Wright-Fisher model of genetic drift

From Wikipedia:

Wright-Fisher model is a model of genetic drift (or allelic drift), which is the change in the frequency of a gene variant (allele) in a population due to random sampling. Genetic drift may cause gene variants to disappear completely, and thereby reduce genetic variation.

Consider a gene with two alleles, A or B. In diploid populations consisting of N individuals there are 2N copies of each gene. An individual can have two copies of the same allele or two different alleles. We can call the frequency of one allele p and the frequency of the other q. The Wright-Fisher model assumes that generations do not overlap. For example, annual plants have exactly one generation per year. Each copy of the gene found in the new generation is drawn independently at random from all copies of the gene in the old generation. The formula to calculate the probability of obtaining k copies of an allele that had frequency p in the last generation is then

${\frac {(2N)!}{k!(2N-k)!}}p^{k}q^{2N-k}$

where the symbol "!" signifies the factorial function. This expression can also be formulated using the binomial coefficient,

${2N \choose k}p^{k}q^{2N-k}$

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