Population Substructure
Definition
The condition in which allele frequencies differ systematically between subgroups within a nominally single population. Substructure means that individuals within a subgroup share alleles more often than random mating would predict, which can cause a database to under-estimate or over-estimate match probabilities for members of a specific subgroup.
- Effect
- Allele frequencies differ between subgroups
- Risk
- Miscalculated match probabilities for subgroup members
- Correction used
- Theta (Fst) correction
Common questions
Why does substructure risk both over- and under-estimating probabilities?+
Members of a tighter subgroup share alleles more often than random mating would predict, so ignoring substructure can make a shared profile look rarer or commoner than it really is depending on which subgroup and alleles are involved.
How do forensic statisticians correct for it?+
They apply a theta correction, derived from Wright's Fst statistic, to the match probability calculation to account for the extra allele sharing within subgroups rather than treating the population as uniformly mixed.
Related terms
- Allele Frequency
- The proportion of a specific allele variant at a given genetic locus in a defined population. Forensic match probability calculations multiply the...
- Hardy-Weinberg Equilibrium (HWE)
- The expected genotype frequency in a large, randomly mating population with no selection, mutation, or migration. Under HWE, the frequency of a...
- Minimum Allele Frequency (Minimum Frequency Ceiling)
- A conservative lower bound applied when an allele is absent or very rare in the database, to prevent an artificially inflated match...
- Reference Population Database
- A curated collection of allele frequencies or feature measurements from a defined population, used to estimate the probability that a randomly chosen...
- Theta Correction (FST or Coancestry Coefficient)
- A statistical adjustment applied to DNA match probability calculations to account for expected allele-sharing within subpopulations due to substructure. A theta value...