Editing Genetic drift (section)

===Wright–Fisher<!--this has c. 6x as many hits as Fisher-Wright, so please leave it alone--> model===

Consider a gene with two alleles, '''A''' or '''B'''. In [[Ploidy#Diploid|diploid]]y, populations consisting of ''N'' individuals have  2''N'' copies of each gene. An individual can have two copies of the same allele or two different alleles. The frequency of one allele is assigned ''p'' and the other ''q''. The Wright–Fisher model (named after [[Sewall Wright]] and [[Ronald Fisher]]) assumes that generations do not overlap (for example, [[annual plant]]s have exactly one generation per year) and that 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<ref name="Hartl_p112">{{harvnb|Hartl|Clark|2007|p=112}}</ref><ref>{{harvnb|Tian|2008|p=11}}</ref>

:<math>\frac{(2N)!}{k!(2N-k)!} p^k q^{2N-k} </math>

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

:<math>{2N \choose k}  p^k q^{2N-k} </math>