A theta adjustment is a mathematical correction applied to a frequency calculation when both alleles at a locus are the same (known as a homozygous state). It is not applied when alleles are different at a locus (known as a heterozygous state). This correction adjusts the frequency slightly upwards to account for the presence of subpopulations in a general population database that might otherwise cause the genotype frequency to be underestimated at that locus.