title: Beyond 2/3 and 1/3: The Complex Signatures of Sex-Biased Admixture on the X Chromosome.
abstract: Sex-biased demography, in which parameters governing migration and population size differ between females and males, has been studied through comparisons of X chromosomes, which are inherited sex-specifically, and autosomes, which are not. A common form of sex bias in humans is sex-biased admixture, in which at least one of the source populations differs in its proportions of females and males contributing to an admixed population. Studies of sex-biased admixture often examine the mean ancestry for markers on the X chromosome in relation to the autosomes. A simple framework noting that in a population with equally many females and males, two-thirds of X chromosomes appear in females, suggests that the mean X-chromosomal admixture fraction is a linear combination of female and male admixture parameters, with coefficients 2/3 and 1/3, respectively. Extending a mechanistic admixture model to accommodate the X chromosome, we demonstrate that this prediction is not generally true in admixture models, although it holds in the limit for an admixture process occurring as a single event. For a model with constant ongoing admixture, we determine the mean X-chromosomal admixture, comparing admixture on female and male X chromosomes to corresponding autosomal values. Surprisingly, in reanalyzing African-American genetic data to estimate sex-specific contributions from African and European sources, we find that the range of contributions compatible with the excess African ancestry on the X chromosome compared to autosomes has a wide spread, permitting scenarios either without male-biased contributions from Europe or without female-biased contributions from Africa.
authors.affiliation: Department of Biology, Stanford University, Stanford, California 94305-5020 agoldb@stanford.edu.
authors.forename: Amy
authors.initials: A
authors.lastname: Goldberg
authors.affiliation: Department of Biology, Stanford University, Stanford, California 94305-5020.
authors.forename: Noah A
authors.initials: NA
authors.lastname: Rosenberg
mesh_terms: D001741:Black or African American / Q000235:genetics*; D000465:Algorithms; D041321:Chromosomes, Human, X / Q000235:genetics*; D005260:Female; D063426:Human Migration; D006801:Humans; D008297:Male; D008957:Models, Genetic; D011156:Population Density; D012737:Sex Factors
keywords: African-American genetics; X chromosome; admixture; mechanistic model; sex bias