DeltaDAF calculation between two populations

Question

[seta]

Hi all,

I came across with this paper, which described the difference in derived allele frequencies (deltaDAF) between two populations was computed as DS − DNS, where DS and DNS are the DAF (derived allele frequency) in the studied geographic region and in the remaining ones, respectively. I'm in a doubt about it; in fact, doing the Fisher's test on the count (not frequency) of derived alleles of two population is in my mind. Could you please share me your idea about this issue?

Many thanks