究介绍了一种推断法医学感兴趣的单核苷酸多态性 (SNP) 型 DNA 混合物中证据权重 (WoE) 的方法。首先，我们重新定义了一些代数公式，以接近似然和似然比 (LR) 的半连续计算。为了解决等位基因丢失问题，我们将峰高比指数（“h”，杂合状态可信度指数）纳入半连续公式中，作为计算“分裂丢失”模型的替代方法。其次，通过评估赋予分型方案任何可能的排列的任何基因座的独特基因型的 DNA 量，推断出感兴趣的个体（POI）进入混合物的原始比例（MRex）（独特基因型是指在相关排列中仅出现一次的基因型）。我们使用几种（1-χ2）检验将这个预期比例（MRex）与混合物中所有其他排列的混合比例（MRobs）进行比较，以根据定量标准评估每个排列存在于混合物中的概率。在每个排列状态的水平上，我们将（1-χ2）值乘以基因型频率和 h 指数。最后，将所有排列状态的乘积相加，得出一个考虑混合物三个独立特性的似然值。由于（1-χ2）指数和 h 指数，这种方法是一种完全连续的 LR 计算方法。我们将我们的方法得出的 MR 和 LR 与 EuroForMix 软件 ver.3.0.3 生成的 MR 和 LR 进行了比较。当真正的贡献者作为 POI 进行测试时，我们的程序生成了高度区分的 LR，与 EuroForMix 不同，这些 LR 从未超过相应的单源 LR。当假贡献者作为 POI 进行测试时，我们得到的 LR 值比 EuroForMix 低得多。这两个发现表明，我们的计算方法比 EuroForMix 更可靠和现实。
