A note on the distribution of admixture segment lengths and ancestry proportions under pulse and two-wave admixture models

TL;DR

This paper derives mathematical distributions of ancestry segment lengths and proportions in admixed populations under pulse and two-wave admixture models, aiding inference of admixture history.

Contribution

It provides explicit formulas for segment length distributions and ancestry proportions under complex admixture scenarios, including a two-wave model.

Findings

Recent admixture data under two-wave and pulse models are often indistinguishable.

Derived formulas enable better inference of admixture timing and sources.

The models help interpret genetic data in admixed populations.

Abstract

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.