MadSci Network: Genetics

Re: RE: Genetics

Area: Genetics
Posted By: Tom Wilson,
Date: Tue Jun 25 16:08:20 1996
First, let me apologize for the long delay in answering your question.  
Unfortunately, things got lost in the shuffle, which we try to avoid, but 
sometimes it happens.  I hope you are still interested in the answer to your 

Eye color is a very complicated and interesting phenomenon.  Many people know 
that brown eyes are "dominant" over blue eyes.  In other words, you only need
one copy of a "brown eyed gene" to have brown (dark) eyes, but to have blue 
(light) eyes you need two copies of the "blue" version of the gene.  For a long
time, it was thought to be that simple.  Unfortunately, it is actually quite a 
bit more complicated, and frankly, we don't understand in detail all the things
that affect eye color.

The simplest thing known to affect eye color is the production of a pigment
called melanin in the iris of the eye.  This is the same pigment that makes your 
skin brown.  Some people have genes that give higher melanin production in the 
eye, resulting in dark eyes, and since you only need one copy of the high-producing 
gene to get pigment production, dark eyes are "dominant".  These same people tend 
to have dark skin as well, as is common experience.  However, it is very possible 
to have black individuals with blue eyes, and all sorts of other interesting 
combinations.  So melanin production in the iris doesn't seem to be absolutely 
linked to skin color.

Further, it is clear that many more subtle features of the eye affect its apparent
color, beyond just the level of pigment.  Specifically, the architecture of the 
iris affects the way it reflects light, and thus the color that it appears.
Interestingly, this is similar to the way that bird feathers appear so many 
different colors - they often do not contain colored pigment, but simply look 
colored by how they reflect the light.  For the eye, we don't know exactly what 
all these factors are, nor how they are inherited.  We call such traits 

But WHY do we have different colored eyes - what purpose does eye color serve?  
The simple answer is none.  Even though people continue to try to show a 
difference in the ability to do certain tasks based on eye color, there is really 
no good reason to think that the color of your eye affects any aspect of vision.  
The purpose of melanin in general is to protect the skin from damaging UV light, 
and presumably this happens in the iris too.  However, people with blue eyes 
don't seem to have a problem!  Finally, it is also true that eye color is like 
any other trait of appearance.  We may not be able to give a specific function, 
but these traits certainly affect the way other people see us, how they react to 
us, and ultimately, how likely we are to be able to reproduce and pass on more of 
our blue-eyed genes to our children!

Hope this helps.  I also hope I understood correctly that the questioner here was 
a teacher of grades 4-6, and not a student.

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