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Friday, October 21, 2011

The Answer is in the Eyes

Let's take a minute to discuss eye color, because whenever it becomes a major feature of a work of fiction the author is doing it wrong.

Let's start with what most people think they know about the genetics of eye color:

1 - Like most physical traits there is a dominant and a recessive gene and a mix gives you a third color (like flowers being red, white, or pink).

2- Brown is the dominant color and that's why so many people have brown eyes.

3- Blue is the recessive color.

4- Green must be a mix of brown and blue.

Options one through four are all lies, in case you hadn't guessed the surprise twist of this post.

There are not two alleles that code for eye color, there is a combination of at least three (on different chromosomes) and some studies cite as many as 17 different alleles that factor into eye color selection. Those genetic factors code for more than eye color, they code for pattern and pigment depth, because your eyes are really layers of color not a single solid sheet of color.

Brown eyes are dominant, but not because of the standard reasons certain features are dominant (like the genetics behind hair color - which is another post for later). Brown eyes are considered the base color for humans. Congrats! If you don't have brown eyes you are a mutant!

But eye color is not a set base like the red/white/pink flower analogy so often used to illustrate Mendelian genetics.
In basic biology classes students learn that traits look like this. Big A is dominant, little a is recessive, and a cross will be a blend or the dominant Big A will, well, dominate. Not so with eye color. First, you need a much bigger punnett square. Second, eye color is on a spectrum. The basic spectrum is roughly:

black - brown - light brown -gray - blue - green

Falling outside the spectrum are violet and red. True purple eyes are associated with albinism, as are red eyes. Elizabeth Taylor, the famous violet-eyed actress - actually had deep blue eyes and excellent makeup (poor color quality in some of her films didn't hurt either).

That debunks all the books where someone realizes the child is theirs because they have blue eyes and the kid has blue eyes but the "parents" both have brown eyes. Brown-eyed parents who are heterozygous (Aa type of thing) can have children with a variety of eye colors.

The other common problem I see in books is that Green Eyes = Irish Lass. Or some heroine from the highlands. Green Eyes and Red Hair seem to be two things people can't seperate in their minds. Which is sad, because the statistics say the Irish Lass only has a 16% chance of being Green Eyed. Now a woman from Iceland, she has an 87% chance of having blue or green eyes. File that under useful information.

Brown eyes are the dominant color world wide, but in Asia and Africa pigment is deeper than the "honey-eyed" European variants. Gray eyes are most common in the Middle East, blue and green eyes are common in Siberia and Scandinavia (also the Scots and Irish have some but that's genetic history and cross-breeding and invasions... long story).

The good news is that as we build international relations we're cross-pollinating our little hearts out. Sci-fi writers can decide if there are still phenotypes (physical appearances) that can flag genetic history, or if the characters have blended into a broader worldwide appearance.

Personally, I like giving my futuristic characters profiles we would consider mixed-racial in modern terms. I think it's unrealistic to say that vast majorities of people will have strong regional/ethnic features 900 years from now. There will be trends, and there will be mutations. But the more borders and mixed-racial taboos fall the more alike humans are likely to look.

Of course, if you're writing a historical novel you best pay attention to what your character is going to look like. The isolation that arose from limited travel and generations without waves of invasions (yes- it happened) gave rise to mutations like the blue eyes and to breeding for features in humans. That's why you can look at someone and guess their genetic nationality today. And why you probably won't be able to in a few hundred years.


  1. Very interesting. I'm a blue eyed mutant. What's read hair say?

  2. Read hair? Definitely a mutation!

    Red hair is a different category. Brown and blonde are coded by the same allele, it's just a difference in how much pigment is laid down in the hair. That's why babies with light hair often have darker hair as adults. It's also part of the reason blondes attract people (lighter hair = younger = better for breeding).

    Red hair is a separate allele and recessive. The coding for red hair and lighter eyes are close together, so you often see the two in combination. :o)

  3. My aunt had an interesting experience in Biology class. The teacher was discussing hereditary traits and said something that wasn't quite right about eye color.

    My aunt said, "Wait, my brother [who is my dad] has green eyes and his wife has grey eyes. They have two brown-eyed kids and two blue-eyed kids."

    The teacher told my aunt that it was genetically impossible and that we couldn't possibly be the children of my parents.

    Ah, morons. Life would be so boring without you.

  4. Emily- I've explained it more than once to strangers who ask which of the kids is mine and which are DH's. But it worries me even more when authors use it as a way to solve a murder or something in a book. I remember reading one years ago where the heroine realized the hero was the missing heir based on eye color. There were other problems with the book, but that's what stuck out.

    Then tonight I saw and editor tweeting about a submission with a violet-eyed heroine with inserted eye roll. You don't need to make your characters have stunning eye colors to make them interesting. And if all that makes them interesting is the eye color there is a problem with your book.

  5. Ha ha! My sci-fi characters have so many different eye and skin (and occasionally hair) colors I don't even bother mentioning genetics most of the time. But this is good to know if I get around to writing my historical fantasy. =)

  6. @Lianna, I saw that same tweet, LOL, and so when I saw this post on Critique Circle I had to check it out. It annoys me that the heroine always has to have some exotic eye color to make her gorgeous. I made mine have brown eyes.

  7. Nick -Sci-fi genetics are *so* much easier. Although sometimes the cross-species hybrids have me asking questions.

    Angela- I have a variety of colors, but not all my characters are human so I don't feel guilty. A good percentage of my human characters have brown eyes though. :o)

  8. Very interesting. I have green eyes, but am no Irish Lass. My kids coined the color of my eyes as 'Kitty Puke Green.' Lucky me...

    Angela @ The Bookshelf Muse

  9. My maternal grandparents both had brown eyes but two of their children have blue eyes. People seem to forget about recessive genes.

    Genetics is the one topic I was interested in and understood back when I did biology in high school. I have a character who has heterochromia, so I did the research on the likelihood of inheritance.

    The only thing I know about violet eyes is that the character of Alanna has them in Tamora Pierce's Song of the Lioness Quartet.

    Eyes are of course fantastic features but I have brown eyes and I think I have pretty good ones. You don't need a character to have violet or topaz eyes to be interesting. There's this extra thing called 'personality' that a lot of writers forget about.

    Fantastic post. It really gives a lot to think about.

  10. Angela - Genetics adds a lot of spice to the guessing game of "What Will Baby Look Like?"

    Keri - I think there's a desire for characters to be special or exotic. The writer wants to make them stand out, and dazzling eye color is one way of doing that. And, of course, there are only so many good descriptors for brown eyes: chocolate, warm... um??? I have brown eyes and the best you can compare them to is bark or dirt. #Earthtones