A Look into hazel eyes
Feb. 28th, 2009 12:23 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
sereneorange
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 |
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�one 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.
Before launching into this, we need to remember one more thing. We have two copies of most of our genes�one from mom and one from dad. What this means is that there are actually a number of ways of combining genes to end up with various eye colors.
For brown, green, and blue eyes, the possibilities using bey2 and gey are:
| BB bb | Brown |
| BB Gb | Brown |
| BB GG | Brown |
| Bb bb | Brown |
| Bb Gb | Brown |
| Bb GG | Brown |
| bb GG | Green |
| bb Gb | Green |
| bb bb | Blue |
As you can see, it is possible to have brown eyes and have a B and a b version of the bey2 gene. Or green eyes and have a G and a b version of gey. These people are carriers for blue eyes.
Now imagine a modifier gene that can give you hazel eyes by having gey make more melanin. This gene comes in two flavors�M increases the amount of melanin gey makes and m has no effect.
OK, so to have hazel eyes you need a G from the gey gene and an M from our modifier gene. M would not give hazel eyes with b. Why? Because b is really a broken version of G�b makes so little melanin because it doesn't work. M can't fix a gene�it can only affect how much melanin a working gey gene makes.
So what are the genetic combinations that give various eye colors using M? To simplify things, we'll ignore bey2 and just concentrate on green, blue, and hazel.
| GG MM | Hazel |
| Gb MM | Hazel |
| GG Mm | Hazel |
| Gb Mm | Hazel |
| GG mm | Green |
| Gb mm | Green |
| bb MM | Blue |
| bb Mm | Blue |
| bb mm | Blue |
Now we're finally ready to look at some examples of how hazel eyes might be inherited. First, the easiest, a blue-eyed parent with bbmm and a hazel-eyed parent with GGMM.
The blue-eyed parent can only give bm to his children and the hazel-eyed parent can only give GM. So, all of their children will be GbMm or hazel-eyed carriers for green and blue eyes. (See below for a more detailed explanation of where these results came from.)
Let's look at a more interesting example. A blue-eyed parent, bbMM, and a green-eyed parent, GGmm.
This time, the blue-eyed parent can only give bM. The hazel-eyed parent can only give Gm. The end result is all GbMm or hazel eyes! A blue and a green-eyed parent will have all hazel-eyed kids.
This is one of the reasons I like the modifier gene explanation so much. It can help explain how green and blue-eyed parents might have hazel-eyed kids.
Finally, let's tackle a tough one. The faint of heart can skip down to the added links at this point if they want�
Imagine two hazel-eyed parents GbMm. What would their kids look like? For this, we need to bring out the old Punnett square.
The way a Punnett square works is you make a table. You put all the possible gene combinations for the egg on top, and all the gene combinations for sperm on the side. (Click here for a more detailed explanation of Punnett squares.) For our example, you'd get something like this:
| GM | Gm | bM | bm | |
| GM | ||||
| Gm | ||||
| bM | ||||
| bm |
The next step is to match up squares. This will figure out all possible combinations and how likely they'll be.
| GM | Gm | bM | bm | |
| GM | GGMM | GGMm | GbMM | GbMm |
| Gm | GGMm | GGmm | GbMm | Gbmm |
| bM | GbMM | GbMm | bbMM | bbMm |
| bm | GbMm | Gbmm | bbMm | bbmm |
From this the results are that there is a 4 in 16 chance for blue eyes, a 3 in 16 chance for green and a 9 in 16 chance for hazel. Even though this looks awful, it might be possible to figure things out if this were all that was involved.
Now imagine adding the brown gene to the mix. And another modifier that decreases melanin from bey2 instead of increasing melanin from gey. And now sprinkle in different modifier genes that increase or decrease melanin made by different amounts. And modifier genes that affect the modifier genes. And�
And in reality, eye color may be a result of all of these ideas�hazel eye color genes, modifier genes, and different versions of bey2 and gey! As you can see, it all gets complicated pretty quickly. We should be thankful that green, blue, and brown are as simple as they are.
Scientists from Australia describe it differently
They found that just a few "letters" out of the six billion that make up the genetic code are responsible for most of the variation in human eye colour.
The research, by a team of scientists from Queensland, Australia, will appear in a forthcoming issue of the American Journal of Human Genetics.
The findings are based on a genetic study of nearly 4,000 individuals.
 |  One of the changes is like switching the light on and off, while the other is like changing the light bulb from brown to green  Richard Sturm, University of Queensland |
SNPs represent a change of just one letter in the genetic sequence. These changes, or mutations, in our DNA can have important consequences for how the gene gets physically expressed.
All the SNPs are located near a gene called OCA2. This gene produces a protein that helps give hair, skin and eyes their colour. And mutations in OCA2 cause the most common type of albinism.
Brown and blue
The study, which focused on twins, their siblings and parents, shows - conclusively - that there is no "gene" for eye colour.
| THE DNA MOLECULE  The double-stranded DNA molecule is held together by four chemical components called bases Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G) Groupings of these "letters" form the "code of life"; there are about 2.9 billion base-pairs in the human genome wound into 24 distinct bundles, or chromosomes Written in the DNA are about 20-25,000 genes which human cells use as starting templates to make proteins; these sophisticated molecules build and maintain our bodies |
In short, these combinations strongly influence the colour of a person's eyes, but they are not the final word
Dr Richard Sturm and his colleagues found three SNPs near the start of the OCA2 gene that were linked to blue eye colour.
"The SNPs we've identified in themselves are not functionally causing the eye colour change, but they are linked very, very closely to something that is," Dr Sturm, from the University of Queensland, told BBC News.
"When OCA2 is knocked out, there is a loss of pigmentation. The position of these SNPs right at the start of the gene means it is possible we're looking at a change in the regulation of the gene in people with blue eye colour."
Functional change
So these SNPs, at the start of OCA2, probably regulate how much of the pigmentation protein is produced by the gene. People with brown eyes might have a lot of this protein, while people with blue eyes have less.
However, the single letter changes involved in green eyes may actually produce functional changes in the pigmentation protein.
The researchers found SNPs at another position in the OCA2 region - linked to green eyes - that resulted in changes to amino acids (the building blocks of a protein).
"To use an analogy, one of the changes is like switching the light on and off, while the other is like changing the light bulb from brown to green," said Dr Sturm.
Altogether, the single letter changes identified in the study accounted for 74% of total variation in eye colour, the researchers said.
The study was a collaboration between researchers at the Queensland Institute of Medical Research and the University of Queensland, both in Brisbane.