  Admixed populations are formed by the merging of two or more ancestral
populations, and the ancestry of each locus in an admixed genome derives from
either source. Consider a simple "pulse" admixture model, where populations A
and B merged t generations ago without subsequent gene flow. We derive the
distribution of the proportion of an admixed chromosome that has A (or B)
ancestry, as a function of the chromosome length L, t, and the initial
contribution of the A source, m. We demonstrate that these results can be used
for inference of the admixture parameters. For more complex admixture models,
we derive an expression in Laplace space for the distribution of ancestry
proportions that depends on having the distribution of the lengths of segments
of each ancestry. We obtain explicit results for the special case of a
"two-wave" admixture model, where population A contributed additional migrants
in one of the generations between the present and the initial admixture event.
Specifically, we derive formulas for the distribution of A and B segment
lengths and numerical results for the distribution of ancestry proportions. We
show that for recent admixture, data generated under a two-wave model can
hardly be distinguished from that generated under a pulse model.

Populations and Evolution