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
Regressive *genes* do not diminish in prevalence each generation. It’s just that there are chances for them to not be expressed.
Imagine you have a blue eyed person married to a homozygous brown eyed person, and they have four kids, and we assume eyes are as simple as a single gene. bb x BB => Bb Bb Bb Bb. That’s four brown-eyed kids, but they aren’t homozygous like their brown-eyed parent. They are heterozygous and all carry the blue-eyed gene. 50% of all genes in this group are still blue-eyed genes. Nothing made blue-eyed genes disappear; they just got spread out so that they never get to take effect.
Imagine in the next generation, each of these carriers of blue-eyed-ness marries another carrier. Bb x Bb => BB Bb Bb bb. Half the genes are still blue! And now they are less spread out. One child of the two carriers is blue-eyed again, and two children are carriers.
Brown-eyed-ness overrides blue-eyed-ness in a single individual, but it is no more likely to be passed on to that individual’s children, so it never disappears, and it keeps coming back in later generations.
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