I like eyes and this has bothered me for a while.
OK, I know that the gene for blue eyes is recessive, and the gene for brown eyes is dominant. Therefore, if one parent is brown-eyed (say both their parents were brown-eyed), and one parent is blue-eyed, you’ll have a brown-eyed child. In the same case, if the brown-eyed parent has one blue-eyed parent, the child may have brown eyes (50% chance) or may have blue eyes (50% chance). That is, if there are 3 out of 4 possible alleles floating around for blue eyes, half the offspring will be blue-eyed.
But how does the model for eye-colour inheritance explain all those other eye-colours that you see? I know people with hazel, green, yellowish-green, greyish-green, and so on, irises (irids?) all the time. Where do these eye-colours come from?
Nope. Just because both their parents were brown eyed, doesn’t mean he’s got two “brown genes”. Say mom has brown/blue–passes on blue, dad has brown/brown–passes on brown. Now the child has brown eyes but carries brown/blue and can pass on either of those to his own kids, so if he ends up with a blue eyed spouse the chances of a kid also being blue is 50/50, not at all impossible. Just because the original mom and dad are brown eyed, it doesn’t mean they had homozygous pairs.
To your main question though, eye color is influenced by three gene pairs (that we know of) not just one. There’s a brown/blue, a blue/green, and a brown only, and it’s the way the three interact that determines ultimate eye color, which is really caused by how much or how little melenin gets left on the iris. A lot, and you’ve got brown eyes, none and you’ve got blue, and then all the ranges in between.
You don’t know that, you simply believe it, which is not quite the same thing.
Eye colour is not a simple recesive dominant trait based on a two alleles on separate chromosomes as you believe. It’s actualy quite complex and still not entirely understood even by geneticists.
It certainly isn’t as simple as brown = dominant, blue = recessive, although that is very broadly true. There are some good websites available that explain the genetics of eye colour inheritance if you care to use Google. I’m personally to lazy.
Basically there are numerous genes for eye colour scattered over numerous chromosomes. And they aren’t all performing the same function. IIRC the ‘base’ human eye colour, as for most mammals, is blue and that;s caused not by a pigment but by the refractive effect of the material the iris is made of. Some of those genes code for a yellow pigments that blend with the blue to produce greens. Other genes code for brown pigmants. Normally the brown pigments, being darker, will swamp the yellows and as a result brown tends to dominate.
Note however that this situation is of a totally different class to the simple dominance recesive of somehting like earlobe attachment where either the personhas a functional copy of agene or they don’t. In the case of eye colour both the yellow and brown genes are perfectly functional but the brown literally swamps the yellow. Because that is the situation the eye can be coloured by different degrees of yellow or brown and hence be shades of greenish brown.
That’s all made more complex by the fact that there are other genes that control the placement of the pigments. So a person can have a brown inner iris and a blue outer iris, or a predominantly green iris with brown flecks.
Sarah I’m not awake enough to think much yet, but here are 1 2 3 other threads that explain the whole eye color thing . Have fun 
I think there’s a great deal of misunderstanding about the color hazel. Some think it includes a mixture of blues and brown, but hazel is more properly defined as greenish-brown or yellowish-brown.
Think hazel, think hazelnut.
Then what’s blue-green? That’s what I thought hazel was. I have that color eyes. My wife has brown eyes. All three of my children have blue eyes.
turquoise? aqua? teal? sea-green? Or maybe it’s just called blue-green.
I’ve also heard of people (rarely) having naturally purple eyes or yellow eyes. Yellow kind of makes sense – maybe there’s just so much yellow pigment that it overwhelms the blue base (or perhaps there’s a separate gene that makes the blue base-color lighter so it doesn’t show through the yellow as much?) But what about purple? Does that mean there must also be a red pigment? Could someone conceivably have irises that are completely red?
I don’t think it’s actually a true purple. More like a very reddish blue. Like oh, see the dark blue on the Straight DopeTM banner? Or like Elizabeth Taylor. Technically, her eyes are blue, but they look almost purple.
Thank you for that explanation. I’ve always wondered how my daughter ended up with greenish brown eyes when both her father and I are blue eyed. I told him it wasn’t the pool guy.
My dad’s eyes are hazel. Mine are officially hazel. Often, they decide they’d rather be green. I don’t understand them at all.
Eye color can change depending on the available light. My wife (named available light by the way) has eyes that are sometimes green, sometimes brown. On the other hand, mine are sometimes brown and sometimes, uh, darker brown.
As for red eyes, I knew a guy who had one blue and one red.
Mine are classified as hazel, but they change. There are slightly overlapping rings of brown and green around the pupil, and the relative sizes (and the size of the overlap area) change. Often, they go completely brown.
I’ve been told that the color changes with my mood and/or with what I’m wearing. I have a really hard time believing either of those, but I have no idea what actually does cause the change. Does anybody know what causes this?
I have a friend who is Polish, and her eyes are yellow, like a cat’s. I think that people who are albino have “red” eyes, because their irises have no colour in them at all, and so the blood vessels show through.
I guess I don’t quite understand why there are so many colour possibilities - what would be the genetic, or evolutionary, reason for this?
Albino people usually have blue eyes that can look pink in certain light and from certain angles.
I have hazel eyes and never heard anyone say hazel was brown/blue. Weird. I wonder why someone would confuse that that? I’ve got no vlue at all, but the brown, green, and a hint of yellow flecking is there.
put simply, why not?
mutation (the origin of alleles, or different forms of genes) is of course random, unless induced in a lab. mutations just happen in response to mutagens, and there does not need to be any “reason” for a mutant allele to proliferate in a population; any allele will do so as long as it does not cause death or severely decreased survival of the organism before reproductive age. asking what the “reason” for an evolutionary development is implicitly presupposes that evolution follows a plan, but the reality is that mutations just happen, and if they happen to be useful or at least not crippling to the organism, they spread.
is this an answer to your question? or are you curious as to specific biochemical mechanisms for production of individual colors resulting from mutations?
I think it’s just random. Natural selection only comes into play if having a certain trait makes a difference in your chances of procreating. If having blue eyes is just as good as having brown eyes (as far as your chances of surviving long enough to procreate), then there’s no evolutionary reason to have one or the other.
I think yellow is an North Eastern European thing. We have yellow eyes in the family and it runs on the side of the family from that part of the world, Poland included.
From Russia With Love also includes a yellow-eyed character in the first chapter.
Yes, that’s the answer to my question - what the evolutionary angle on it was. I thought that the number or size of melanin cells would determine how much “brown” pigment there would be in the iris, like with skin colour. And as Cecil tells us, green eyes are green because of fat deposits 
Hey, that’s interesting! My father’s side of the family is all Polish, which may go a long way towards explaining why I have green eyes which are yellow near the centre. (When my pupils are small my eyes tend to look yellow-green, and the rest of the time they’re green-blue). Mum’s eyes are a very dark green and my dad’s are pale blue - mine are mid-depth between the two - but neither have yellow, nor do any relatives that we’re aware of. I’ve wondered where I got it from.