Populations whose mating pairs have levels of similarity in phenotypes or genotypes that differ systematically from the level expected under random mating are described as experiencing assortative mating. Excess similarity in mating pairs is termed positive assortative mating, and excess dissimilarity is negative assortative mating. In humans, empirical studies suggest that mating pairs from various admixed populations – whose ancestry derives from two or more source populations – possess correlated ancestry components that indicate the occurrence of positive assortative mating on the basis of ancestry. Generalizing a two-sex mechanistic admixture model, we devise a model of one form of ancestry-assortative mating that occurs through preferential mating based on source population. Under the model, we study the moments of the admixture fraction distribution for different assumptions about mating preferences, including both positive and negative assortative mating by population. We demonstrate that whereas the mean admixture under assortative mating is equivalent to that of a corresponding randomly mating population, the variance of admixture depends on the level and direction of assortative mating. We consider two special cases of assortative mating by population: first, a single admixture event, and second, constant contributions to the admixed population over time. In contrast to standard settings in which positive assortment increases variation within a population, certain assortative mating scenarios allow the variance of admixture to decrease relative to a corresponding randomly mating population: with the three populations we consider, the variance-increasing effect of positive assortative mating within a population might be overwhelmed by a variance-decreasing effect emerging from mating preferences involving other pairs of populations. The effect of assortative mating is smaller on the X chromosome than on the autosomes because inheritance of the X in males depends only on the mother’s ancestry, not on the mating pair. Because the variance of admixture is informative about the timing of admixture and possibly about sex-biased admixture contributions, the effects of assortative mating are important to consider in inferring features of population history from distributions of admixture values. Our model provides a framework to quantitatively study assortative mating under flexible scenarios of admixture over time.

交配对在表型或基因型方面具有与随机交配预期水平有系统差异的相似性水平的种群被描述为经历分类交配。交配对中的过度相似被称为正分类交配，而过度不同被称为负分类交配。在人类中，实证研究表明，来自各种混合种群的交配对——其祖先来自两个或多个源种群——具有相关的祖先成分，表明在祖先的基础上发生了积极的分类交配。概括两性机械混合模型，我们设计了一种通过基于源种群的优先交配发生的祖先分类交配形式的模型。在模型下，我们研究了关于交配偏好的不同假设的混合分数分布的矩，包括按人口分类的正和负分类交配。我们证明，虽然分类交配下的平均混合相当于相应的随机交配种群的平均混合，但混合的方差取决于分类交配的水平和方向。我们考虑按种群进行分类交配的两种特殊情况：第一，单一混合事件，第二，随着时间的推移对混合种群的恒定贡献。与正分类增加种群内变异的标准设置相反，某些分类交配方案允许混合物的方差相对于相应的随机交配种群减少：对于我们考虑的三个种群，种群内积极的分类交配的方差增加效应可能会被其他种群配对的交配偏好所产生的方差减少效应所淹没。同种交配对 X 染色体的影响小于对常染色体的影响，因为雄性 X 的遗传仅取决于母亲的血统，而不取决于交配对。因为混合物的方差可以提供关于混合物时间和可能关于性别偏见混合物贡献的信息，所以在从混合物值的分布推断种群历史特征时，要考虑分类交配的影响。我们的模型提供了一个框架，可以随着时间的推移在灵活的混合场景下定量研究分类交配。