Abstract
We estimate the population genetics parameter (also referred to as the fixation index) from short tandem repeat (STR) allele frequencies, comparing many worldwide human subpopulations at approximately the national level with continental-scale populations. is commonly used to measure population differentiation, and is important in forensic DNA analysis to account for remote shared ancestry between a suspect and an alternative source of the DNA. We estimate comparing subpopulations with a hypothetical ancestral population, which is the approach most widely used in population genetics, and also compare a subpopulation with a sampled reference population, which is more appropriate for forensic applications. Both estimation methods are likelihood-based, in which is related to the variance of the multinomial-Dirichlet distribution for allele counts. Overall, we find low values, with posterior 97.5 percentiles when comparing a subpopulation with the most appropriate population, and even for inter-population comparisons we find . These are much smaller than single nucleotide polymorphism-based inter-continental estimates, and are also about half the magnitude of STR-based estimates from population genetics surveys that focus on distinct ethnic groups rather than a general population. Our findings support the use of up to 3% in forensic calculations, which corresponds to some current practice.
Highlights
We analyse an extensive new data set of the short tandem repeat (STR) profiles of individuals with worldwide origins, to estimate FST for national-scale subpopulations relative to continental-scale populations
We use two approaches to estimating FST, which differ according to the choice of reference population: a direct method that is appropriate for forensic applications, and an indirect method that reflects current population genetics practice
The use of an FST adjustment can be regarded as allowing for additional uncertainty arising from the fact that the available database does not fit the circumstances of the case perfectly, which logically reduces confidence in the result, reflected in the reduced likelihood ratio (LR)
Summary
We analyse an extensive new data set of the short tandem repeat (STR) profiles of individuals with worldwide origins, to estimate FST for national-scale subpopulations relative to continental-scale populations. We use two approaches to estimating FST, which differ according to the choice of reference population: a direct method that is appropriate for forensic applications, and an indirect method that reflects current population genetics practice. Larger values of FST imply greater coancestry and so a greater probability that the profiles of X and Q are similar. This results in a lower likelihood ratio (LR), meaning that ignoring coancestry between X and Q is unfavourable to the defendant. The use of an FST adjustment can be regarded as allowing for additional uncertainty arising from the fact that the available database does not fit the circumstances of the case perfectly, which logically reduces confidence in the result, reflected in the reduced LR
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