Hi, Etienne. To echo what Vance said in his comment, establishing any meaningful genealogical evidence to the Nordic countries (or to mainland Europe, for that matter) from an ancestral lineage in the British Isles based solely on DNA "ethnicity estimates" (more technically correct and less emotionally charged: admixture) is extremely difficult. This is simply because the British Isles historically were a melting pot of both invaders and settlers, from the Romans to the Saxons, the Norse to the Normans.
Strong genetic signatures from the Norse, for example, still persist today. I wrote a brief article in 2018 about one particularly interesting study. The lead author of that research published, almost two years later, another that focused primarily on Scotland: Gilbert, et al. "The Genetic Landscape of Scotland and the Isles." Proceedings of the National Academy of Sciences 116, no. 38 (17 September 2019): 19064–70. That paper's abstract includes:
"Extensive geographic structuring is revealed, from broad scales such as a NE to SW divide in mainland Scotland, through to the finest scale observed to date: across 3 km in the Northern Isles. Many genetic boundaries are consistent with Dark Age kingdoms of Gaels, Picts, Britons, and Norse.... We document a pole of Norwegian ancestry in the north of the archipelago (reaching 23 to 28% in Shetland) which complements previously described poles of Germanic ancestry in the east, and “Celtic” to the west. This modern genetic structure suggests a northwestern British or Irish source population for the ancient Gaels that contributed to the founding of Iceland."
It's a fascinating subject for those of us with ancient roots in the isles. A couple of additional research papers (in case you ever have trouble falling asleep) include Byrne, et al. "Insular Celtic Population Structure and Genomic Footprints of Migration" (2018), and Patterson, et al. "Large-Scale Migration into Britain during the Middle to Late Bronze Age" (2022).
The net message here is that there is a reason that AncestryDNA, for example, has shifted admixture estimates multiple times between Scots, Irish, and Norse percentages on our test results.
That said, since your first Tait ancestor is a great-grandmother, that estimate of 57% Scandinavian--assuming MyHeritage isn't way off on the numbers--would not have come solely from her.
Any given great-grandparent will, on average, account for about 12.5% of our DNA. There can be a significant amount of variance to that number, but 57% is a literal impossibility unless there was pedigree collapse among her children. Other than the difference in size between the X and Y chromosomes, we get exactly 50% of our DNA from each parent. If the 57% is correct, at first glance I would expect you to have two grandparents who both have strong Nordic ancestry back a few or several generations.
Perhaps the best way to try to approach the question is to build out what you can of your own "genomic map." DNA Painter is a very handy and free (extra features are available for a small fee) utility that can help you do this.
The idea is simple: begin with your closest DNA matches, meaning largest shared amounts, then enter those data into the tool. MyHeritage and other services--with the very notable exception of AncestryDNA--provide the chromosome number and segment start and stop points that you need. As you identify exactly how you're related to each of your matches, you can begin to color-code the segments based upon the ancestor from whom they came. If you don't have a great deal of information about your matches to begin, I'd recommend just color-coding for grandparents. As you get more detail, you can refine the coding so that the colors indicate great-grandparents.
In other words, start with the data, and see where it leads you. Understanding the genealogies of your matches will eventually lead you see where the Scandinavian roots possibly lie...with the caveat that, as per some of those research papers I cited, Scandinavian roots back several hundreds of years ago might mean anything from the British Isles to the Netherlands and Germanic Northern Europe to the Eastern Baltic.
Once you're a bit of the way along, another methodology that can help is called "clustering." Conceived and coded by Evert-Jan Blom, this is a super-cool tool that companies like MyHeritage and GEDmatch adopted and offer directly from their own sites. Evert-Jan's own website for his DNA tools is https://geneticaffairs.com/.
I wouldn't take all the information at face value, but if you have a GEDmatch account there is a feature that may help give you a bit of insight when coupled with a map at DNA Painter. The caveat is that all the admixture "projects" on GEDmatch are quite dated--few more recently updated than a decade ago--and the reference genome libraries they use are fairly small given what's available today. Nevertheless, it may provide a clue.
In the "Free Tools" section you'll find a selection titled "Admixture (heritage)"; select that. On the next screen will be two options to set: "Select the project" (choose "Eurogenes"), and "Select how you want to process it" (choose "Chromosome Painting").
Click the "Continue" button, then enter your kit number and the "calculator model" of "Eurogenes K36". That option offers the most granular breakdown. What you get is a colorful "heatmap" of each of your chromosomes. The display will be quite wide, so you'll need to scroll back and forth. The maps will be labeled in increments of 10 million base pairs.
There will undoubtedly be overlap between admixture values, but the ones for "Fennoscandian" and "North_Sea" should be the most indicative. The displayed heat maps represent the percentage of admixture coverage for a particular stretch of each chromosome. What you might want to do is make a note of where the percentages are predominately one of those two values. For example, here's a snippet of my Chromosome 4 where the peach color represents Fennoscandian:
That would tell me that on Chr 4 I have a high percentage of saturation of Fennoscandian admixture starting at about position 143 million through about 151 million, and then again from about 156 million through just under 160 million.
So as I build up my genomic map data in DNA Painter, I can watch for segments that overlay those ranges on Chr 4. Those matches will share those "high Fennoscandian" segments with me. This is an imperfect approach, but with your reported percentage of Scandinavian admixture you're likely to have fairly long sections of your chromosomes that show as distinctly Fennoscandian or North_Sea. If nothing else it might provide you a way to prioritize your research and highlight cousins who are most likely to share those same chromosomal areas of admixture with you.
Happy hunting!
