EFFECTS OF SUBPOPULATION STRUCTURE ON PROBABILITY-CALCULATIONS OF DNAPROFILES FROM FORENSIC PCR ANALYSIS

DNA typing for forensic identification is a two-step process. The firs t step involves determining the profiles of samples collected at the c rime scene and comparing them with the profiles obtained from suspects and the victims. In the case of a match that includes the suspect as the potential source of the material collected at the crime scene, the last step in the process is to answer the question, what is the likel ihood that someone in addition to the suspect could match the profile of the sample studied? This likelihood is calculated by determining th e frequency of the suspect's profile in the relevant population databa ses. The design of forensic databases and the criteria for comparison has been addressed by the NRC report of 1996 (National Research Counci l, 1996). However, the fact that geographical proximity, migrational p atterns, and even cultural and social practices have effects on subpop ulation structure establishes the grounds for further study into its e ffects on the calculation of probability of occurrence values. The iss ue becomes more relevant in the case of discrete polymorphic markers t hat show higher probability of occurrence in the reference populations , where several orders of magnitude difference between the databases m ay have an impact on the jury. In this study, we calculated G values f or all possible pairwise comparisons of allelic frequencies in the dif ferent databases from the faces or subpopulations examined. In additio n, we analyzed a set of 24 unrelated Caucasian, 37 unrelated African-A merican, and 96 unrelated Sioux/Chippewa individuals for seven polymor phic loci (DQA1, LDLR, GYPA, HBGG, D7S8, GC, and D1S80). All three set s of individuals where sampled from Minnesota. The probability of occu rrence for all seven loci were calculated with respect to nine differe nt databases: Caucasian, Arabic, Korean, Sioux/Chippewa, Navajo, Puebl o, African American, Southeastern Hispanic, and Southwestern Hispanic. Analysis of the results demonstrated marked differences in the probab ilities of occurrence when individuals were compared to the different populations and subpopulation databases. The possible genetic and fore nsic consequences of subpopulation structure on probability calculatio ns are discussed.