If blue eyes are regressive (overridden by brown eyes) then, how could millions of north Europeans end up with blue eyes, supposing that blue eyes come from a single mutation?
In: Biology
They key here is that eye color isn’t an entirely independent trait. Blue eyes is the lack of melanin, so any genes affecting melanin production in general will also affect eye color. People living in places with less sunlight need to have lighter skin (aka less melanin) for better vitamin D production, and lighter eyes happen as a side effect
Even if it’s a recessive gene, the survival benefits means it’ll spread through the population. High melanin genes can’t dominate if the people are dead because of vitamin D deficiency.
https://en.wikipedia.org/wiki/Eye_color#Genetic_determination
> OCA2 Associated with melanin producing cells. Central importance to eye color.
> HERC2 Affects function of OCA2, with a specific mutation strongly linked to blue eyes.
https://en.wikipedia.org/wiki/HERC2#Protein_function
> SNPs of HERC2 are **strongly associated with iris colour variability in humans**. In particular, the rs916977 and rs12913832 SNPs have been **reported as good predictors of this trait, and the latter is also significantly associated with skin and hair colour**.
https://en.wikipedia.org/wiki/Vitamin_D#Deficiency
> Dark-skinned people living in temperate climates have been shown to have low vitamin D levels.[32][33][34] Dark-skinned people are less efficient at making vitamin D because melanin in the skin hinders vitamin D synthesis.
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