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It actually really peaks in certain typical Germanics and is more broadly Northern European than specifically Northwestern in concentration. British Isles populations are not more than around 5% behind so the difference is far from extreme. On gnomAD v2.1.1 the carrier rate for the Swedish aggregate of 12,910 individuals is 21.26% which is higher than the 15.5% for Irish in the study you referenced:
Spoiler!
https://gnomad.broadinstitute.org/va...et=gnomad_r2_1
Danes are somewhat closer to British than Swedes are with R151C being somewhat more frequent even though R160W is still the most common R variant. The overall carrier rate of R variants is similar to Swedes (just over 40%) while it can reach 50% among Insular Celts:
Spoiler!
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4372390/
20.6% of the UK Biobank sample is a carrier for R151C, which is the most common R variant in the British Isles. For R160W the carrier rate is 16.5%. This study makes direct reference of UK Biobank data: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6548228/
For Swedes at least these rates are more or less reversed. As per the Samuel Andrews (Old TA account: https://www.theapricity.com/forum/me...49-Fire-Haired) spreadsheet all Scandinavians are more frequently carriers of R160W than R151C. This is apparently in fact the case with Northern/Central Europeans at large. The gnomAD data is probably better representative of Swedes than the more limited but otherwise quite extensive data he obtained from 23&me.
Here is his spreadsheet although it has some limitations especially regarding sources. You probably have seen it before:
https://docs.google.com/spreadsheets...uNtXGrCtw/edit
The combination of a greater tendency of Scandinavians towards R160W over R151C and a high incidence of very light hair pigmentation leads golden-red hair to be proportionally more common than deeper red. Dense pigmentation can mask rufosity. Where a dark-haired person would only have a rufous cast to their hair in certain light, a fair haired person might be reddish-blond. The redness of more obscured reddish-brown shades (not dark red which is fully expressed) can seem hard to discern in different lighting. Neither the Fischer nor Fischer-Saller scales include such shades as reds but the later includes “red-blond” shades which are more likely to be associated with carrying only one copy of an R variant. For this reason when reddish-blond shades are counted there is more overlap between Scandinavia and the British Isles in red hair.
Here is an example of someone with masked rufosity. My dad has a high school friend whose hair can change from medium to dark brown (~Fischer #5) with essentially no apparent red tint to fairly reddish, almost like a light auburn. He may possibly carry one copy of an R variant, but until the eumelanin is degraded by photobleaching his rufosity does not clearly show:
Spoiler!
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