Using AncestryDNA results from over a quarter million people, the AncestryDNA science team set out to perform a “genetic census” of the United States. [. . .]As I mentioned, the "Irish" estimates are likely inflated in much of the country, with Scotch-Irish, Scottish, and Welsh probably contributing a considerable part of the "Irish" component outside of the Northeast.
Solely using ethnicity estimated by DNA, these maps reveal spatial patterns that are telling of the ancestral origins of present day Americans: where they came from and where they eventually settled. [. . .]
For example, let’s look at the Scandinavian map. Scandinavian immigrants – from Sweden, Norway, and Denmark – tended to settle in the upper Midwest where geography, culture, and local economics felt familiar to life in the old country.
On the map, these are the greenest regions: the states with the highest amounts of Scandinavian ancestry. In other words, DNA also suggests localized migration of individuals of Scandinavian origin to North Dakota, Minnesota, and neighboring states, with little migration to other U.S. regions. History agrees with genetics!
Look at the Irish ancestry map as another example. The highest statewide averages are concentrated in Massachusetts and other states in the Northeastern U.S. – where many Irish immigrants, forced to leave their homes and lands, settled in the 19th century. Growing numbers of Irish that arrived after the 1820s were often poor and common laborers, and took jobs in the construction of buildings, canals, roads, and railways in cities in the eastern United States.
Many of these cities still show the highest average amounts of Irish ethnicity in the U.S. today! DNA affirms that many descendants of Irish immigrants still live where their ancestors initially settled – in the Northeast.
GREAT BRITAIN AND WESTERN EUROPE ETHNICITY
If you look at the maps for Great Britain and Europe West, you see that other ancestries are more widespread across the whole country. Leading up to the Boston Tea Party and the Declaration of Independence in 1776, large numbers of Europeans arrived in what is now the U.S., in some cases to escape religious persecution. While there were subsequently many waves of immigration, individuals primarily from Western Europe and Great Britain were our first Americans.
That we see British ancestry in many people of the U.S. may be evidence of the long history of individuals from Great Britain migrating to the United States, and far and wide across those states.
States with the highest Irish ancestry
First, for all AncestryDNA ethnicity estimates of people born in the same state, we averaged their fractions of Irish ethnicity. Then, we found the U.S. states whose residents have the highest, and lowest, amounts of Irish ancestry.
On the map are the top five states with the highest average Irish ancestry. Massachusetts is #1, and all of the other top states are also in the Northeast.
AncestryDNA estimates its Massachusetts-born customers average 28.5% Irish genetically, which is reasonably close to my surname-based estimate of 26% (using 1940 census data).
AncestryDNA's estimates of Irish ancestry for much of the rest of the country are likely inflated, however. AncestryDNA's "Irish" cluster spills over into Scotland and Wales, and to a lesser extent even into England and France. While (in an analysis shown in the AncestryDNA white paper) 95% of Irish are placed into the "Irish" cluster, only something like 60% of British are placed into the "Great Britain" cluster (with most of the rest presumably being placed into either the "Irish" or "Europe West" clusters). AncestryDNA's estimates rely on ADMIXTURE, an allele frequency-based approach, whereas I think very large data sets and an approach that makes use of haplotype information will be needed to clearly dissect recent ancestry within Northwestern Europe.