August 05, 2011

Genetic structure of Swedish population

PLoS ONE 6(8): e22547. doi:10.1371/journal.pone.0022547

The Genetic Structure of the Swedish Population

Keith Humphreys et al.

Patterns of genetic diversity have previously been shown to mirror geography on a global scale and within continents and individual countries. Using genome-wide SNP data on 5174 Swedes with extensive geographical coverage, we analyzed the genetic structure of the Swedish population. We observed strong differences between the far northern counties and the remaining counties. The population of Dalarna county, in north middle Sweden, which borders southern Norway, also appears to differ markedly from other counties, possibly due to this county having more individuals with remote Finnish or Norwegian ancestry than other counties. An analysis of genetic differentiation (based on pairwise Fst) indicated that the population of Sweden's southernmost counties are genetically closer to the HapMap CEU samples of Northern European ancestry than to the populations of Sweden's northernmost counties. In a comparison of extended homozygous segments, we detected a clear divide between southern and northern Sweden with small differences between the southern counties and considerably more segments in northern Sweden. Both the increased degree of homozygosity in the north and the large genetic differences between the south and the north may have arisen due to a small population in the north and the vast geographical distances between towns and villages in the north, in contrast to the more densely settled southern parts of Sweden. Our findings have implications for future genome-wide association studies (GWAS) with respect to the matching of cases and controls and the need for within-county matching. We have shown that genetic differences within a single country may be substantial, even when viewed on a European scale. Thus, population stratification needs to be accounted for, even within a country like Sweden, which is often perceived to be relatively homogenous and a favourable resource for genetic mapping, otherwise inferences based on genetic data may lead to false conclusions.



Jim H. said...

This mirrors something I noticed when mapping the distribution of mtDNA K1a10 (Google Map at ). K1a10 in Sweden has only been found in a band in the center, and the same is true of Norway. The suggestion of spread eastwards from Norway is inescapable. Maybe the people of central Norway and central Sweden are genetically about the same.

eurologist said...

This, like previous studies that have shown three different populations in Sweden, can also be interpreted as:

(i) a southern population that is dominated by agriculturalists from the south (i.e., mostly adjacent northern Germany),
(ii) a central group that like much of Norway has retained a larger fraction of the indigenous central/southern Scandinavian people,
(iii) and a population that has a higher percentage of Finnish and/or Sami background.

Måns Sjöberg said...

About 12,000 Finns immigrated from the Finnish provinces of Savolax and northern Tavastland to the Swedish provinces of Värmland, Dalarna, Västmanland, Hälsingland, Medelpad, and southern Lapland from the late 16th to the early 17th century. I can't see how this could render so strange data for Dalarna. The special thing about Dalarna is its (historical) isolation and age-old language. To me it seems much more likely that they kept a large fraction of indigenous genetic material.

Måns Sjöberg said...

Also, Norrbotten and Västerbotten have a large Saami population, which might explain their peripheral position on the map.

Jämtland, Härjedalen and northwestern Dalarna have a smaller Saami population speaking a more old-fashioned Saami language.

The bottom left provinces are close to Denmark and some of them were even part of Denmark up to 1658.

princenuadha said...

"(ii) a central group that like much of Norway has retained a larger fraction of the indigenous central/southern Scandinavian people,"

Thanks, that does help explain the unusual position of dalarna, which isn't far from the south and isn't genetically in the direction of the fins.

A three population model seems like a good scheme. And while dalarna may be genetically far from both southern and northern Sweden because of isolation they are still part of the line that southern Sweden makes. Do you remember when dienekes showed that map of east Asia and how the isolated populations were far apart but also all over the place. Because of that last difference I don't think dalarna is just the result of isolation.

It would be really great to have a close up of Norway and surrounding populations to get a better idea.