Re: Why are both eyes the same colour?

Hi James,

  First, some terminology.  The colored part of the eye is the iris (plural irides).  The 
color of the iris in any individual is the result of two pigments, eumelanin which is 
black, and phaeomelanin which is yellow.  Eye color is determined by the ratio and 
relative amounts of each pigment.  Low eumelanin and phaeomelanin leads to 
blue irides, low eumelanin and high phaeomelanin leads to green or even hazel 
irides, and high eumelanin and pheaomelanin leads to brown irides.

  Within an individual, the relative production of eumelanin and phaeomelanin is 
dependent on a series of enzymes.  Every person carries two copies (alleles) of 
each of these enzymes, one from Mom and one from Dad.  Thus overall pigment 
production in the iris reflects the combined pigment producing capacity of all of the 
enzymes in the pathway.  

  Usually, the pigment producing capacity of every cell in the body is the same, 
because every single cell carries exactly the same information (alleles) of the 
pigment producing enzymes inherited from Mom and Dad.  However, the human 
body is composed of billions of cells, all originally derived from one cell, the 
fertilized egg.  Every time a cell divides, it has to make a new copy of the DNA, so 
that each of the two daughter cells carries the same information that the original cell 
did.  In the process of millions of divisions, sometimes a mistake is made in copying 
the DNA, referred to as a somatic mutation.  In this case, the genetic material in the 
individual is not exactly the same in every cell.  The proportion of  cells carrying the 
somatic mutation depends on when in development the somatic mutation occured.  
Somatic mutations early in development can affect as much as 50% of the body, 
whereas somatic mutations late in development can affect just a few cells.  Given a 
few dozen somatic mutations per cell division, we all carry millions of somatic 
mutations, but usually only in a few cells, and usually in segments of the DNA that 
don't matter ("junk" DNA).

  Getting back to your question, let's say that a somatic mutation happens in one of 
the pigment producing enzymes, and that this mutation occurs in a cell that 
eventually gives rise to the left eye.  The pigment producing capacity of the left iris 
will therefore be different from the right iris.  This is quite frequent in some breeds of 
dog, and isn't too rare in humans, either.  Just take a close look at David Bowie 
(http://www.bowiewonderworld.com/).  It's also possible to have a somatic mutation 
that only affects a portion of one iris, sometimes referred to as "sectored iris" or 
"bicolor iris".  Bicolor irides can be associated with some genetic diseases 
(Waardenburg syndrome comes to mind) but most of the time are more akin to a 
birthmark, with only cosmetic effects.

  Frankly, I think bicolor irides are kind of cool, but with modern tinted contacts 
anyone can achieve the same effect...

  Chris