However, the admixture process may distort patterns of LD, break up runs of homozygosity, and combine allele frequency distributions from distinct parental populations. Similarly, recombination breaks up ancestry tracts over generations therefore, more recent admixture events are expected to generally have longer tracts of a single ancestry within individuals.Ĭlassic statistical methods in population genetics typically rely on allele frequencies, patterns of linkage disequilibrium (LD), and interpopulation sequence differences. For example, drift may increase the variance in ancestry proportion within a population and across loci in small populations. Summaries of ancestry-such as the mean proportion of ancestry across individuals in a population, the proportion of ancestry at different genetic loci in a population, and the length of ancestry tracts within individuals-are shaped by the demographic history of a population. Genetic ancestry often varies along a chromosome within an individual and between individuals within an admixed population ( Fig 1). This compilation is a limited selection of work that exemplifies recent key advances and stimulates discussion about priorities for the future.īroader genetic sampling of populations worldwide is increasingly being combined with advances in theory and computational methods to elucidate human history. Within this theme, we have curated a collection of PLOS Genetics publications. The term “admixture” encompasses multiple models of migration and population interactions we focus on scenarios of human admixture with moderate to large contributions from at least 2 source populations, emphasizing the role that admixture has played throughout human evolution. Here, we highlight recent progress and discuss future directions for the study of admixed human populations ( Fig 1). This is related to, but distinct, from genetic similarity and from genealogical ancestry, with the relationship further discussed by Mathieson and Scally. In this context, we use ancestry to describe “genetic ancestry” -the population origins of material within a genome. For example, ancestry patterns in admixed populations have been used to infer recombination rates and to identify epistatically interacting or adaptive alleles. The mosaic ancestry patterns of admixed populations can also be used to elucidate the mechanisms and timescales of evolution in humans more generally. Recent empirical work has shown that studying the genetics of a wider set of human populations can yield historical insights as well as medically relevant information about health and phenotypes. Therefore, genetic studies provide an opportunity to understand population history and the forces generating variation. Many migration events have occurred through colonization or forced displacement, and ancient admixture often predates historical records. Human population movement frequently lacks historical records. (Bottom) To leverage these patterns of admixed ancestry to better understand human history and phenotypic variation, we highlight key areas of recent progress and possible future directions in the study of human genetic admixture. The distribution of this variation is governed by the demographic and selection history of the admixed population and its sources. Generations of recombination between these haplotypes lead to an admixed population with genetic ancestry that varies between individuals and along haplotypes. (Top) Large-scale movements of individuals allow haplotypes from previously isolated populations to come together in a combined gene pool. Recent empirical studies of admixed populations have emphasized inclusion of populations that have historically been excluded or underrepresented in genetic studies, producing important insights into human genetic and phenotypic variation. Despite their ubiquity and importance, admixed populations remain understudied in population and medical genetics, especially from a theoretical perspective. Indeed, admixture is one of the fastest evolutionary processes to dramatically change the composition of a population. These processes shape modern human genetic and phenotypic variation and may lead to differences in disease risk between populations. Admixed populations are formed as moderate- to large-scale movements of individuals allow the exchange of genes from 2 or more previously isolated populations, creating populations with ancestors from multiple sources ( Fig 1). One of the major insights from the modern genomic era is the ubiquity of migration and admixture throughout human history.
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