Why aren't green eyes more prevalent, given that the green allele is dominant over the blue one?
My understanding is that human eye colour is determined by two genes:
1) HERC2, with alleles Bx, BB or xx,
2) gey, with alleles GG, Gb or bb,
- B means brown
- x means not brown (this is less confusing than using b for blue here, because having no B allele in HERC2 doesn't necessarily give someone blue eyes; their eyes will be either blue or green depending on the gey gene)
- G means green
- b means blue.
Capital letters stand for alleles that are dominant; lower case, ones that are recessive.
So the 'algorithm' goes:
- 1) look at HERC2; if it's anything other than xx, you've got BROWN eyes
- 2) if HERC2 is xx, look at gey; if it's anything other than bb, you've got GREEN eyes
- 3) still here? then you've got BLUE eyes
My question is this: why is it that green eyes are so rare if the green allele is dominant over the blue one? When a blue-eyed person and a green-eyed person interbreed, their offspring will either be equally likely to have green eyes as blue (if the green-eyed parent is Gb) or always have green (if that parent is GG).
But many more people have blue eyes than green eyes, despite blue being recessive to green.
The obvious answer is that people with blue eyes (xx bb) and people with green eyes (other xx-ers) have tended to avoid breeding with each other to a large extent. Is any other explanation in the ring? Perhaps the greens are catching up?