Statistical genetics: Allele frequencies that follow a Dirichlet distribution
Largely an informed guess based on properties of distributions rather than specific knowledge of statistical genetics: a beta distribution is useful for modeling the frequencies of two alleles at one locus. A Dirichelet distribution, which is a multivariate generalization of a beta distribution, would thus be useful for modeling a set of loci.

The flip side is to think of beta as just a special case of Dirichelet: if you only have one locus, beta works. If you have multiple loci, then Dirichelet.