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Forensic Biology · Tools
Derive allele frequencies from genotype counts and test for equilibrium with chi-square.
p² + 2pq + q² = 1
Observed genotype counts
Chi-square (1 df)
0.1600
p = (2·AA + Aa) / 2N = 0.6250, q = 0.3750
The critical chi-square value at one degree of freedom and α = 0.05 is 3.841. A deviation can indicate population structure, non-random mating, selection, or a typing artefact such as allele drop-out.
The chi-square test is unreliable when any expected count falls below five, which happens easily with rare alleles; an exact test is preferred there. Consistency with Hardy-Weinberg does not validate a frequency database, and a single locus says little about the population as a whole.
Hardy-Weinberg equilibrium is the assumption behind every random match probability: that genotype frequencies are predictable from allele frequencies as p², 2pq and q². Testing it is how a frequency database is validated before use in casework.
A significant chi-square indicates the population is not in equilibrium, which can reflect genuine population structure or a typing artefact such as allele drop-out.