If blue eyes are a recessive gene and green eyes are a dominant gene, why are there more people with blue eyes?
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Green eyes are just brown eyes. So it’s rare to have that exceptional pigment combination. Blue eyes were a mutation.
Along with the other answers, I also once read a study which concluded that people with blue eyes are more likely to reproduce with partners who are also blue eyed.
Blew my mind, and I thought about it and tried to apply it to myself to figure out some sort of “why”. My wife has blue eyes and I do too. And yep, imagining myself being with a woman with not blue eyes feels weird. It’s nothing conscious. I’ve got nothing against green or brown eyes, or the people that have them. But when I try to imagine that *intimate* feeling with someone that doesn’t have blue eyes, it feels… Mildly uncomfortable. Something deep down inside my mind is actively steering me away from “wrong” eye colors. It’s so fucking weird. I’m sure if I’d clicked with some absolutely amazing woman with brown eyes, it wouldn’t have been a big deal to override that instinct, but the fact that it **is** a factor is pretty fascinating regardless.
It’s more complicated than that. The eye has multiple genes for multiple pigments. There’s eumelanin (brown) and pheomelanin (yellow), plus the back of the eye reflects as a blue color so the lack of either melanin means blue. There’s no green pigment, instead, the blue of the eye plus the yellow of pheomelanin makes green, with no brown pigment meaning quite green, but some brown pigment means more hazel. Different combinations make different colors than those I’ve mentioned.
The fault assumption is here is that an allele being recessive implies something about the frequency of the allele. It does not. Recessive does not mean less evolutionarily fit, dominant does not mean more evolutionarily fit. They are descriptors for alleles in terms of how they translate from genotype to phenotype, nothing more.
Because less people have the green eye gene.
It doesn’t matter if a gene is dominant or recessive if you don’t have it in the first place.
There is no gene for green eyes. Green eyes occur because of DNA recombination. When you have two dominant or two recessive genes, they can recombine.
Green is tricky because it has a more difficult genetic recipe.
Blue eyes – nothing is added, the blue is from light scattering unimpeded.
Brown eyes – lots of melanin, amount of melanin is what decides how brown the eyes are.
Green eyes – a small amount of melanin + yellow pigment called Lipochrome.
Hazel eyes – same as green eyes but more melanin.
Hazel and Green are on a spectrum determined by the amount of melanin present. And since brown eyes are the most common eye color in humans, there’s usually a higher concentration of melanin in the genetic soup so eyes will be more hazel then green. Since green eyes have only a small amount of melanin, it becomes more rare.
Multiple genes are responsible for eye color, so it’s very complex.
Eye colour is determined by a single factor – melanin. This is the same pigment that determines human skin and hair colour.
Eyes colour does not come in discrete categories like “blue”, “green, or “brown”, but in a spectrum ranging from pale blue* to dark brown. What we call green is in the middle.
Several genes contribute to eye colour, all by affecting melanin pathways and the regulation of other genes.
So I would guess that “green” eyes are relatively rare because there is a narrow range of melanin concentrations that give that effect, and a small combination of alleles that underlying that.
*Or albino if you lack melanin at all.
Sturm RA, Duffy DL, Zhao ZZ, Leite FP, Stark MS, Hayward NK, Martin NG, Montgomery GW. A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color. Am J Hum Genet. 2008 Feb;82(2):424-31. doi: 10.1016/j.ajhg.2007.11.005. Epub 2008 Jan 24. PubMed: 18252222. Free full-text available from PubMed Central: PMC2427173.
Sturm RA, Larsson M. Genetics of human iris colour and patterns. Pigment Cell Melanoma Res. 2009 Oct;22(5):544-62. doi: 10.1111/j.1755-148X.2009.00606.x. Epub 2009 Jul 8. Review. PubMed: 19619260.
White D, Rabago-Smith M. Genotype-phenotype associations and human eye color. J Hum Genet. 2011 Jan;56(1):5-7. doi: 10.1038/jhg.2010.126. Epub 2010 Oct 14. Review. PubMed: 20944644
I have green eyes, my second brother has one green and one blue, and the third of us has blue eyes. I can’t remember what color my parents’ eyes were, except not brown or hazel, that is, either green or blue.
So, scientists have discovered **at least** 16 genes that contribute to eye colour. They are pretty sure there’s more.
Best I can remember, one is for melanin production (which affects how brown your eyes are), and so is another, and a third is the consistency of the goopy stuff in your eye. Changing the consistency of the goopy stuff changes light refraction which changes what we see when we look at a persons eyes. Cannot remember what the others are, but long story short – there are 16 different genes all having an impact. This gives us the huge variation from grey to blue to green, Hazel, brown and dark brown.
It is definitely not as simple as blue being recessive and brown being dominant.
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