Population distribution and factors affecting individual DNA shedding propensity

With most DNA casework now involving contact traces, scientists are often asked to explain the absence of detected DNA on a touched object, or to evaluate different scenarios on passive (secondary) DNA transfer. Both situations require information on an individual’s propensity to leave DNA behind after contact (shedder status). Investigators, prosecutors, defense attorneys, and ultimately judges and jurors depend on forensic biologists to provide scientific guidance and realistic assessments of hypothetical scenarios. Despite 15 years of research on this topic there are many gaps in our knowledge on factors influencing DNA shedding, the population frequency of high versus low shedders, and the reproducibility of shedder status determinations. This proposal has two aims: our main objective is to determine biological baseline shedder status frequencies in four US ethnic groups, and correlate DNA shedding to biological and medical characteristics of these populations. But prior to starting on this goal, it will be necessary to eliminate testing variation and develop a standardized method to determine shedding propensity. Method standardization will use adhesive disks for skin surface collection and test participants before and after washing on dominant and non-dominant hand, another friction ridge skin area, and sebaceous skin. Testing will also explore the value of replicates. The population study will target 100 individuals from different ethnic groups. Subsequent data analysis will provide percentages of high, intermediate, and low shedders, and test skin surface measurements (hydration, sebum density, and color) and factors captured in the questionnaire (e.g. gender, age, body mass index) for effect on individual shedding propensity. John Jay College with its diverse student and faculty is well suited for this endeavor. In response to the NIJ “Developing a future-focused workforce” priority forensic science graduate students will be recruited as research assistants. One anticipated outcome is a standardized method to test for shedding propensity, which theoretically could be used to test persons of interest in a case and provide case specific answers. The information on population distribution and factors affecting shedding propensity will be helpful for answering more general questions on DNA deposit and transfer probabilities in court. A future direction could be the use of this data set to develop a predictive model for shedder status.