  The sample frequency spectrum of a segregating site is the probability
distribution of a sample of alleles from a genetic locus, conditional on
observing the sample to have more than one clearly different phenotypes. We
present a model for analyzing quadri-allele frequency spectrum, where the
ancestral population diverged into three populations at a certain divergence
time and the resulting mutations on the branches of the coalescent tree gave
rise to three different derived alleles, which could be observed in the present
generation along with the ancestral allele. The model has been analyzed for
non-constant population size, assuming we had a certain number of extant
lineages at the divergence time and no migration occurs between the
populations.

Probability;Quantitative Methods