…in the united states! (just as you all suspected.) amongst white folks anyway (that’s who was included in the study below).

from a study published in 2009, Measures of Autozygosity in Decline: Globalization, Urbanization, and Its Implications for Medical Genetics:

This research has definitively shown the existence of a trend for decreasing autozygosity with younger chronological age in the North American population of European ancestry. The ROHs we identified, larger than 1 Mb, are clearly representative of autozygosity due to distant consanguinity in our outbred populations, and not chromosomal abnormalities or common copy number variants. Using our predictive models of decreasing Fld, we show a quantifiable decrease in consanguinity over the twentieth century. Based on data provided in Carothers et al, this decrease in Fld found in our discovery population is on the order of individuals transitioning from a single inbreeding loop 4–5 generations prior, to no inbreeding loops within <6 generations. We postulate that the increased mobility, urbanization and outbreeding in North America in the last century has led to less consanguinity (and thus less homozygosity and homogeneity) in younger individuals.”

the researchers looked at two different sets of genomes — one from the ninds repository @the coriell institute, the other from the baltimore longitudinal study of aging (blsa). the blsa is, obviously, biased towards people on the east coast of the u.s. (in and around baltimore). glancing through the list of submitters to ninds, there’s also something of an east coast bias there, although many samples do come from other areas of the country (see the list of locations at the end of this post).

amongst the findings in this study are that 1) the number of runs of homozygosity (roh) has decreased in white americans over the last one hundred years or so, and 2) the lengths of the roh have shrunk as well. both of these are good indicators of outbreeding.

here are a couple of tables/charts from the paper (click on images for LARGER views):

measures of autozygosity in decline - table 02

measures of autozygosity in decline - percent of genome in roh

what’s interesting to contemplate, i think, is what this might mean wrt selection pressures on americans going forward? especially, what might it mean in light of european-americans encountering other, newer groups within american society that are not outbreeding so much (at least not at the moment) — newly arrived immigrants from many muslim countries, for example — or even, perhaps, latin americans (although i’m not 100% sure about how much they’ve been inbreeding over the past few hundred years or so — stay tuned!). how is that all going to play out? interesting times.

possibly related footnote — here is an abstract from the 2013 ashg conference:

“Reconstructing the Genetic Demography of the United States”

“The United States (U.S) is a complex, multiethnic society shaped by immigration and admixture, but the extent to which these forces influence the overall population genetic structure of the U.S is unknown. We utilized self-reported ancestry data collected from the decennial U.S Census 2010 and allele frequency data from over 2000 SNPs for over 40 of the most common ancestries in the U.S. that were available from the Pan Asian Single Nucleotide Polymorphism (PASNP), Population Reference Sample (POPRES), 1000 Genomes, and Human Genome Diversity Panel (HGDP) databases. We utilized the relative proportions of individuals of each ancestry within each county, state, region and nation and calculate the weighted average allele frequency in these areas. We reconstructed the genetic demography of the U.S by examining the geographic distribution of Wright’s Fst. Shannon’s diversity index, H was calculated to assess the apportionment of genetic diversity at the county, state, regional and national level. This analysis was repeated stratifying by race/ethnicity. We analyzed households with spouses, using the phi-coefficient as a measure of assortative mating for ancestry. This analysis was repeated stratifying by age of the spouses (older or younger than 50). Most of the genetic diversity is between ancestries within county, but this varies by race/ethnicity, and ranges from 95% for Whites to 43% for Hispanics illustrating that the White ancestries are relatively homogeneously scattered throughout the U.S whereas the Hispanic ancestries show significant clustering by geography. Analysis of the mating patterns show strong within ethnicity assortative mating for American Indian/Alaska Natives, Asians, Blacks, Hispanic, Native Hawaiians/Pacific Islanders, and Whites, with φ = 0.30, 0.864, 0.92, 0.863, 0.478 and 0.832 respectively (P<1×10-324 for each) and significantly less correlation in the younger cohort. These results show demographic patterns of social homogamy which are slowly decreasing over time. One major implication is that data collected from different locations around the U.S are susceptible to both within- and between-location population genetic substructure, leading to potential biases in population-based association studies.”
_____

origin cities of the ninds samples (from a quick-ish glance):

Burlington, VT
Lebanon, NH
Boston x 10
New York x 7
Albany
Rochester, NY x 4
New Haven x 3

Bethesda x 7
Baltimore x 5
Philadelphia
Washington, D.C.

Winston-Salem, NC
Charleston, SC

Atlanta
Birmingham x 3
Augusta

Jacksonville x 4
Tampa
Gainesville

Cincinnati x 5
Cleveland
Lexington
Louisville
Memphis
Indianapolis, IN
Ann Arbor

Chicago x 3
Springfield, IL
Rochester, MN
Minneapolis
Englewood, CO
Kansas City

Houston x 4

Phoenix
Salt Lake City

Los Angeles
Irvine, CA
Fountain Valley, CA
San Diego x 2
San Francisco x 3
_____

previously: runs of homozygosity and inbreeding (and outbreeding) and runs of homozygosity in the irish population and western europeans, runs of homozygosity (roh), and outbreeding and russians, eastern europeans, runs of homozygosity (roh), and inbreeding

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