Department of Mathematical Sciences, Aalborg University, Fredrik Bajers Vej 7G, DK-9220 Aalborg East, Denmark.
Forensic Sci Int Genet. 2013 May;7(3):327-36. doi: 10.1016/j.fsigen.2013.01.005. Epub 2013 Feb 27.
Y chromosome short tandem repeats (Y-STRs) are valuable genetic markers in certain areas of forensic case-work. However, when the Y-STR DNA profile is weak, the observed Y-STR profile may not be complete--i.e. locus drop-out may have occurred. Another explanation could be that the stain DNA did not have a Y-STR allele that was detectable with the method used (the allele is a 'null allele'). If the Y-STR profile of a stain is strong, one would be reluctant to consider drop-out as a reasonable explanation of lack of a Y-STR allele and would maybe consider 'null allele' as an explanation. On the other hand, if the signal strengths are weak, one would most likely accept drop-out as a possible explanation. We created a logistic regression model to estimate the probability of allele drop-out with the Life Technologies/Applied Biosystems AmpFlSTR(®) Yfiler(®) kit such that the trade-off between drop-outs and null alleles could be quantified using a statistical model. The model to estimate the probability of drop-out uses information about locus imbalances, signal strength, the number of PCR cycles, and the fragment size of Yfiler. We made two temporarily separated experiments and found no evidence of temporal variation in the probability of drop-out. Using our model, we found that for 30 PCR cycles with a 150 bp allele, the probability of drop-out was 1:5000 corresponding to the average estimate of the probability of Y-STR null alleles at a signal strength of 1249 RFU. This means that the probability of a null allele is higher than that of an allele drop-out at e.g. 4000 RFU and the probability of drop-out is higher than that of a null allele at e.g. 75 RFU.
Y 染色体短串联重复序列（Y-STRs）在某些法医案件工作中是有价值的遗传标记。然而，当 Y-STR DNA 谱微弱时，观察到的 Y-STR 谱可能不完整——即可能发生了基因座缺失。另一种解释可能是，污渍 DNA 没有可检测到的 Y-STR 等位基因，该等位基因是“无效等位基因”。如果污渍的 Y-STR 谱很强，人们就不会轻易将缺失视为缺乏 Y-STR 等位基因的合理解释，而可能会将“无效等位基因”视为一种解释。另一方面，如果信号强度较弱，则很可能会将缺失视为可能的解释。我们创建了一个逻辑回归模型，以使用 Life Technologies/Applied Biosystems AmpFlSTR(®) Yfiler(®)试剂盒估计等位基因缺失的概率，以便可以使用统计模型来权衡缺失和无效等位基因。用于估计缺失概率的模型使用有关基因座失衡、信号强度、PCR 循环数和 Yfiler 片段大小的信息。我们进行了两个暂时分开的实验，没有发现缺失概率随时间变化的证据。使用我们的模型，我们发现对于 30 个具有 150bp 等位基因的 PCR 循环，缺失的概率为 1:5000，对应于在 1249 RFU 的信号强度下 Y-STR 无效等位基因的平均估计概率。这意味着在例如 4000 RFU 的信号强度下，无效等位基因的概率高于等位基因缺失的概率，而在例如 75 RFU 的信号强度下，缺失的概率高于无效等位基因的概率。
