How do the genes for uncommon eye colors work? - The Tech Interactive (2024)

Editor’s note (7/20/2021):Some information in this article is out of date. Scientists now know that eye color is acomplex trait, influenced by at least 50 different genes.

The short answer is that we don’t know. 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. Hopefully you'll get an idea of 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?

Another reason is that the inheritance is probably complicated, and not 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 more about eye color.

Eye color comes from genes that make melanin

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

Two genes,BEY2andGEY, work together to make brown, green, or blue eyes. Each gene comes in two versions oralleles.

One form ofBEY2makes lots of melanin (and is usually referred to asB) while the other form makes only a little (b). One form ofGEYmakes some melanin (G) while the other makes only a little (b).

So how do you get eye color from all of this? If you haveByou get brown eyes,G(but noB)you get green eyes and if you only haveb, 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 fromGEYandBEY2. Something likeHEYfor hazel. And maybeHEYis a bit likeBEY2andGEYin 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 hadB, hazel eyes if you hadHbut notB,green eyes if you hadGbut notHorBand blue eyes if you only hadb.

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.

Another possibility is a variation on this theme. Maybe hazel eyes come from different versions ofBEY2orGEY. 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, we don’t know. 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 melaninBEY2orGEYmake. For example, you could get a gene that hasGEYmake more melanin orBEY2make less. The end result would be hazel eyes.

What might this inheritance pattern look like? Pretty complicated.

Gene combinations and eye color

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 usingBEY2andGEYare:

Now imagine a modifier gene that can give you hazel eyes by havingGEYmake more melanin. This gene comes in two flavors --Mincreases the amount of melaninGEYmakes andmhas no effect. As you can see, it is possible to have brown eyes and have aBand abversion of theBEY2gene. Or green eyes and have aGand abversion ofGEY. These people are carriers for blue eyes.

OK, so to have hazel eyes you need aGfrom theGEYgene and anMfrom our modifier gene.Mwould not give hazel eyes withb. Why? Becausebis really a broken version ofG--bmakes so little melanin because it doesn't work.Mcan't fix a gene -- it can only affect how much melanin a workingGEYgene makes.

So what are the genetic combinations that give various eye colors usingM? To simplify things, we'll ignoreBEY2and just concentrate on green, blue, and hazel.

Inheritance of eye colors

Now we're finally ready to look at some examples of how hazel eyes might be inherited. First, let’s imagine a blue-eyed parent withbbmmand a hazel-eyed parent withGGMM.

The blue-eyed parent can only givebmto his children and the hazel-eyed parent can only giveGM. So, all of their children will beGbMmor hazel-eyed carriers for green and blue eyes.

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 givebM. The hazel-eyed parent can only giveGm. The end result is allGbMmor 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. Imagine two hazel-eyed parentsGbMm. 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 one parent on top, and all the gene combinations for the other parent on the side. For our example, you'd get something like this:

The next step is to match up squares. This will figure out all possible combinations and how likely they'll be.

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 fromBEY2instead of increasing melanin fromGEY. And now sprinkle in different modifier genes that increase or decrease melanin made by different amounts. And modifier genes that affect the modifier genes.

In reality, eye color may be a result of all of these ideas -- hazel eye color genes, modifier genes, and different versions ofBEY2andGEY! 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.