eye color

Anthony Tomlinson tomlinson at pplros.demon.co.uk
Wed Dec 18 06:33:44 EST 1996


In article <32AF89EE.2ED1 at chi1.uncfsu.edu> Floyd Waddle,
fwaddle at CHI1.UNCFSU.EDU writes:
>The short answer to the question of what will brown X green produce? is 
>Just about anything. The long answer is that eye color genetics in humans 
>is not well described in the textbooks. The older books claim brown is 
>dominant to blue which isn't true.  The reason is that blue is not the 
>genetic alternative to brown. Brown eyes is due to the presence of 
>melanin in either the front half of the iris or the entire iris - the 
>literature I've read is confusing here.  Whichever it is, the genetic 
>alternative may be "presence of melanin" (dominant) vs "absence of 
>melanin" (recessive). Moreover there are two types of melanin, eumelanin 
>and phaomelanin.  In mammalian hair, eumelanin produces black and brown, 
>phaomelanin produces yellow.  Lack of melanin produces blue eyes due to 
>light refraction. Presence of melanin produces green to dark brown.  
>(Green = yellow + blue) If this explanation is true so far, then the 
>actual eye color of pigmented eyes is controlled by modifier genes which 
>determine amount and placement of pigment. I suspect the full explanation 
>is somewhat different than this since this does not account for dark vs 
>light blue eyes.  If you check enough biology and genetics textbooks, you 
>might find a better explanation.  This at least accounts for why a green 
>eyed person and a blue eyed person can have a brown eyed child. The blue 
>eyed person cannot express genes that determine how much melanin is 
>present, but can pass such genes on to offspring.

Just to muddy the waters, my eyes are hazel, but vary in colour from
green to
brown at different times.  This has nothing to do with lighting or
anything like
that; it seems to be a genuine phenomenon.  Any ideas?

Anthony



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