Chi squared and its p-value

Chi squared is calculated as shown in Genetic Data Analysis by Bruce S. Weir (Sinauer Associates), equation 3.11 on page 117. If n is the number of individuals (2n is the number of chromosomes)

chi squared Χ² = (2n) D² / f_A f_a f_B f_b

The probability of the chi squared statistic is given by the area under the chi squared distribution between Χ² and infinity. If we are considering a pair of biallelic markers there is 1 degree of freedom and the p-value is calculated as follows:

(Χ²)^-0.5 e^-_Χ2_/2 / (2π)^0.5