Abstract
BackgroundIn order to isolate an individual’s genotype from a sample of biological material, most laboratories use PCR and Capillary Electrophoresis (CE) to construct a genetic profile based on polymorphic loci known as Short Tandem Repeats (STRs). The resulting profile consists of CE signal which contains information about the length and number of STR units amplified. For samples collected from the environment, interpretation of the signal can be challenging given that information regarding the quality and quantity of the DNA is often limited. The signal can be further compounded by the presence of noise and PCR artifacts such as stutter which can mask or mimic biological alleles. Because manual interpretation methods cannot comprehensively account for such nuances, it would be valuable to develop a signal model that can effectively characterize the various components of STR signal independent of a priori knowledge of the quantity or quality of DNA.ResultsFirst, we seek to mathematically characterize the quality of the profile by measuring changes in the signal with respect to amplicon size. Next, we examine the noise, allele, and stutter components of the signal and develop distinct models for each. Using cross-validation and model selection, we identify a model that can be effectively utilized for downstream interpretation. Finally, we show an implementation of the model in NOCIt, a software system that calculates the a posteriori probability distribution on the number of contributors.ConclusionThe model was selected using a large, diverse set of DNA samples obtained from 144 different laboratory conditions; with DNA amounts ranging from a single copy of DNA to hundreds of copies, and the quality of the profiles ranging from pristine to highly degraded. Implemented in NOCIt, the model enables a probabilisitc approach to estimating the number of contributors to complex, environmental samples.
Highlights
In order to isolate an individual’s genotype from a sample of biological material, most laboratories use polymerase chain reaction (PCR) and Capillary Electrophoresis (CE) to construct a genetic profile based on polymorphic loci known as Short Tandem Repeats (STRs)
Alternative methods that employ complex, continuous models of the signal have been developed to facilitate the interpretation of challenging forensic samples; these models can be used within a likelihood ratio (LR) framework to evaluate the strength of the evidence [12,13,14]
We used over 1200 single source empirically derived multiplex STR profiles from pristine, degraded or damaged DNA, or inhibited PCR processes to develop the model [32]
Summary
In order to isolate an individual’s genotype from a sample of biological material, most laboratories use PCR and Capillary Electrophoresis (CE) to construct a genetic profile based on polymorphic loci known as Short Tandem Repeats (STRs). For samples collected from the environment, interpretation of the signal can be challenging given that information regarding the quality and quantity of the DNA is often limited. The genetic profile(s) of the contributor(s) are deduced using a combination of presence/absence rules [9] This method has been shown to result in inaccurate interpretation of forensic samples that contain (i) a low mass of DNA, (ii) a mixture of DNA from several individuals, or (iii) damaged or degraded DNA [8, 10, 11]. Alternative methods that employ complex, continuous models of the signal have been developed to facilitate the interpretation of challenging forensic samples; these models can be used within a likelihood ratio (LR) framework to evaluate the strength of the evidence [12,13,14]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
