The appropriate use of subpopulation corrections for differences in endogamous communities

Zhivotovsky et al. [L.A. Zhivotovsky, S. Ahmed, W. Wang, A. Bittles, The forensic DNA implications of genetic differentiation between endogamous communities, Forensic Sci. Int. 119 (2001) 269-272] gave an estimate of 0.13 for the coancestry coefficient, theta in three co-resident Pakistani communities. Wang et al. [W. Wang, S.G. Sullivan, S. Ahmed, D. Chandler, L.A. Zhivotovsky, A.H. Bittles, A genome-based study of consanguinity in three co-resident endogamous Pakistan communities, Ann. Hum. Genet. 94 (2000) 41-49] explain that the sampling is from extended families but conclude that "the present findings show that differentiation resulting from genetic stratification by genetic relatedness can be very pronounced in specific populations and thus be important in the evaluation of forensic DNA evidence." In this paper we present simulation results that show that sampling from families will be expected to produce high values of theta regardless of the population value for theta. A theta value estimated in this way cannot be used in formulae suggested by NRC II [NRC II, National Research Council Committee on DNA Forensic Science, The Evaluation of Forensic DNA Evidence, National Academy Press, Washington, DC, 1996] (Eqs. (1) and (2)) to estimate the match probability for a DNA profile in a subpopulation. Whilst it may have some application to estimating the match probability from the pedigree itself there are superior methods for assigning match probabilities for relatives.