The Genetic Structure of Norway

[Grace O'Malley]

Abstract
The aim of the present study was to describe the genetic structure of the Norwegian population using genotypes from 6369 unrelated individuals with detailed information about places of residence. Using standard single marker- and haplotype based approaches, we report evidence of two regions with distinctive patterns of genetic variation, one in the far northeast, and another in the south of Norway, as indicated by fixation indices, haplotype sharing, homozygosity, and effective population size. We detect and quantify a component of Uralic Sami ancestry that is enriched in the North. On a finer scale, we find that rates of migration have been affected by topography like mountain ridges. In the broader Scandinavian context, we detect elevated relatedness between the mid- and northern border areas towards Sweden. The main finding of this study is that despite Norway’s long maritime history and as a former Danish territory, the region closest to mainland Europe in the south appears to have been an isolated region in Norway, highlighting the open sea as a barrier to gene flow into Norway.

https://www.nature.com/articles/s41431-021-00899-6.pdf

[Grace O'Malley]

Not a great deal in this study but as has been noted before Norwegians and Danes obviously have some differences.

Kinship to Denmark and Sweden
We explored the mean sum of autosomal haplotype sharing (IBD > 3 cM) between Norwegian and Swedish counties, and Denmark as a whole (Supplementary Figs. S6 and S7). We find a distinct pattern of low degree of shared ancestry between Norway and Denmark (3.1 cM), including the South/ Southeast of Sweden (Skåne = 3.3 cM). At the opposite end, the northernmost county in Sweden, Norrbotten, shared 13.1 and 8.1 with Finnmark and Troms, respectively. Further, we detected elevated haplotype sharing between the counties on the border of Norway and Sweden. Noteworthy, the former disputed county of Jämtland, conquered by Sweden in 1679, stands out for having a relatively high IBD sharing with NordTrøndelag of 6.6 cM.

Norway has close historical ties to Denmark, as Norway became a vassal state of Denmark in 1380, lasting 443 years, until 1814. The PCA (Supplementary Fig. S3) and IBD analyses (Supplementary Fig. S6) strongly suggest that the counties in southern Norway have diverged from the rest of the Norwegian population due to isolation, rather than gene flow from Denmark or some other neighboring populations. We speculate that the isolation in the Norwegian south may be caused by several factors. (1) The region has an unusual coastline, without deep fjords, common elsewhere in Norway. Historically the fjords have played a critical part in the transportation of goods and people. The absence of fjords may have increased isolation (2) late development of infrastructure like railroad and roads in the last 100 years (3) failure to recruit economic
migrants.

For the first time, we document restricted gene flow in the southern part of Norway, which is contradicting a commonly held notion of Danish admixture. We next aimed to characterize the detailed population structures in the Norwegian population further using rare variants, as rare variants are more geographically clustered, due to their more recent origin.

[Grace O'Malley]

Figure S1a: PCA of the dataset from this study (black) merged (SNPs = 58,457) with public datasets of selected and colored European samples, including one single Sami sample from a public dataset (left legend). The size of the black circles (right legend) represents the percentage of East-Asian ancestry (CHB) calculated by ADMIXTURE.


Figure S1b: Due to sample size bias, a second PCA of the dataset from this study (black) merged (SNPs = 58,457) with public datasets of selected and colored European samples. Here a maximum threshold of 20 samples per county was used, and not max. 200 as in S1a.