Red-headism is more Nordic, isn't it. Irish, Scottish, English and Nords are red. "Eric the Red".
Do you look at this picture and think: "Polish"?
Here is the Wikipedia entry on Red Hair:
Today, red hair is most commonly found at the northern and western fringes of Europe; it is associated particularly with the people located in the United Kingdom and in Ireland (although Victorian era ethnographers claimed that the Udmurt people of the Volga were "the most red-headed men in the world").[7] Redheads are common among Germanic and Celtic peoples.Redheads constitute approximately 4% of the European population.[8] Scotland has the highest proportion of redheads; 13% of the population has red hair and approximately 40% carries the recessive redhead gene.[9] Ireland has the second highest percentage; as many as 10% of the Irish population has red, auburn, or strawberry blond hair.[10] It is thought that up to 46 percent of the Irish population carries the recessive redhead gene. Red hair reaches frequencies of up to 10 percent in Wales.[11]
Red-hair is found commonly amongst Ashkenazi Jewish populations, possibly due to the influx of European DNA over a period of centuries, or in the original founding of their communities in Europe,[12] although both Esau[13] and David[14] are described in the Bible as red-haired. In European culture, prior to the 20th century, red-hair was often seen as a stereotypically Jewish trait: during the Spanish Inquisition, all those with red-hair were identified as Jewish.[15] In Italy, red hair was associated with Italian Jews, and Judas was traditionally depicted as red-haired in Italian and Spanish art.[16] Writers from Shakespeare to Dickens would identify Jewish characters by giving them red-hair.[17] The stereotype that red-hair is Jewish remains in parts of Eastern Europe and Russia.[18]
In the United States, it is estimated that 2-6% of the population has red hair. This would give the U.S. the largest population of redheads in the world, at 6 to 18 million, compared to approximately 650,000 in Scotland and 420,000 in Ireland.[citation needed]
Red or reddish-tinged hair is also found in other European populations particularly in the Nordic and Baltic countries as well as parts of the Netherlands, Belgium, France, Greece, Turkey, Portugal, Spain, Italy, Germany, Russia and South Slavic countries.[citation needed]
(Note: Poles are the Western Slavs, the one main Slavic group not mentioned as having red hair- JB
Because of migration from Europe from the 16th to the 21st centuries, red-haired humans are also found all around the world, such as in North America, South America, Australia, New Zealand, South Africa, Siberia, etc.
The Berber populations of Morocco[19] and northern Algeria have occasional redheads. Red hair frequency is especially significant among the Kabyles from Algeria, where it reaches 4 percent.[20][21] Emigration and invasion from North Africa to Southern Europe (especially Spain and Portugal) added to the number of natural red-haired humans in that region.[citation needed] The Queen of Morocco, Lalla Salma wife of king Mohammed VI, has red hair. Abd ar-Rahman I also had red hair, his mother being a Christian Berber slave.
In Asia, darker or mixed tinges of red hair can be found sporadically from Northern India, the northern Middle East (such as Iran, Lebanon and the countries of the Levant), and in rare instances on the Island of Hirado, Japan[22] and the South Pacific. Red hair can be found amongst those of Iranian descent, such as the Persians, Lurs, Nuristanis and Pashtuns. Emigration from the these people as well as parts of the Middle East, Central Asia, North India, and North Africa added to the population of red-haired humans in the Americas, Australia, New Zealand, South Africa, and other parts of Africa and Europe.
In Argentina, Brazil and Chile, people with red hair also make up a small portion of the population.[23]
In Australia, small isolated populations of Indigenous Australians (mainly in Western Australia and Australia's Northern Territory) have been found with red hair.[citation needed]
I've never thought about this but I think the "Jewish" theory makes more sense, but of course I haven't read the historical analysis called out in the article. And it's also quite common in Europe that the Kings are not that closely related to the people they rule, as Kings marry off their children to the princesses from adjacent nations, not the boy or girl next door.