An evaluation of potential allelic association between the STRs vWA and D12S391: implications in criminal casework and applications to short pedigrees

An evaluation was carried out to determine the effect on routine forensic calculations when incorporating STRs D12S391 and vWA. These loci are co-located on the same arm of chromosome 12. It has been suggested that allelic association could result in over-estimates of strength-of-evidence calculations. In the first place, we argue that is very unlikely that genotypes collected from typical cosmopolitan forensic databases can provide meaningful information about effects attributable to physical linkage. Since admixture is the most likely cause of allelic association in modern populations we specifically evaluate this effect. We use computer simulation as the preferred approach to generate populations with disequilibrium and observe the effect on match probability. Although we have specifically evaluated the linkage between D12S391 and vWA, the methods described in this paper can be extended and generalized to evaluate linkage effects between any pair of loci where the recombination rate is known. Many jurisdictions apply a subpopulation correction following the standard method of Balding and Nichols. Such corrections would appear to be more than adequate to compensate for any increase in match probability that we were able to create by this admixture. Linkage is likely to have an appreciable effect on relatedness calculations in short pedigrees in some but not all instances. We examined those circumstances where an effect is likely and give formulae for some common situations. The complexity of these calculations is a cause for concern in some laboratories. We discuss possible strategies that might be employed and plausible effects.