  We develop a global and hierarchical scheme for the forward Kolmogorov
(Fokker-Planck) equation of the diffusion approximation of the Wright-Fisher
model of population genetics. That model describes the random genetic drift of
several alleles at the same locus in a population. The key of our scheme is to
connect the solutions before and after the loss of an allele. Whereas in an
approach via stochastic processes or partial differential equations, such a
loss of an allele leads to a boundary singularity, from a biological or
geometric perspective, this is a natural process that can be analyzed in
detail. Our method depends on evolution equations for the moments of the
process and a careful analysis of the boundary flux.
