To determine if the X2 value we've calculated is statistically significant, i.e. the observed values deviate significantly from the expected values, we must compare it to values in a chi-square distribution table. In order to make this comparison, we must first calculate the d.f. The formula for calculating degrees of freedom for H-W equilibrium is: d.f. = k-1 - m, k = the number of genotypic classes m = the number of independent allele frequencies estimated. In this case, only one of the allele frequencies is independent, e.g. Ft. Although we also calculated the frequency of AF508, it is not independent of F because together the two frequencies must sum to 1