"Michael J. Kane" wrote:
> Jeff (or anyone else), if blue eyes result from a lack of
> pigment in the iris, then why do albinos (who presumably
> lack pigment in their iris) have *pink* eyes, not blue?
>
The only human genetics text I have available to me this afternoon was
published in 1980 (_Genetics: Human aspects_ by A. P. Mange & E. J.
Mange; Saunders College/Holt, Rinehart, & Winston). Neverthless, I think
that the basics of what is going on in albinism were pretty well worked
out by that date. There are two subtypes of albinism:
tyrosinase-negative and tyrosinase-positive oculocutaneous albinism.
People with the tyrosinase-negative subtype lack functional
tyrosinase--an enzyme that leads to the production of the pigment
melanin. Thus, these people have no melanin in their eyes, skin, or
hair. According to Garrod (1909; quoted in Mange & Mange, p. 362):
"Pigments of other kinds are not wanting, such as the lipochromes which
impart their yellow tints to fats and blood serum, and haemoglobin and
its derivatives." Mange & Mange state that the irises are GRAY TO BLUE
because they contain no pigment; but that, "in direct light the eyes
appear to glow red" (p. 363). This red eye-color, if I remember
correctly, is due to the fact that the blood cells in the back of the
eye still are red (as Garrod stated, hemoglobin is normal). Such
red-reflectance does not occur in nonalbino individuals with blue eyes,
if I remember correctly, because they have more pigment in their irises
(even if still only a minimal amount) than albino individuals.
On the other hand, people with the tyrosinase-positive subtype have, as
the name suggests, functional tyrosinase. The cause of the problem in
melanin production is not known (at least in 1980). These people often
do have some pigment production:
"Some pigmentation may develop over a period of years.... The hair may
turn from white or cream-colored in infancy to yellow or red in
adulthood; the skin may gradually change from pink to cream-colored...;
freckles and pigmented moles may occur too. Eye color may change from
BLUE TO BROWN, and red reflectance of the retina usually diminishes or
disappears in later years. (p. 363; emphasis added).
This suggests that, as more pigment is added to the eye, the red
coloring arising from light reflecting off of blood vessels will
disappear. The iris color, however, is blue (in albinos with no
pigmentation).
Does all of this sound correct? Since I am working with incomplete
material here, I do not feel completely certain of all my inferences.
Jeff
P.S. I am sending this at 2:50 pm MST. Some of my posts are taking
several hours to get posted to you. So, if this appears after someone
else has answered the question, please forgive me.
--
Jeffry P. Ricker, Ph.D. Office Phone: (480) 423-6213
9000 E. Chaparral Rd. FAX Number: (480) 423-6298
Psychology Department [EMAIL PROTECTED]
Scottsdale Community College
Scottsdale, AZ 85256-2626
"The truth is rare and never simple."
Oscar Wilde
"No one can accept the fundamental hypotheses of scientific psychology
and be in the least mystical."
Knight Dunlap
