rigleyman@aol.com wrote:
Yes, eye colour can change. But only in limited ways, and the genes do
not change:
A young individual does not necessarily lay down all the pigment right
away and it may take till age 5 or more to get full eye pigmentation.
For example I had blue eyes till I was five by which time they started
to turn green then hazel.
It is the same pigment that makes several colours of eye according
to how it is laid down - so genetically I have hazel eyes - the blue
ones I started with are actually eyes with no pigment to speak of -
which causes them to look blue. There is no blue pigment. As pigment
quantity increases, the colour turns green - then hazel. (More pigment
would make brown).
Some people are born with the pigment all laid down and will start
out hazel if they are genetically hazel. There is a gene mechanism in
eyes whereby the *migration* of melanocytes (pigment holding cells) is
affected in the time it takes to get where it is going, determined by
polygenes.
[Polygenes are multiple small genes which act much like pigment in a
paint pot - the more you inherit - the higher the *intensity* of either
a colour expression of a major gene - or the higher the intensity/speed
of migration of melanocytes. Polygenes affect intensity of many things
determined by more discrete major genes. So major genes determine eye
color - but polygenes determine how intense the color is and how fast it
gets where it is going.]
Later in life, as we age, the pigment fades, and you see the reverse
process. So for example brown will become a paler brown, and hazel may
show more green (from reduction of melanin or phaeomelanin, not from
addition of green - there is no green pigment - it is the way light
works with a specific amount of pigment).
So it is not really a "change of color" and it is definitely not a
change in the genetic coding - it's just a variation in the rate of
laying down pigment when young - or the result of fading pigment when old.
You can see the eye colour effects in cats rather well, where people
manipulate the polygenes by selection. Black, blue and red Persian cats
are selected to have orange eyes. Except for polygenes for
intensity/amount of pigment, the colour is actually the same genetically
as the green eyes of say a Chinchilla persian.
If you cross a green-eye chinchilla and an orange-eye black persian
you get a yellowy-green eye colour in the kittens, due to the mixing of
high quantity and low quantity intensity polygenes, resulting in
"average" pigment expression namely yellow-green.
When they age, the black, red and blue persians will have faded eye
color, but the green eye persians will stay bright green. The reason is
that green involves very little pigment, so no change when it fades -
but orange involves a lot - so orange is more subject to fading with
age. As is brown in people.
[Note: Cat eye colour genetics varies somewhat from human in the major
genes but in the matter of pigment fading and intensity - that is
comparable. Hope the example was useful.]
Namaste,
Irene
(Author of "Cat Genetics - What will the kittens look like?"
--
Irene de Villiers, B.Sc AASCA MCSSA D.I.Hom/D.Vet.Hom.
P.O. Box 4703 Spokane WA 99220.
www.angelfire.com/fl/furryboots/clickhere.html (Veterinary Homeopath.)
"Man who say it cannot be done should not interrupt one doing it."
