Feb. 28th, 2009

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I have hazel eyes (although I would prefer big deep dark eyes). I get my eye color from my mother, but I was wondering how hazel eyes happen. I was born with blue eyes like my father (but not as vibrant) but they started switching around when I was about 4 (I think). My sister's have green eyes. Linda's were like lime sherbet, and Karen's are kind of a pistachio ice cream color. My brother's eyes are brown. He got the color from his mother.

I found an interesting read of the genetics of hazel eyes (and any eye color ) over at Understanding Genetics.

Unfortunately, very little work has been done on eye colors other than blue, green, and brown. For this answer, I'll focus on where hazel eyes might fit into the picture. I think once you read this over, you'll get a good feel for how quickly genetics can get too complicated to figure out easily.

So why don't we know more about the genetics of hazel eyes? Part of the reason comes from the difficulty of defining hazel. In other words, when is hazel actually brown? Or green?

People are working on coming up with ways to more precisely define these different eye colors. Hopefully they won't open up a whole new can of worms by giving us a bunch of new eye colors (brown-hazel, hazel, green-hazel, etc.).

Another reason is that the inheritance must be pretty tricky. It must not be as "simple" as blue, green, and brown eyes.

So how might something like hazel eyes work? No one knows for sure but I'll discuss some possibilities. But before that, it is important to go into a little detail about eye color.

Brown, green and blue eye color comes from a pigment called melanin. Brown eyes have a lot of melanin in the iris, green eyes have less, and blue eyes have little or no pigment.

Two genes, bey2 and gey, work together to make brown, green, or blue eyes. Each gene comes in two versions or alleles.

One form of bey2 makes lots of melanin (and is usually referred to as B) while the other form makes only a little (b). One form of gey makes some melanin (G) while the other makes only a little (b).

One of his eyes has B in some
cells and only b in others
So how do you get eye color from all of this? If you have B you get brown eyes, G but no B, green eyes and if you only have b, then you get blue eyes.

Most likely, hazel eyes simply have more melanin than green eyes but less than brown eyes. There are lots of ways to get this level of melanin genetically.

It may be that hazel eyes are the result of genes different from gey and bey2. Something like hey for hazel. And maybe hey is a bit like bey2 and gey in that it comes in two forms´┐Żone that makes enough melanin for hazel eyes (H) and one that makes little or no melanin (b).

If this were true, the scheme for eye color would have to be changed. In the new scheme, you would have brown eyes if you had B, hazel eyes if you had H but not B, green eyes if you had G but not H or B and blue eyes if you only had b.

My gut tells me this probably isn't the answer. Even though this sounds pretty complex, it seems like it wouldn't be that much harder to tease out than green and brown eyes. So it is probably something else.

Another possibility is a variation on this theme. Maybe hazel eyes come from different versions of bey2 or gey. I said at the outset that there were two versions of each gene. But what if there were more? What if there were many versions that result in the various shades of color we see?

This is certainly plausible and some recent research suggests that this might be part of the story. But again, I'm just not sure. I would think the genetics again would be easy enough that it would have been figured out by now.

Another possibility is that there may be modifier genes. These are genes that would affect how much melanin bey2 or gey make. For example, you could get a gene that has gey make more melanin or bey2 make less. The end result would be hazel eyes.

What might this inheritance pattern look like? Pretty complicated.
More. . .  )

Scientists from Australia describe it differently

No gene for eye color )


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